The secret to have bulletproof mental math in just a few days

Bulletproof Mental Math is a short course that will give you the tools and practice to do effortless, error-free mental math in your case interviews

Don't let a simple math mistake get between you and the offer!

The type of math you need in consulting interviews is simple: adding, subtracting, multiplying and dividing.

You need to be able to do that with regular numbers, percentages and large numbers.

Straight forward, isn't it? 

I'm pretty sure that if you're able to get an interview with a consulting firm, you can also do those operations.

So, why do people mess up with simple math?

If you've ever been to a case interview (or even done a mock interview), the answer is easy: 

For the first time in your life, you're expected to do math quickly, accurately, under pressure, out loud and perhaps without even using paper.

School didn't prepare you for that.

(In fact, it prepared you for the opposite! Slow, quietly, pen-and-paper, and without a ton of pressure…)

And because of these nuances, candidates often make simple math mistakes in their cases that derail them completely.

See, interviewers won't outright reject you just because you made a small arithmetic mistake.

But they WILL reject you if you:

  • Get so anxious from making a mistake (or getting stuck) that you tank your performance in the case from then on…

  • Make tons of small math mistakes and never catch them, making them think the same would happen in the real job…

  • Take so long fixing your math that you can't even get to the end of the case (that happened to me once)...

Because of this, learning to do error-free mental math quickly is a small but important part of your case interview preparation.

This means you'll want to learn this skill, but also do so without investing a lot of time!

How to rapidly improve your mental math skills

Most candidates I've met do some kind of mental math practice during their preparation.

The problem?

They waste too much time.

They spend hours per week FOR MONTHS AND MONTHS practicing this skill.

And even after all that, they still can't rely on their skills.

In other words, the ROI is terrible.

Now, the reason this happens is because people work on this in a haphazard way.


  • Download a mental math app to start practicing during the day
  • Get a couple tips from a friend that improves things a little bit
  • Apply those tips during cases, but keep making careless mistakes, wishing it to go away eventually

No wonder they don't improve that much…

What I've found over the years is that you can learn to effortlessly compute numbers in your head without making errors in just a few days, as long as you have a learning system with 3 things in place:

Let's break this down:

THING #1: Work ONLY on case-related math problems.

There are specific types of math operations that you'll need for your case interviews. 

Some are very easy, so you just need to learn to be agile and to not make mistakes (e.g.: the 4 basic operations, percentages, operations with large numbers like millions and billions)

Some are a bit trickier, such as financial math and compounding, the Rule of 72, etc.

There are also many types of operations that you won't need: square and cubed roots, logarithms, etc.

If your focus is to improve your mental math for case interviews, it'd be wise to find use a method focused only on what you needed. You don't want any gaps, but you also don't want to waste time with difficult problems that won't be useful.

(This eliminates most mental math books, btw…)

One way to do that is to do "freestyle" practice – just practice your mental math as you do cases.

The only problem is that this misses the other two factors, so you're unlikely to improve that much unless you're already a "natural" on mental math.

THING #2: Learn ONLY the most effective mental math techniques.

Books focused on mental math do this well – they teach you specific techniques for computing numbers faster and without making mistakes.

Of course, they don't have as many practice problems and they teach you types of problems you'll never use in a case interview…

… Which is why you want a resource that gets the best techniques these books teach that apply to the problems you'll get in your cases.

We never learn these techniques in school, so it's no wonder we can't do mental math fast and free of error.

THING #3: Get plenty of practice!

This is the path many people take: they download a mental math app and churn out calculation after calculation in their downtime.

17 x 32

1,082 + 517

832 / 12

While this is better than doing nothing, you're not improving your technique to solve these problems, nor are you focusing on the right types of calculations.

You just do more and more math, and "try to get better next time".

At the end you're just tired, numb and frustrated.

Because none of these isolated solutions incorporate all three elements, candidates take months to improve their mental math skills. 

Not to say, many still mess up even after all that practice.

But every candidate I have seen who have put these three things into place (the right problems, techniques and practice)...

… They all improve DRASTICALLY in a matter of days.

So far, no resource had this balance of these three things right for case interview prep.

This is why we've created Bulletproof Mental Math… 

Introducing... The only practice-based crash course that helps you master effortless, fast and error-free mental math in just a few days.

Why Bulletproof Mental Math

  • Learn to do fast mental math

    Real consultants calculate quickly and don't need paper. You should strive for the same.

    We'll give you the tools and practice to do math typical of case interviews quickly, effortlessly and without needing pen and paper.

  • Make your math bulletproof

    More important than being quick and doing it in your head is to not make mistakes (or to at least catch them quickly).

    This course incorporates error-checking methods in all of its techniques.

  • The quickest way to learn

    Let's be honest, you won't be hired because you're a human calculator. You just don't want math to get in the way.

    This is why this course was designed to be short, and its concepts were hand-picked to be easy to learn and extremely effective in your interviews.

What you'll get inside Bulletproof Mental Math:

Module 1: 

Mental Math Fundamentals

In this Module, you'll learn the basic fundamentals of doing quick, accurate mental math, and how it is COMPLETELY DIFFERENT than the math you learn in school (which I call "paper math").

Mental Math Fundamentals are important in an interview context even if you use paper, and the reason is that it makes the math easier to explain (from your perspective) and easier to follow (from your interviewer's perspective).

Specifically, you'll learn:

  • The three mental math myths that make candidates look green and naive to interviewers.

  • The top three techniques to do math quickly, accurately and without using paper (we call them the "Crystal Ball Technique", the "Rounding Hierarchy" and "Numerical Breakdown").

  • The basic math "primitives" that you need to be more fluent and comfortable with math than 99% of candidates.

This module is the 20% of this course that will get you 80% of the results.

Many candidates can already greatly increase the mental math skills of most candidates, because what gets them stuck if often a lack of one or two mindsets/techniques.

Module 2: 

Fluent "5th Grade" Math

You've learned most of the math you need in consulting by 5th grade – I'm talking about the 4 basic operations.

Yet, case interviews pose a new challenge: being extremely fluent while talking through addition, subtraction, multiplication and division.

In this module, you'll learn AND practice how to apply the Mental Math Fundamentals into these basic operations, so that you can compute them quickly, accurately and effortlessly.

You'll learn:

  • The ONLY 3 techniques you'll ever need to add and subtract with MBB-level numeracy and fluency.

  • Why doing more calculations is often necessary to make the math easier, and how that relates to the processing power of your brain.

  • The "10% Trick", the "Division-Muliplication Inversion", and other 3 key techniques to multiply and divide effortlessly in your next case.

Improving in these 4 operations may sound silly to you, but if you look at your past mistakes, most of them will be with one of them.

Module 3: 

Mastering Large Numbers and Percentages

Case interviews and client problems alike often face large numbers and percentages.

We're talking about Millions, Billions, as well as tiny rates expressed in percentage terms.

And with these varying orders of magnitude, comes a big source of mistakes:

You keep missing the number of zeroes.

We'll make this a problem of the past, by showing you:

  • How to eliminate mistakes with large and tiny numbers with 2 key techniques and 2 key principles.

  • The ONE KEY TECHNIQUE every consultant uses to instantly compute unintuitive percentages (that'll make you sound like one of them).

  • Four other techniques to deal with percentages that'll be your safety net for harder cases.

This module will save you from making the gross order-of-magnitude mistakes that are the most dangerous when doing math in case interviews.

Not making these mistakes will give your interviewers the assurance that you're going to be someone reliable enough to be left with clients without being micro-managed (aka: their next hire).

Module 4: 

Fluent Financial Math

Financial math is especially tricky in case interviews.

It deals with formulas like ROI, CAGR, and NPV.

It also involves small percentages that compound over time.

In this module, you'll learn to be fluent in this kind of tricky math that's especially common in investment cases.

Specifically, you're gonna learn:

  • How to use the Rule of 72 to effortlessly manipulate compound interest across many years with just pen and paper.

  • The only few numbers you need to memorize to turn most boring 5-minute compound interest calculation tables into a single, 15-second solution.

  • The 6 financial formulas you need to know to be up to speed with the MBAs and finance geeks that are interviewing in the room next to you.

This module will make financial math less scary to you, as well as help you shave off several minutes of computations in interviews that require this kind of math.


Applied Case Math

This course is powerful because it helps you deal with NUMBERS in a more fluent, elegant, and less risky way.

However, we also want to bridge the gap between manipulating numbers and using them in REAL CASE PROBLEMS.

In this module, we bring some of the concepts taught in our other courses related to analytics (Analytics Academy and Master Market Sizing) to show you how the mental math techniques taught here relate to specific types of math equations and structures you'll find in your interviews.

Specifically, you're gonna learn:

  • The 3 calculation patterns that you'll use the most in market sizing questions in order to estimate faster, make less mistakes and deflect curve-ball questions from your interviewer.

  • The tricky math behind break-even questions, McKinsey's favorite math question type.

  • The "hidden" percentage-based type of math question that almost no candidate is aware of, and how to use the techniques from this course to solve it with ease.

This bonus module will bring you back down to earth and teach you how to apply the techniques in this course in common, challenging real case interview situations.

It'll also give you a sneak peek of the methods and techniques we teach in our other courses on how to structure tricky case math questions.


Worksheet Video Solutions

Every core module of this course has one or more worksheets so you can practice the techniques learned.

Because we don't want you to get stuck anywhere, in this module we'll solve the WHOLE worksheets with you.

This way, you can follow an expert solving each of the math problems in the worksheets out loud, so you can emulate the application of the techniques to each individual problem.

This is an optional module -- it may well be the case that you'll feel confident to solve the worksheets all by yourself right after learning the techniques in each module.

Yet, should any part feel difficult to you, the video solutions here will help you get over the hump.

WARNING: Bulletproof Mental Math is a PRACTICE-BASED COURSE

Like in all our other courses, you'll find just the best systems and techniques to improve your mental math. And just like in our other courses, you'll also find A TON OF PRACTICE.

Every single module of this course has worksheets with math problems that help you apply the techniques you've just learned.

And on top of that, you'll find video solutions to these worksheets, to see us solving these problems in real-time.

Yes, there are other places you can find techniques to do math faster and without using paper…

And there are also a variety of tools and apps to practice doing math out there…

But I know of no other resource that has only the techniques that are most useful for case interviews INTERTWINED with the practice you need to immediately internalize those techniques.

Some people spend months improving their mental math for their cases – which is a big waste of time.

This course was made for you to drastically improve your math in just a few days.

And the way this is done is by putting techniques and practice together, and focusing on just the things that matter specifically for case interviews.

You'll learn just what you need to learn and then immediately apply these into practice problems.

It does take some work, but it's exactly this work that'll enable you to effortlessly power through math problems without making mistakes in your next interview.

Frequently Asked Questions

  • Why do consulting firms care about mental math if everyone has a calculator in their pockets nowadays?

    Good question, and one of the most common we get in the comments of our Youtube channel.

    They don't really care about mental math per se.

    What they care about is "numerical comfort": how comfortable you are working with numbers.

    Doing simple math quickly, without showing effort, without making mistakes, and, ideally, without needing paper is a proxy they use to see how comfortable you are with numbers.

    Think about it…

    Which person is more comfortable with numbers?

    PERSON 1: Automatically picks up their phone (or a piece of paper) when asked what is 78 + 81.

    PERSON 2: Quickly gives an (approximate) answer when asked what is 1,102 / 12%.

    If you had taken Bulletproof Mental Math, you'd answer 8,816 immediately to the second question because you'd have internalized that dividing by 12% means (roughly) multiplying by 8, and you'd be able to easily multiply 1,102 by 8 by doing math from left to right.

    You'd do that before the other person could even unlock their phone. And you'd also know the result is slightly imprecise and that to get to the precise result all you needed to do was to sum ⅓ of 1,102 to the result.

    This is numerical comfort. And your interviewer wants to see that.

    Being able to quickly and effortlessly do mental math is akin to being able to work on a spreadsheet using only keyboard commands.

    On the surface, it doesn't matter much because the person who reaches for the mouse all the time can create the same model as you. But in practice, it makes a big difference.

  • How long will I need to spend on this course?

    The whole course has 3h45m of core content (which includes video lessons and mini-exercises within the lessons) and worksheets to help you internalize the techniques.

    Our estimated time for you to complete the worksheets is 45 minutes in total.

    So you can complete the whole course in 4h30m, including doing the worksheets.

    In case you have a bit more difficulty with the worksheets, there are video solutions of all worksheets, that amount to about 2 hours of extra video. These are optional and only necessary if the techniques from a specific module are especially difficult for you to apply.

    So at the top of the range, you'll spend 6-7 hours doing everything.

    This is by design.

    We wanted the course to take you to a Top 5% level of mental math in just a few days.

    Of course, you can take the course at a slower pace if you have more time than that – by this point, it's your choice.

  • My interview is in only 2 days, is it enough?

    Depending on your schedule, you may not be able to go through the whole course (unless you spend 2-3 hours per day on it), but even if you do ~50% of it, you'll be way more fluent in doing math.

    I don't know about you, but I'm not a completionist… If I can do 50% of something and that something helps me achieve my goals, that's good enough.

    That said, be sure that whatever you do in these two days is the critical thing you should work on.

    If you know with good certainty that computations / mental math is the thing that'll help you the most in these interviews, I don't see anything that'll help you more than this course.

    If, however, you think other things such as creating frameworks / structuring equations / estimations is more important, I wouldn't join this course for now and focus on those other things instead.

  • I've always been bad at math… Will this work for me?

    If you're the type of person who sees it as realistic to get an interview with a consulting firm, you can learn to do fast, error-free math without using paper.

    In fact, I've once had a coaching client who had trouble with arithmetic since 5th grade, and I was able to solve her problems overnight by teaching her just two techniques – techniques you'll find in this course.

    The truth is, these techniques could be taught to 7th graders and they'd be able to easily do consulting-level mental math. Unfortunately, school prioritizes other things like sin/cos functions, but that's a topic for another day…

    The point is: the techniques are simple and effective. With a little bit of practice (which you'll also find in this course), 99.9% of the people who get a consulting interview are able to internalize them in under a week and completely change the way they solve quick math problems.

  • I don't have trouble with the calculations, but with structuring the equations. Is this the right course for me?


    This course focuses almost exclusively on doing computations fast, effortlessly and without making mistakes.

    You can use these techniques either on paper or using just your mind.

    If you're having trouble with structuring the equations, a step prior to computing the numbers, we have two other courses that are focused on that.

    Master Market Sizing is our course focused on Estimation problems. It teaches you how to structure, create assumptions and reality-check your estimations (market sizing problems, for example).

    Analytics Academy is our course focused on Quantitative Analysis and Chart Interpretation problems. It teaches you how to structure analyses when you already have the data (so it's not an estimation), and how to interpret the results of analyses or charts to move the case forward.

    These two courses are longer than Bulletproof Mental Math because the skill of structuring these types of problems is more complex and take more time to learn.

    They're important, though, because if you can't structure your estimations or analyses, it doesn't matter how fast you can compute numbers.

    Which course is for you merely depends on what skill you want to get better / which skill you struggle the most with right now.

  • My recruiters said I could use pen and paper to do math… Why should I learn to do mental math then?

    Even if you use pen and paper to solve all your math problems in your interviews, your interviewer's still looking at how you solve them.

    They want you to solve math problems in a way that's efficient, easy to communicate and easy to avoid (or catch) mistakes.

    Aka: all the techniques taught in this course.

    The techniques in this course work for both pen and paper math and mental math.

    It just so happens that, once you learn them, mental math becomes as easy as pen and paper math, so you won't even bother using paper most times.

    That said, you still need some competency in mental math for when these two scenarios eventually happen:

    1) You get a math problem so easy your interviewer would cringe if you had to reach for pen and paper (e.g.: "82 + 150"; "60 * 11%"; "1,152 / 2").

    2) Your interviewer (usually a partner) explicitly asks for you to run a set of calculations in your head, out loud and without using paper.

    The first scenario will happen several times per interview, because most interviews have a ton of computations, many of them very simple. If you have to use paper to do all of them, your interviewer may see you as numerically challenged.

    So it pays to do at least some of the computations mentally on your interviews.

    The second scenario is rarer, and will likely only happen in final rounds.

    But it has happened to me and to several people I know.

    And believe me, you don't want to be stuck on a final round unable to do this.

  • I've heard you tell me not to worry much about calculations in other courses. Should I worry about them or not? How important is mental math in the grand scheme of things?

    In the grand scheme of things, arithmetic skills are less important than others such as problem structuring and structuring math problems.

    The reason why we emphasize this so much in other courses is that a lot of beginner candidates spend as much time on mental math as on those other, more important skills.

    Now, the truth is that if you can't solve the key skills well, being fast on mental math is pretty much useless. You won't even get to the point of calculating numbers.

    That said, being able to do bulletproof mental math is pretty useful – your case will flow better, your interviewer will be more impressed and you'll be more confident you won't make a dumb mistake that derails your case.

    It's just down on the priority list compared to those other skills (which are the 6 Building Blocks we teach in Case Interview Fundamentals).

    This is the primary reason why we designed this course to be FAST.

    It's useful to learn it, but only if you don't spend weeks / months to do so. You've gotta learn it quickly for the ROI to be positive.

    Otherwise, it's better to spend all this time on the more fundamentals skills.

  • Can't I just get a mental math book?

    Of course you could.

    And it'd probably help you.

    There's only two problems:

    1) They tend to spend a lot of time on irrelevant things (for case interviews), like vedic math, how to quickly multiply 8-digit numbers and party tricks.

    2) They don't have enough practice, especially practiced focused on the types of numbers you'll see in cases.

    All in all, these are books written by and for math enthusiasts who get excited about, well, solving math riddles.

    It's cool reading for the summer if you're that kind of person, but it's an inefficient way to upskill for your case interviews.

    We've created this course because a lot of people want to learn this specifically for passing interviews as quickly as possible.

    If that's you, this course is a better fit.

    The price difference is small too, and here you'll get exactly what you need.

    So, these are the pros and cons of each. In the end, it's your call of course.

  • What about other courses and resources specific to case interviews?

    We've used them all and didn't find a mental math course / resource that:

    1) Has the right problems, the right techniques and the right practice intertwined to teach exactly what you need.

    2) Is focused on helping you be able to effortless, fast and error-free as quickly as possible.

    We wanted you to have that, thus we've built this course.

    If you've ever used any other of our resources and liked it, this one won't disappoint you.

Try Bulletproof Mental Math for 30 days, 100% risk-free!

We've worked hard to make the only program focused on building your mental math skills for case interviews FAST (in just a few days)

We've battle-tested every technique inside the course with real candidates. Many went from constantly making math mistakes to fluently and reliably doing mental math without much effort. 

We know it works, and we know it will help you compute faster and more reliably than 99% of candidates.

And we want you to see it by yourself - which is why we offer you to try the whole course risk-free for 30 days.

If after 30 days you don't LOVE this course, we want you to have 100% of your money back.

Just send us an e-mail, show us you did the work and we will return you 100% of the investment of the course. We'll even eat the credit card fees.

This guarantee lasts for 30 days, which is more than enough to go through the whole course.

You can learn AND practice our mental math techniques for the basic operations, large numbers, percentages, and financial math so that doing math stops getting in the way -- all 100% risk-free.

Get instant access to Bulletproof Mental Math now for just $70

You'll get 12-months of access to the whole material and will be able to start learning and practicing right now

Join Bulletproof Mental Math Now!

What if you could do mental math just like your interviewer does?

In my last interview at McKinsey (right before I got the offer), something my interviewer said stood out to me...

I was doing a series of math computations and as I announced the final result, the Partner looked at me, mesmerized, and said:

"It's interesting... You do math just like I do."

Needless to say, my confidence skyrocketed and it's not a stretch to say this helped me throughout the rest of the case.

More importantly: that Partner saw me as one of them.

(And called me to announce the offer to me while I was still stuck in traffic, going home from that interview.)

Now, you might be thinking that "it must be nice" to be so good at dealing with numbers.

But the truth is I wasn't always good at mental math.

In fact, the first time I applied to McKinsey, I got stuck for 10-15 minutes in ONE SINGLE DIVISION.

That's how nervous I was...

But one thing I noticed during my many interviews (and later confirmed once I started working at the Firm) is that every consultant seemed to do math similarly.

They'd do it from left to right (instead of the traditional right to left)...

They'd break down complicated numbers into easy-to-work numbers, and seamlessly put it all together at the end...

They'd do rough math with simpler numbers before doing the precise math (so they knew if they made a mistake or not)...

Because I saw those patterns, I attempted to emulate them throughout my preparation. At first, it was hard, but after many months of practice (and a lot of time seeing how they'd do the math themselves) I learned to do it.

Hence the praise I received from that one McKinsey Partner that called me to tell me I was getting the offer.

And let me tell you...

The difference between struggling with doing simple computations vs. effortlessly and fluently doing math in the same way that your interviewers do is NIGHT AND DAY.

Not only does it affects their perception of how good a fit you are with the profession...

... It also dramatically increases your confidence!

You get more confident because you know you won't get stuck.

Because you know if you make a mistake, you'll catch it before they even see the mistake.

Because you know the math won't stand in the way of you solving the case problem (or your client's problem).

And the best part?

It only takes a few hours with this course plus a few days applying what you learned in it to master this skill.

Get instant access to Bulletproof Mental Math now for just $70

You'll get 12-months of access to the whole material and will be able to start learning and practicing right now

Join Bulletproof Mental Math Now!