How to Do Case Interview Mental Math
Fast, Accurately, and while Under Enormous Pressure

Some things in life are NOT natural. Doing math computations in your head, while an interviewer is staring at you, with the prospect of a $100,000 or $225,000 job offer at stake and your entire career future at stake, is NOT a natural act.

I have received numerous emails from PhDs from MIT in Physics and Engineering who failed a case interview because they made a computational error. Obviously people with perfect GRE scores can do math taught to 11-year-old children, but doing it UNDER PRESSURE with another person in the room, staring at you, waiting impatiently for the answer, is an entirely different experience.

In a case interview, 95% of the time a single mental math error results in an automatic rejection. This is because firms like McKinsey, Bain and BCG can not take the risk that you will make a mental math error in front of a client that’s paying them $1 million a month for your and your colleagues’ time.

If you are even considering a career in consulting, I can guarantee you that you already know enough math concepts to pass the case interview. It’s called addition, subtraction, multiplication and division. In consulting, we also often use percentages. That’s 90% of the math we use in the case interview. Clearly, you already know how to do that.

What you may not know how to do, or have enough real world experience doing, is doing such computations in your head, using large numbers.

For example, what is the approximate answer to 500 million divided by 150 multiplied by 15%?


One second… two seconds… three seconds…

Do you know the answer?

If you haven’t figured out the correct answer by the time you read this sentence, your current mental math skills may be too slow or not accurate enough to pass a case interview.

Incidentally, the precise answer is 500,000, but any answer between 400,000 – 600,000 would have been acceptable by most interviewers.

Here’s the big secret to doing mental math fast (and accurately). You do NOT do mental math in the same way you do math using paper and pen, or with a calculator. The sequence of operations is totally different. In mental math, you actually take MORE steps than when you do math on paper or via calculator. Yes, you actually make the math problem LONGER mentally, but in the process you make the problem SIMPLER to solve. THAT is the counter-intuitive secret to doing mental math both faster and more accurately.

Because so many people struggle with the mental math aspects of the case interview, I’ve put together a very useful and immediately applicable video tutorial on how to do case interview math quickly and accurately while under pressure. 

Deluxe Mental Math Fast and Math Guide with LOMS

  • Deluxe Mental Math and Math Guide
  • LOMS

Deluxe Mental Math Fast and Math Guide

  • Mental Math Fast
  • Math Guide

Mental Math Fast

  • Mental Math Fast

Read what some of our 120,000 students have to say:

"I found your Deluxe Mental Math and Math Guide Toolkit absolutely useful, concise, time effective and it covers the core math for cases I've been exposed to while practicing - particularly interviewer-led cases which demand a lot of math calculations."
- Ignacio, Argentina
"Your resources (LOMS, case interview math and success stories/emails) that I've been using for the last 5 months have enabled me to finally get an offer from LEK for the life sciences specialist role! Thank you so much! I was literally starting case prep from a sub zero standard. I listened to your LOMS videos almost every single day while working, over and over again. I found that re-listening to them really helped me learn something new each time and/or reinforce points about structure and synthesis, which I would then incorporate while solving cases. Thank you once again!"
"I would like to thank you for the materials provided in your book and on your website which have been a great help to me. Only 2 months ago I knew very little about consulting and had never done or practiced a case interview. Fast forward 6 weeks and I received an offer to join McKinsey. Using your book and online tutorials as a starting point I began practicing and spent about 6 hours per day on case interviews (video calls with case partners around the world), mental math (the tool on your website is very useful and I practiced about 30 minutes with it every day), preparing the personal experience examples and reading news articles and thought pieces."

Got questions ?

The Mental Math Fast program consists of:

  • A video tutorial showing you these mental math strategies and tricks, how and when to use them, and how to practice them in preparation for a case interview
  • Transcripts from the video tutorial
  • Presentation slides for easy reference
  • Instant Recognition Math Reference Handout – a quick guide to figuring out which mental math approaches to use in specific situations

The Mental Math Fast program is delivered to you digitally within minutes of your purchase, allowing you to start using the program immediately.

Yes, the Mental Math Fast program is backed by our 30-day No Questions Asked Money Back Guarantee.