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Is the usage of calculator(or perceived well) for market sizing and/or math questions portion of the case interviews?

Anonymous A asked on Jan 07, 2018 - 3 answers

Hi PrepLounge Community,

The other day I was reading a book called Ace Your Case III. And the book suggests that usage of calculator to do quantitative section of the case interviews such market sizing is actually OK. Here is the excerpt from that book dedicated for the market-sizing cases and how to manhandle the these sort of cases:

" Rule 3: Write it down.

If you feel more comfortable writing everything down and using a calculator, do! Most interviewers will not care if you use a pencil and paper to keep your thoughts organized and logical. And if pulling out the HP to multiply a few numbers keeps you from wigging out, then by all means do it. Your interviewer will be more impressed if you are cool, calm, and collected, and if using props helps you, then go for it."

Would PrepLounge community that have experience with real case interviews suggest following recommendation or do you think it is not acceptable given the expectations from interviewee?

I appreciate your time in advance,

Kind regards,


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replied on Jan 07, 2018
Former BCG decision round interviewer with 300+ real interviews in 8 years
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In my experience at Bain, BCG and McKinsey no use of calculator during in-person interviews is generally allowed. However the suggestion of writing down the math and calculation is one that I second wholeheartadly because makes it easier for the interviewer to:

-Follow what you are currently doing

-Go back to your notes and recall what you did

Even better would be to explain out loud what calculations is being done and why. E.g.: "I am multiplicating the US population of XXXM by Y.Y%, the percentage of population between the age of 18-35 which is the target demographic to find out the total size of the target population"

Hope it helps,


replied on Jan 08, 2018
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You are not allowed to use a calculator.

Basically, you need to develop 3 calculation skills:

1) Learn how to multiply double digit numbers (google fast math tips or The Veda math).

2) Learn how to work with zeros. Best way - always use 10^power instead of zeros


300x9000 = 3*10ˆ2 x 9*10ˆ3=3x9*10ˆ(2+3)=27*10ˆ5

Handwritten it looks not that complicated. If you get used to writing all the numbers that way, you will never loose zeros and all multiplications/divisions will be replaced with + or -.

3) Use math tools (Mimir math for iOS, Math tool on Viktor Cheng website) to practice. Train, train, and train again

PS, Additionally I suggest to learn how to make the division mentally:

4) Learn the division table up to 1/11 (i.e. 5/6 = 83.3%). It will help you calculate any percentage problems

Good luck!

Anonymous replied on Feb 23, 2018

Hey anonymous,

I've never come across any consulting firm that allows to use a calculator (not only MBB, but all firms that either I or someone one come across). Most likely, that's just one very outdate and wrong piece of preparation advice.



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