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Calculations: When is an estimate not good enough?

Anonymous A

I have been on 4 cases with 2 other newbies and in all cases, we struggled a little bit (or more) with the calculations.

E.g. How much is 43 of 90 in % (yes not super complicated but we need to get used to it. Would it have been ok to suggest in a real interview to calculate with 1EUR instead of 90Cents?

So, when is simplifying ok or when is simplifying def not ok?



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Sidi replied on 04/28/2018
McKinsey Engagement Manager & BCG Consultant | Interviewer at McK & BCG for 7 years | Coached 35+ candidates secure MBB offers

Hi Anonymous,

first of all, let me state that one result of proper case interview preparation will be that this question will become completely irrelevant to you. ALL calculations in Consulting case interviews are utterly simple! You just need to internalize these important points and act accordingly:

1. You NEVER do calculations in your head! Interviewers are not at all interested in your ability to do calculus in your head! The only thing that counts is that you can be trusted to consistently come up with the correct result.

2. Don’t worry about speed! Again, the interviewer wants to see you come to the correct result in a very secure way - NOT in a speedy way

3. As a consequence of the above, concentrate on practicing written calculations of all sorts. All 4 operators (+, -, x, /). Thereby you will quickly get to the level where it becomes completely irrelevant whether you solve for 43% of 90 or out of 100.

If you still want to get a rule of thumb on rounding, I think it is permissible if well below 10%. In your example 100 is clearly more than 10% above 90, so it seems like a stretch to me.

replied on 05/01/2018
Former BCG decision round interviewer with 300+ real interviews in 8 years at BCG. First session $249, following $219

Alkways ask for permission. If it's not given then proceed with long form calculation. If it's given make sure you do not do approximations of approximations (e.g. 43/90, ok if they let you to approximate to 4/9 but then is not acceptable to approximate that to 4/10).

Hope this helps,


Vlad replied on 04/28/2018
McKinsey / Accenture / Got all BIG3 offers / More than 300 real MBB cases / Harvard Business School


  • Ask the interviewer 1-2 times to get a sense of how comfortable he is with rounding numbers and how many zeroes he requires.
  • Continue with an assumption that you should keep that level of preciseness throughout the case. Worst case - he'll tell you to change the approach

I don't necessarily agree with the previous advice - most of the calculations you can do in your head pretty fast:

  • Learn how to multiply double digit numbers ( - it takes a day to practice how to calculate it mentally
  • Learn the division table up to 1/11 (i.e. in your casw 43/90 = a bit more than 4/9. And you know that 4/9 = 44.4%)
  • Learn how to work with zeros (Hint: 4000000 = 4*10ˆ6)
  • Use math tools (Mimir math for iOS), Math tool on Viktor Cheng website to practice


Francesco replied on 04/29/2018
Ex BCG | MBB Specialist | #1 Expert for coaching sessions (1300+) and recommendation rate (100%)

Hi Anonymous,

in general terms:

  • you will be allowed to approximate in market sizing, since the numbers used there are already an approximation. The interviewer will actually expect you do so. Suitable approximations are usually in the single digits percentage, as mentioned by Sidi.
  • you won’t be allowed to approximate in business cases, since the interviewer will have the exact numbers for the solution and will check them. Having said that, you can always ask – asking if you can approximate is never a mistake. Just be aware that you are expected to know how to solve the math in business cases without approximation.

It’s important that in both cases – thus also in a market sizing - you ask the interviewer if it is indeed ok to approximate the calculation. If you just round without asking, it could be perceived as a mistake in your math, or at the bare minimum a mistake in communication since you did not align upfront.



Anonymous A replied on 04/28/2018

Super Sidi, thanks à lot for your help and quick response!

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