Consulting Case Math – Why It’s Not Rocket Science

Updated: Oct 21, 2019

You are currently preparing your consulting interview? Then mastering case math is essential from the start!

Why? Because making more than 3-4 mistakes in your cases might actually reduce your chances significantly, even if you are really good at the more qualitative side of cases.

How Many Mistakes Can You Get Away With?

I have heard people get away with up to 2-3 smaller mistakes in a ‘Superday’ involving 3-4 case interviews, depending how they did in the other (more qualitative) parts of case.

While there’s no specific cut-off for math mistakes, you should aim at minimizing mistakes at all costs.

When you are worried that you have never been good at math, STOP worrying! Case math tends to be very simple arithmetic and includes NO advanced math such as differential equations.

What you SHOULD NOT do:

  • Doing the entire math in your head and not writing down anything

  • Guessing and quick shots

  • Overcomplicating things

What you SHOULD DO:

  1. Present your approach to your interviewer and write down as much as possible

  2. Simplify numbers/ calculations by involving and getting reassurance from the interviewer

  3. Spend some time practicing some of the short-cuts and frequent calculation types using our Free 4-page Consulting Math eBook

In order to succeed you’ll need to focus on the following (in the stated order):

1) Accuracy: this is key, even if you’re not THAT FAST, rarely will the interviewer push you to increase your speed. But if you keep on making mistakes this will significantly reduce your chance to move on to the next round

2) Speed: you need to make sure to be at least average in your speed. So this is more of a hygiene-factor, but nothing that will win you offers

To succeed you should have a good feeling for numbers, which you can learn by working with some tricks shown below.

Simplifying large numbers

Large numbers are notoriously difficult to deal with and are often a source of mistakes. One of the tricks is to actually divide by multiples of 10 to simplify. But write those 10s down on paper, because it quickly happens that you forget to add them back at the end of a complex calculation.

Example calculation:

Your car company Fessla is selling 120,000 cars of Model Z per year and the car costs around $35,000.

You need to solve: 120,000 x $ 35,000 = ?

1) Step 1: divide each number by 1,000 which gives you: 120 x $35

2) Step 2: 120 x $10 = $1,200; $1,200 x 3 = $3,600 + $600 (5* 120 = 1200/2) = $4,200

3) Step 3: Add back zeros: $4,200 + 000 + 000 = $4,200,000,000 or $4.2bn

With those 3 simple steps you’re done.

Splitting up percentages

Say you need to calculate: 35% of 150 for in your case interview.

In this case it makes sense to focus on multiples of 10%. So instead of doing the exercise in one step, just divide the 35% in to multiples of 10%. It could look something like this:

1) 10% x 150 = 15

2) 5% x 150 = 15/2 = 7.5

So in order to solve the question: you need to calculate 15 x 3 (for the 3 x 10%) and add 7.5 = 45 + 7.5 = 52.5

Rounding up and down

In case interviews you often need to estimate revenues and market sizes. Let’s say you have found that the case is asking you to estimate daily revenues of a small restaurant that serves around 75 guests with an average bill of USD 65.

Question: $65 x 75 = ?

Instead try rounding with the up/ and down approach $60 x 80 = (10 x 80) = 800 x 6 = 4,800

Doing the original calculation would have only led you to 4,857, off around 1.2%.

However, make sure not to round too much (i.e. more than 10% up/ down). Also, before doing so explain to the interview how you are approaching the calculation. This way you’ll make sure that he can step in, if he thinks that your are oversimplifying or reducing accuracy too much.


Let’s say you need to quickly add a range of numbers such as: 17, 9, 7, 1, 13, 1 and 18. Then instead of just adding them up as they are listed, try an alternative approach:

18 (17+1) + 10 (9+1) + 20 (7+13) + 18

= 2x 18 + 10 + 20 = 66

Write out these groupings on paper and you’ll see that your ability to quickly and accurately add up numbers will improve significantly.

For an in-depth review of Consulting Case Math Download Your Free 4-page Consulting Math eBook Today including 10+ example questions and solutions.

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