How good does my math *really* have to be to succeed at case interviews?

Hi there! 

Great question. 

Let me first provide some context on what these firms are looking to evaluate. 

They are looking to determine whether you are generally comfortable with numbers and can quickly do rough math if needed. E.g., if a client told you that they have 17% share of a $23B market growing at 8%. You will want to be able to do some quick math in your head w/o taking out your phone :). Similarly, can you sense check numbers coming out of an excel model? The model tells you that the revenue for product XYZ is $1.3B. Is that a reasonable number or is something in the model broken. 

So given that what is most important in the interview: 

a. Be structured 

b. Demonstrate that you are looking for / can find shortcuts to get to (approximate) answers

c. Be solid at executing the math with pen and paper

d. Can do (very) simple math in your head (e.g., 3.5*7 or 22/3) 

e. Sense-check your answer: Is this number in the right ballpark? 

f. Walk the interviewer through your thinking and math - make it easy for them to guide and help you (vs. silently do some math and then give them final answer of a multi-step calculation will make it very hard for them to guide / help) 

Less important: 

a. Being able to do math w/o pen and paper 

b. Being extremely quick 

c. Obviously try to avoid making calc mistakes, but if they happen: fix them and move in. I made plenty math error in interviews that resulted in offers

Happy to have a chat to help you calibrate.

Cheers & best of luck! 

-Andreas

Hi there, 

It’s a great question and one that a lot of candidates underestimate.

The short answer: you don’t need to be a human calculator to ace case interviews, but you do need to be fast, accurate, and structured with the level of math you’ll encounter.

In real interviews, the math is rarely about complex formulas. It is about interpreting numbers in context, making quick approximations, and explaining your thought process clearly. Often, 80 to 90 percent accuracy is enough if your logic and communication are solid. Exact precision only matters when the case explicitly calls for it.

A few signs your math is “good enough” for interviews:
• You can break down problems into manageable steps under time pressure.
• You are comfortable with percentages, ratios, growth rates, weighted averages, and quick mental approximations.
• You can check your work with a back-of-the-envelope method in under a minute.

For example, in your “1,475,000 ÷ 12” question, the important thing is to:

1. Approximate quickly (1.5M ÷ 12 ≈ 125k)
2. Then refine if time allows (~122.9k)
3. State your logic so the interviewer follows your reasoning.

If you want to test yourself, try problems like:
• 3,750 × 0.28 (aim for 15 to 20 seconds)
• 2.4M ÷ 48 (aim for 20 to 30 seconds)
• A product’s price increases from $40 to $46. What is the percent change? (aim for 10 seconds)
• A market grows from $120M to $160M in 3 years. What is the CAGR? (aim for 45 seconds)

If you can handle these within the suggested times while explaining out loud how you got there, you are in a very good place. Check out the math section on PrepLounge, it provides a lot of helpful suggestios. 

Remember, interviewers care more about how you think than whether you shave off the last decimal place. The key is practicing so your math feels effortless and your mind is free to focus on the bigger strategic picture.

Happy to walk you through a live math drill and give feedback. I offer a free intro call if you would like to explore that.

Great question—this is something many candidates struggle with, and it’s normal early in prep.

You don’t need perfect mental math to succeed in case interviews. What matters most is your approach, accuracy, and clarity of explanation. Interviewers are not just looking for the right number—they’re paying close attention to how you solve the problem. That includes:

• Defining what you’re solving for
• Identifying the right variables
• Explaining your process clearly as you solve

For example, when solving something like 1,475,000 ÷ 12, it’s not just about getting to ~122.9. It’s about showing your logic: “I’ll divide 1.2M ÷ 12 = 100K, then adjust for the remaining 275K…”

If you can solve that kind of problem in under 60 seconds while explaining it clearly, you're likely in a strong place.

To benchmark yourself, try solving problems like these within 60–90 seconds each:

1. What is 3.75% of 480,000?
2. Estimate 890,000 ÷ 16
3. A company has 250 stores. Each store serves 1,200 customers/month. What’s the annual customer volume?
4. A product sells for $30, costs $12, and fixed costs are $180,000/month. How many units to break even?

Always practice talking through your math out loud—structure and communication matter as much as the final result. Keep practicing, and you’ll improve both speed and confidence.

Hey there :)

It’s less about solving “trick” problems and more about showing you can work quickly and confidently with business-relevant numbers. You need to be comfortable with large numbers, rounding, estimating, and spotting when a number “looks off”, that’s part of showing business sense, which is essentially the daily work of a consultant. If you can do calculations like your example in under a minute with clear, logical steps and communicate them while thinking about the implication of the number, you’re in a good place. Accuracy matters, but equally important is interpreting what the result means for the case.

best, Alessa :)

Math matters a lot less than people assume. And in fact, it's starting to matter even less.

Consulting firms are aware that what you need is an okish sense for maths. They don't want math geniuses. What they rather want are consultants who are effective at working with a client through a math problems and arriving together at the same conclusion. 

Other skills, such as first principles structuring are comparatively a lot more important. 

For maths, you might find the following cheatsheet useful:

• Cheatsheet: The Must-Know Consulting Terms for Interviews

Best,
Cristian