Where to Practice Case Interview Quant / Math?

You are absolutely right to prioritize this—mental math under pressure is where many candidates who structure cases perfectly still trip up and lose momentum. It’s a massive differentiator.

The reality is that there isn't a single, perfect, proprietary website that charges $50 and perfectly replicates the logic jumps required in a live case. The math itself isn't difficult; the challenge is quickly translating qualitative data into a calculation, executing it flawlessly under stress, and then interpreting the result back into a strategic recommendation.

The most effective practice is two-fold:

First, build speed and accuracy in isolation. Forget the fancy apps for a minute and build a simple practice engine. Create a Google Sheet or Excel file with two columns: in the first, list 50 simple calculation pairs (e.g., $18M at 12%, 750 units at $14 margin, CAGR from $200M to $350M over 4 years). Use the second column for your timed answer key. Drill this for 15 minutes every morning, forcing yourself to verbalize the steps (e.g., "750 times 10 is 7,500, plus 750 times 4 is 3,000, so 10,500 total"). This targets the simple arithmetic errors that kill interviews.

Second, integrate this back into full case practice. Go through high-quality published cases (like those available from specific firm websites or university consulting clubs) and focus only on the math exhibits. Cover the answer with your hand and force yourself to identify the calculation needed (margin bridge? break-even point?), set up the numbers mentally, calculate, and state the final result verbally. Do this for the same case three times until you nail the numbers instantly. You aren't just practicing calculating 15% of 800; you're practicing hearing "The client wants to know the impact of cutting the cost base by 15%" and immediately formulating (800 * 0.85). That quick translation is the key skill the firms test.

Hope this helps you structure your prep! All the best.

Hey there,

Here are a few approaches that helped me. While mental math is usually not the decisive interview criterion, being strong at it definitely helps you stand out.

Mental math (incl. quick percentage changes, back-of-the-envelope calculations, overall growth rates)

• Re-learn written / long arithmetic (column methods) – very powerful when calculations become more complex and helps keep your work structured and accurate
• Use mental math apps (e.g., Mental Math Cards Game) for 5–10 minutes daily to build speed and accuracy – ideally by replacing passive habits (like social media doom-scrolling) with short, focused drills
• Practice situational math in everyday life instead of using a calculator (e.g., calculating change at checkout, estimating quick percentages, or doing rough back-of-the-envelope calculations)

Market sizing

• Build a solid foundation using business school consulting playbooks (e.g., I liked Chicago Booth materials) or online resources (e.g., PrepLounge’s market sizing articles and example cases)
• Practice by sizing things in your everyday surroundings – for example, estimating the # of people in your country who own an electric car or a bicycle, and then checking the real numbers online

Unit economics / break-even: Practice by doing as many break-even and unit economics cases as possible, and study the structure and framework used afterwards to build sizing patterns in your mind

CAGR: I wouldn’t prioritize getting very fast at multi-period CAGR calculations – this is unlikely to be a major focus in case interviews

Many people like using rocket blocks - you can check that out.

A word of caution though - interviewers are not looking for human calculators.. as someone who was a previous interviewer, I really did not care if someone is able to calculate a CAGR instantly. 

So while its great you want to improve your "math", remember it is about quantitative thinking / reasoning, not calculations.

All the best!

Adding here a collection of the most common formulas and terms that show up in case interviews:

• Cheatsheet: The Must-Know Consulting Terms for Interviews

Best,

Cristian
 

Hi there!

Well, for you and anyone who reads this: This question highlights a very common trap, and it actually hurts many candidates far more than it helps.

The core issue is that people start optimizing for something that is not being assessed. Mental math speed is not a line item on any McKinsey, BCG, or Bain evaluation sheet. Interviewers are not scoring how fast you can compute percentages or do long multiplication in your head. What they are scoring is whether you think clearly, communicate cleanly, and use numbers in a way that is client friendly and decision oriented.

Pure mental math drills train speed in isolation. Case interviews do the opposite. Math is never the objective. It is a tool to support a business judgment. When candidates focus heavily on drills, three problems show up again and again in interviews.

First, they rush into calculations without properly framing the question. Instead of clarifying the objective or setting up a simple structure, they start computing. From the interviewer’s perspective, this looks like activity without direction.

Second, they stop communicating. Many candidates go silent while calculating, then resurface with a number. In a real client setting, that would be unacceptable. Interviewers want to hear your logic, your assumptions, and why the calculation matters.

Third, they lose judgment. They treat every number as equally important and aim for precision where it is not needed. MBB firms care far more about order of magnitude, sanity checks, and whether the conclusion makes business sense than about numerical elegance.

In practice, a slower, well explained calculation that leads to a clear takeaway always outperforms a fast but opaque one. Interviewers are constantly asking themselves, “Would I put this person in front of a client?” Being calm, structured, and interpretable under pressure matters far more than raw calculation speed.

The better way to train is to practice math embedded in full cases. Do it out loud. Start with a clear headline explaining what you are trying to find out. State your assumptions explicitly. Walk through the calculation in a simple, structured way. End with a conclusion that ties the number back to the business question.

If your math does not help a decision maker decide, it was the wrong math to practice.

None of my mentees did isolated "math drills". And more than 600 of them are now in MBB. 

Hope this helps! :)

Sidi

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Dr. Sidi S. Koné

Hi there,

For case interviews, you want speed and clarity, not advanced math. These are the best resources that closely match real case quant

• GMAT Problem Solving (medium level) for percentages, ratios, growth and logic
• Old McKinsey PST questions for business-style math and data interpretation
• Victor Cheng / Case in Point math drills for breakeven, margins and unit economics
• PrepLounge math and market sizing drills for interview-like pacing

How to practice

• Practice out loud and under time pressure
• Round numbers and explain assumptions
• Do short daily drills (15–20 minutes) rather than long sessions

If you want, I can share a simple daily math routine that works well for interviews

Happy to help you prep – feel free to reach out

Best
Evelina

Hi there,

You can try FastMath.