Why Case Math Matters

Consultants deal with numbers every day — market sizing, growth projections, cost structures, and profitability analyses. But in a case interview, interviewers don’t expect you to calculate in your head like a calculator. They’re testing:

• Logic: Can you set up the right equation?

• Clarity: Do you explain each step as you go?

• Sanity: Do your numbers make sense in the real world?

They care more about your thought process than the exact final number.

1. Slow Down and Structure Your Equation

When you get a math problem, don’t rush to multiply. Take five seconds to define what you’re solving for and what data you’ll need.

For example:

“We want to find profit. Profit equals revenue minus cost. Revenue is price times quantity, and cost is fixed plus variable.”

This gives the interviewer confidence that you understand the logic behind the math.

2. Write Neatly and Label Everything

Keep your notes organized. Write units next to every number. Many candidates lose points by saying things like “the market size is 800” — 800 what? Thousand? Million? Per year?

Example:

• “Customer base: 1 million people”

• “Average spend: $50 per month → $600 per year”

• “Market size = 1M × $600 = $600M annual revenue”

It’s clean, structured, and easy to follow.

3. Round Intelligently

Top candidates round numbers strategically to make mental math fast and transparent.

Instead of calculating 487 × 23, you can say:

“Let’s round 487 to 500 for simplicity. 500 × 20 is 10,000, and 500 × 3 is 1,500. That gives roughly 11,500.”

Then pause and sanity check it:

“That seems reasonable given the original numbers.”

It’s not about precision — it’s about directionally correct reasoning.

4. Talk Through Your Math

Always narrate what you’re doing. Imagine you’re walking your interviewer through your whiteboard.

“If 20% of customers buy the premium product at $200, that’s 0.2 × 1,000,000 customers = 200,000 customers, generating $40M in revenue.”

By explaining each step, you show confidence and keep the interviewer aligned. Silence during math creates doubt — speaking keeps you in control.

5. Sanity Check Every Answer

Once you’ve finished a calculation, ask yourself: Does this make sense?

If your math suggests each customer spends $50,000 a year at a coffee shop, that’s a red flag. Take a second to step back and reason through it.

You can even say it out loud:

“That feels high for a consumer product — maybe I misplaced a decimal.”

This kind of self-correction shows business judgment, not weakness.

6. Practice the Right Way

Don’t just do random math drills. Focus on the types of math that come up most in real case interviews:

• Market sizing: Estimate populations, spending, or demand.

• Profitability: Calculate revenues, costs, and margins.

• Breakeven: Find how many units or months until profit.

• Growth: Apply percentage changes and CAGR.

• Investment returns: Calculate ROI or payback periods.

Keep a small notebook of quick math problems and revisit them daily. 10–15 minutes of focused practice is better than hours of passive studying.

Common Case Math Mistakes

• Forgetting to write units (millions vs. thousands)

• Misplacing decimals

• Skipping sanity checks

• Doing silent math instead of walking through it

• Rushing under pressure instead of structuring first

Remember: math mistakes are rarely fatal — poor communication is.

Example: A Quick Market Sizing

Case: Estimate annual U.S. revenue from gym memberships.

1. Population: 330 million people

2. Percent who go to the gym: ~20% → 66 million people

3. Average monthly cost: $50 → $600 per year

4. Revenue = 66M × $600 = ~$40B per year

Wrap up by saying:

“So I’d estimate the U.S. gym membership market is around $40 billion annually, which seems reasonable given what we know about the fitness industry.”

Clean, logical, and confident.

Final Thoughts

Case math isn’t about speed — it’s about structure. If you can explain your setup clearly, calculate in a calm, transparent way, and sanity check your answers, you’ll earn the trust of any McKinsey, BCG, or Bain interviewer.

Practice out loud, write neatly, and think like a businessperson first — not a human calculator.