Mental math is a critical skill in case interviews. Fumbling with numbers signals lack of preparation. Here’s how to get fast and accurate.
Why Mental Math Matters
In a case interview, you’ll need to:
- Perform calculations without a calculator
- Maintain conversation flow while computing
- Catch errors in data or your own work
- Build credibility with the interviewer
Essential Techniques
Percentage Shortcuts
- 10% of anything: move the decimal point
- 5% = half of 10%
- 15% = 10% + 5%
- 1% = move decimal two places
Multiplication Tricks
- Multiply by 5: multiply by 10, divide by 2
- Multiply by 25: multiply by 100, divide by 4
- Multiply by 11: add the digits and place in the middle
Division Shortcuts
- Divide by 5: multiply by 2, divide by 10
- Divide by 8: divide by 2 three times
- Recognize common fractions: 1/3 ≈ 33%, 1/6 ≈ 17%, 1/8 = 12.5%
Estimation Rules of Thumb
- Revenue estimate: # customers × average spend × frequency
- Market size: Population × % relevant × adoption rate × price
- Break-even: Fixed costs ÷ (price - variable cost)
Daily Practice Routine
- Spend 15 minutes daily on mental math drills
- Practice market sizing to combine estimation skills
- Always verify your answer with a quick sanity check
- Time yourself — aim for under 30 seconds per calculation