#Quantitative
Staying precise with numbers when an interviewer is watching
Mental arithmetic under pressure is a learnable skill. These drills, combined with a structured narration approach, will eliminate calculation errors in your next case.
By AdminMarch 14, 2026 5 min read
The pressure problem
Most candidates can do the math. The difference between a good and great candidate is whether they can maintain accuracy while an interviewer watches — and while thinking about what the numbers mean.
Why errors happen under pressure
Quantitative mistakes in case interviews usually come from one of three sources:
- Rushing — Trying to calculate faster than your brain can process
- Losing track — Forgetting which number you're working with
- Not narrating — Keeping calculations in your head without verbalising the approach
The structured narration approach
Top candidates never calculate in silence. Instead, they follow this rhythm:
- Announce what you're calculating: "Now I'm going to estimate the addressable market by multiplying..."
- Round clearly: "Let's call 37 million roughly 40 million for simplicity."
- Calculate step by step: "40 million × 25% gives us 10 million."
- Interpret the result: "So the addressable market is approximately 10 million customers."
Daily drills for mental arithmetic
Practise these for 5 minutes each day before your interview prep:
- Multiplication of two-digit numbers
- Percentage calculations (what is 15% of 240?)
- Market sizing estimates (estimate the total market)
- Growth rate projections (if something grows 8% annually for 5 years)