To do mental maths well under pressure, simplify the numbers before you calculate, say each step out loud, and sense-check every result against business reality. Speed comes from about 15–20 minutes of daily drilling on a small set of techniques (zeros, percentages, division shortcuts and growth rates), not from being a natural.
Why does case maths feel harder than it is?
The arithmetic in most case interviews is GCSE-level: multiplication, division, percentages and occasionally compound growth. It feels hard because you're doing it in front of an evaluator, with large numbers, while also tracking the business logic. Under stress, working memory shrinks. Every technique below reduces the load on working memory.
How should you set up a calculation?
Write it down before you compute
Before calculating, write the equation in words: "Revenue = customers × purchases per year × average price." Then substitute the numbers. This stops the most common error, which is multiplying the wrong things together, and it lets the interviewer follow and correct you early.
Narrate as you go
Say "So that's 4 million customers times 3 purchases, which is 12 million transactions." Narration lets the interviewer step in if you misread a figure, and it shows structured thinking even if you slip on the arithmetic.
What techniques make big numbers manageable?
Handle zeros separately
Strip out the zeros, do the core calculation, then add them back. For 40,000 × 3,500: 4 × 35 = 140, then add seven zeros (four from 40,000 and two from 3,500, plus the one you dropped from 35's place). The answer is 140,000,000. Many candidates prefer scientific shorthand: 4×10⁴ × 3.5×10³ = 14×10⁷ = 1.4×10⁸. Pick one method and use it consistently.
Use units such as thousands, millions and billions in your head rather than counting zeros. "£1.2 billion divided by 48 million customers" becomes 1,200 million ÷ 48 million = 25, so £25 per customer.
Use friendly factors
- Multiplying by 25 is multiplying by 100 and dividing by 4: 36 × 25 = 3,600 ÷ 4 = 900.
- Multiplying by 5 is multiplying by 10 and halving.
- Dividing by 0.2 is multiplying by 5.
Percentages: build from 10% and 1%
To find 7% of 240, take 10% (24) minus 3% (3 × 2.4 = 7.2), which gives 16.8. Or take 7 × 1% = 7 × 2.4 = 16.8. Also remember that percentages swap: 8% of 50 equals 50% of 8, which is 4.
Growth and compounding
The Rule of 72 says something growing at r% a year roughly doubles in 72 ÷ r years. At 6% growth, that's about 12 years. For small rates over a few years, simple addition is close enough: 3% for three years is about 9% (the exact figure is 9.27%). Say that you're approximating.
Should you round, and how much?
Round when the interviewer allows it, and say so: "I'll round 48 million to 50 million to keep this simple. That will slightly understate the per-customer figure." Rounding in opposite directions, one number up and another down, keeps errors small. Don't round where precision drives the decision, for example when a margin is near break-even.
How do you sense-check a result?
Every number you produce should pass a "does this make sense?" test before you move on:
- Order of magnitude: if your UK market size for dog food is £300 billion, you've slipped a decimal place. UK GDP is in the low trillions.
- Per-unit logic: divide totals back down. If your answer implies each UK household spends £5,000 a year on toothpaste, something is wrong.
- Direction: if costs rose and volumes fell, did profit really go up?
Saying "that's about £2,000 per store per day, which seems plausible for a small convenience store" is something interviewers explicitly value.
What if you make a mistake?
Correct it calmly: "Sorry, I dropped a zero there. It's 12 million, not 1.2 million, and that changes the conclusion because...". Recovering well is a positive signal. Ploughing on with an absurd number, or freezing, are the real problems.
How should you practise?
A practical daily routine (about 15–20 minutes):
- Five minutes of drills: multiplication of two-digit numbers, percentage changes, and division with large units.
- Five minutes of case-style word problems: for example, "a factory makes 1,800 units a day at £14 margin; what's the annual profit if it runs 250 days?" (1,800 × 14 = 25,200 per day; × 250 = 6.3 million, so £6.3m).
- Five minutes of out-loud practice: solve two problems while narrating, ideally recorded, so you hear your hesitations.
Practise under realistic conditions as well. Doing maths silently at a desk isn't the same as doing it while someone waits. Practising with an AI interviewer like caize.ai, which asks the numbers question in the flow of a case and expects you to talk through it, gets you closer to interview conditions than a worksheet does.
Do the online assessments change anything?
Yes. Screening tests such as BCG's online case (the "Casey" chatbot) and McKinsey Solve's Redrock module reward speed and accuracy with data, often against a clock. McKinsey also says that candidates invited to interview get access to its AI Prep Tool for practising the quantitative part of its case interviews. Use it if you have access, and re-verify availability before you rely on it.
FAQ
Can I use a calculator in a case interview?
Usually not in live case interviews. Online assessments vary and some allow one. Check your invitation and ask your recruiter if unsure.
How accurate do I need to be?
Accurate enough to support the decision. Rounding is normally fine if you say so, but wrong orders of magnitude aren't.
What's the fastest way to improve?
Daily short drills plus out-loud practice in case context. Consistency beats occasional long sessions.
Should I write everything down?
Write the equation and key intermediate results clearly, and number your pages. Interviewers often look at your notes.
