Case interview math drills
Six moves that cover case math, twenty drills with script-checked answers and methods, the numbers to memorize, and saying the setup before the answer.
Updated October 28, 2026
| Skill | The move | Example |
|---|---|---|
| Percent of a number | Split into 10%, 1% and 5% pieces, then add | 12% of 850: 85 + 17 = 102 |
| Percentage points of a base | Points times base, keep the units | 0.8 points of $4.2B: 0.008 x 4,200M = $34M |
| Breakeven | Fixed cost over contribution per unit (or over margin for revenue) | $6M / ($50 - $30) = 300,000 units |
| Growth rate | End over start, then roots and powers by memory: 1.1 squared is 1.21, 1.08 cubed is about 1.26 | 242/200 = 1.21, so 10% a year |
| Combined changes | Multiply the factors, never add the percentages | 0.9 x 1.15 = 1.035, up 3.5% |
| Big numbers | Count the zeros separately from the digits | 25,000 x $3.2M = 80,000M = $80B |
How do I do case math fast without mistakes?
Say the setup before the answer. “Twelve percent of 850: ten percent is 85, two percent is 17, so 102.” The interviewer hears the method, you catch your own slips, and if the number is wrong the error is visible and cheap. Round early and say that you are rounding. Keep the units attached to every number: millions, points, per unit. Most case math errors are unit errors, not arithmetic.
Then check it against something. A margin should land between 0 and 100 percent; a payback in years should be a small number; a per-employee figure should look like a salary. One sanity check per calculation, out loud, before anyone asks.
Twenty drills
Cover the right column, say the setup out loud, then compare. Twenty seconds each is the target; the method column is how to get there.
| # | Drill | Answer | Method |
|---|---|---|---|
| 1 | A store makes $4.2B of revenue. Shrink rises from 1.8% to 2.6%. How much more is lost? | $34M | 0.8 points of $4.2B: 0.008 x 4,200 = 33.6, so about $34M. |
| 2 | Revenue $850M, margin 12%. What is profit? | $102M | 10% is 85, 2% is 17, total 102. |
| 3 | Profit $102M on revenue $850M. What is the margin? | 12% | 102 / 850: 85 is 10%, 17 is 2%, so 12%. |
| 4 | Fixed cost $6M, price $50, variable cost $30. Breakeven units? | 300000 units | Contribution $20 per unit; 6,000,000 / 20 = 300,000. |
| 5 | Fixed cost $2.4M, contribution margin 40%. Breakeven revenue? | $6M | 2.4 / 0.4 = 6. |
| 6 | Revenue grows from $200M to $242M in two years. Annual growth rate? | 10% | 242/200 = 1.21; the square root of 1.21 is 1.1, so 10% a year. |
| 7 | A market of $12B grows 8% a year. Size in 3 years? | $15.1B | 1.08 cubed is about 1.26; 12 x 1.26 = 15.1. |
| 8 | Price falls 10% and volume rises 15%. Revenue change? | 3.5% | 0.9 x 1.15 = 1.035, so up 3.5%. |
| 9 | 25,000 stores, average $3.2M revenue each. Total? | $80B | 25,000 x 3.2M = 80,000M = $80B. |
| 10 | Share rises from 15% to 18% of a $40B market. Revenue gained? | $1.2B | 3 points of $40B is $1.2B. |
| 11 | Unit cost $18, price $24. Gross margin? | 25% | 6 / 24 = 25%. |
| 12 | $5M investment, $1.25M a year of contribution. Payback? | 4 years | 5 / 1.25 = 4. |
| 13 | Costs are $720M and rise 5%. New cost? | $756M | 5% of 720 is 36; 756. |
| 14 | A plant runs 300 days, 16 hours a day, 1,200 units an hour. Annual output? | 5.76M units | 300 x 16 = 4,800 hours; x 1,200 = 5.76M. |
| 15 | 12% of customers churn a year from a base of 850,000. How many leave? | 102k | 10% is 85,000, 2% is 17,000: 102,000. |
| 16 | Revenue $3.6B, 45,000 employees. Revenue per employee? | $80k | 3,600M / 45,000 = 80,000. |
| 17 | A discount of 20% is followed by a 20% price rise. Net change? | -4% | 0.8 x 1.2 = 0.96, down 4%. |
| 18 | Margin improves from 8% to 11% on $2.5B revenue. Extra profit? | $75M | 3 points of 2,500 = 75. |
| 19 | Volume 1.8M units, price $45, variable cost $27, fixed $20M. Profit? | $12.4M | Contribution $18 x 1.8M = $32.4M; minus 20 = 12.4. |
| 20 | A $600M business grows 6% a year for 5 years. Approximate size? | $803M | 1.06^5 is about 1.34; 600 x 1.34 = 803. |
Which numbers should I know by heart?
| Memorize | Why |
|---|---|
| 1.1 squared is 1.21; 1.1 cubed is 1.33; 1.05 squared is about 1.10 | Growth rates from two or three years of data |
| 1.08 cubed is about 1.26; 1.06 to the fifth is about 1.34 | Projecting a market a few years out |
| Rule of 72: years to double is 72 over the growth percentage | 8% a year doubles in about 9 years |
| One million times one thousand is one billion | Store counts times store revenue |
| A year is 8,760 hours; a working year is about 2,000 hours; a year is about 250 working days | Capacity and utilization |
| US population about 335M; about 130M households; the world about 8B | Market sizing anchors |
How do I practice this?
Ten minutes a day, out loud, with a timer. Two sets of ten drills from the table above, then ten of your own built from a casebook’s numbers. Record yourself once a week and listen for the setup: if you hear an answer with no method before it, that is the habit to fix, because it is the one an interviewer notices. For the sizing version of the same habit, the market sizing walkthrough runs one chain end to end.
Questions
How much math is in a case interview?
Percentages, breakevens, growth rates, combined changes and big-number multiplication, done out loud with rounding. No calculus, no calculator; the method matters more than the last digit.
How do I avoid mistakes in case math?
Say the setup before the answer, round early and say so, keep units attached, and sanity check every result against a ratio you know. Most errors are unit errors.
Are the drill answers checked?
Yes. Every answer on this page was computed by a script in the site’s repository, and the page prints the script’s output; the method column is the mental route to the same number.
Does Casepartner check my math in a case?
When numbers land it shows a math check: the setup, the answer, one implication. It never recommends before the math is checked. In practice, compare its check with yours after each number.