How fast should mental math be in a case interview?

Aim for 10 to 15 seconds on a two-digit percentage or multiplication problem, and 20 to 30 seconds on a multi-step calculation. Faster than that is unnecessary; slower than that breaks the conversation.

Can I use a calculator in a case interview?

No. Case interviews are explicitly designed to test mental and pen-and-paper math without a calculator. You can use scratch paper.

What is the rule of 72?

The rule of 72 is a shortcut for compound growth: a quantity growing at X% per year doubles in roughly 72/X years. So 6% growth doubles in 12 years; 8% in 9 years; 12% in 6 years. It is accurate enough for case interview math.

Guide · Case math

12 case interview math tricks consultants actually use

TL;DR

Case math is graded on speed and accuracy without a calculator. Twelve specific shortcuts — percent flips, rule of 72, halve-and-double, fraction-to-percentage conversions, and CAGR by inspection — cover most of what consultants compute under pressure.

Last updated: May 16, 2026. Written by the CaseXcel team.

The 12 shortcuts

1. Percent flip

"X% of Y" equals "Y% of X." Use whichever is easier.

8% of 25 = 25% of 8 = 2. Faster than 0.08 × 25.

2. The 10% anchor

Compute 10% by moving the decimal one place left. Build other percentages off it.

15% of 320 → 10% is 32, half of that is 16, total 48.

3. The rule of 72

At growth rate X%, a quantity doubles in 72/X years. Use it to estimate compounded growth fast.

Revenue grows 6% per year. It doubles in 12 years. Triples in roughly 19.

4. Multiplication by halving and doubling

Replace a hard multiplication with an easier equivalent: double one factor, halve the other.

18 × 25 → 9 × 50 → 450.

5. Round, then correct

Round to a friendly number, do the math, then add or subtract the correction.

47 × 21 → 50 × 21 = 1,050, then subtract 3 × 21 = 63 → 987.

6. Break revenue into price and quantity

When given revenue and asked to interpret, immediately split into price × quantity. Then you know which lever moved.

Revenue went from $100M to $110M. If price was flat, quantity rose 10%. If volume was flat, price rose 10%.

7. Growth rate approximation

When two growth rates compound, add them. Works well for sub-10% rates.

Volume up 4%, price up 3%. Revenue up roughly 7% (precise: 7.12%).

8. The breakeven shortcut

Breakeven units = Fixed costs ÷ (Price per unit − Variable cost per unit). State it as a formula, then plug numbers.

Fixed costs $1M, price $50, variable cost $30 → $1M ÷ $20 = 50,000 units.

9. Ratios into percentages

Memorize the common fractions as percentages: 1/3 = 33%, 1/4 = 25%, 1/5 = 20%, 1/6 = 17%, 1/7 ≈ 14%, 1/8 = 12.5%, 2/3 = 67%.

"Sales fell to two-thirds of last year" = 33% decline. Skip the calculation.

10. Large-number scaling

When multiplying big numbers, multiply the leading digits and add the zeros separately.

600M × 40 → 6 × 4 = 24, plus 8 + 1 = 9 zeros → 24 billion.

11. CAGR by inspection

CAGR ≈ (end/start)^(1/n) − 1. For case math, use rule-of-72 in reverse: if value tripled in n years, CAGR ≈ ln(3)/n × 100 ≈ 110/n.

Revenue tripled in 10 years → CAGR ≈ 11%. (Actual: 11.6%, close enough.)

12. Sanity-check with order of magnitude

Before stating the final number, ask: "Is this billions, millions, or thousands?" Many wrong answers are off by a factor of 1,000 because the candidate lost a zero.

You get $4.7B for monthly revenue of a regional company. That is wrong by orders of magnitude. Rescale.

How to actually internalize these

Pick one trick per day. Practice 10 problems applying it before moving on.
Drill mixed problems by day three. Reading shortcuts is not the same as applying them under pressure.
Time every problem. Untimed practice is homework, not preparation.
Talk through your work out loud. In the real interview you will narrate; practice that now.

Drill case math at consultant speed

CaseXcel's Case Math and Mental Math drills generate fresh problems at the exact difficulty and format used in real consulting interviews. Five minutes a day for two weeks closes most of the speed gap.

Try a free drill →

12 case interview math tricks consultants actually use

The 12 shortcuts

How to actually internalize these

Drill case math at consultant speed

Walk into your case interview ready.