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Math"What's 15% of 80?" The bill is on the table, the tip needs deciding, and everyone suddenly looks at their phones. But finding a percentage of a number is genuinely a two-step move — and once the pattern clicks, you'll do the easy ones in your head forever.
This guide covers the universal method, the mental-math shortcuts for common percentages, and how to run the calculation backwards.
Try it yourself: Get instant answers with zero mental arithmetic. Percentage Calculator →
The two-step method
Step 1: convert the % to a decimal (divide by 100)
Step 2: multiply by the number
Step 2: multiply by the number
The word "of" in "15% of 80" means multiply. So: 15% → 0.15, then 0.15 × 80 = 12. Done.
The most common error is skipping step 1: 15 × 80 = 1,200, which is 100 times too big. Sanity check: any percentage below 100% of a number must be smaller than the number itself — if it isn't, your decimal point is misplaced.
Worked examples
| Question | Step 1 (decimal) | Step 2 (multiply) | Answer |
|---|---|---|---|
| 20% tip on $64 | 0.20 | 0.20 × 64 | $12.80 |
| 7% tax on $250 | 0.07 | 0.07 × 250 | $17.50 |
| 5% commission on $4,000 | 0.05 | 0.05 × 4,000 | $200 |
| 85% on a test, 60 questions | 0.85 | 0.85 × 60 | 51 correct |
Mental-math shortcuts for the common ones
- 10%: move the decimal one place left. 10% of 240 = 24.
- 5%: find 10%, halve it. 5% of 240 = 12.
- 15%: 10% + 5%. 15% of 240 = 24 + 12 = 36.
- 20%: find 10%, double it. 20% of 240 = 48.
- 25%: divide by 4. 25% of 240 = 60.
- 50%: halve it. 75%: 50% + 25%.
These six cover roughly 90% of real-world percentage questions — tips, taxes, discounts, and splits at the register.
Running it backwards: finding the whole
Sometimes you know the percentage result but not the starting number: "My raise was $180, which is 6% — what was my salary?" Division is the inverse of the forward move:
Whole = part ÷ decimal form of the percentage
— $180 ÷ 0.06 = $3,000 —
— $180 ÷ 0.06 = $3,000 —
Always verify by running forward: 0.06 × 3,000 = 180. ✓
What about percentages of percentages?
"20% of 30% of 500" works left to right: 30% of 500 = 150, then 20% of 150 = 30. Equivalently, multiply the decimals: 0.20 × 0.30 × 500 = 30. This is exactly how stacked discounts and compounded fees behave — see our percentage basics guide for why you can't just add them.
Frequently asked questions
Find 10% (move the decimal one place left), then add half of that for 5%. For 15% of 80: 10% is 8, half of 8 is 4, so 8 + 4 = 12. Two steps, no calculator.
Because "percent" means per hundred — the formula needs the decimal form. 15% is 15/100 = 0.15. Multiplying by 15 instead of 0.15 gives an answer 100 times too big.
Divide the part by the decimal form of the percentage. If 60 is 40% of some number: 60 ÷ 0.40 = 150. Check: 150 × 0.40 = 60. Division is the inverse of the forward calculation.
The whole number itself — 100% of 75 is 75. This sanity check catches decimal errors: if your 15% answer is bigger than the original number, the decimal point is in the wrong place.
Disclaimer: Calculator content is for education only — examples are illustrative and rounded; double-check figures used in financial or contractual decisions.