Percentages show up everywhere — restaurant tips, store discounts, exam scores — yet the mental math still trips people up under pressure. Here's a refresher that sticks.
The core formula
A percentage is just a fraction out of 100. To find X% of a number, multiply the number by X and divide by 100 — or simply move the decimal point two places left and multiply.
- 15% tip on $60 → 60 × 0.15 = $9
- 20% off a $45 item → 45 × 0.20 = $9 discount, so you pay $36
- 42 out of 60 on a test → (42 ÷ 60) × 100 = 70%
Quick trick: 10% of any number just moves the decimal one place left. Once you have 10%, you can double it for 20%, or halve it for 5%.
Why this matters
Being able to estimate percentages quickly helps you catch pricing errors, compare deals, and sanity-check calculator results — even when you use a tool to get the exact number.
PrimeCalculator Editorial Team
Calculator & Finance Writers