How to do percentages in your head (and never need a calculator)

Percentages feel like magic until you see the one rule underneath them: a percent is just a number per hundred. Once that clicks, you can do most percentage problems without touching a calculator.

The one rule

To find X% of a number, multiply the number by X, then divide by 100. That's the whole thing. 15% of 200? 200 × 15 = 3,000, then ÷ 100 = 30.

Three shortcuts that make it mental

• 10% is just moving the decimal. 10% of 240 is 24; 10% of 68 is 6.8.
• 5% is half of 10%. 5% of 240 = half of 24 = 12.
• 1% moves the decimal twice. 1% of 240 is 2.4.

Now build any percent from those. 15% = 10% + 5%. 20% = 10% doubled. 8% = 5% + 1% + 1% + 1%.

Tips and discounts

An 18% tip on a $40 meal: 10% is $4, another 5% is $2, and 1% three times is about $1.20 — roughly $7.20. A 25% discount is a quarter off, so you pay three-quarters of the price.

Why it sticks

You're not memorizing answers — you're learning the moves. Do it a dozen times and your brain starts skipping the paper entirely. That's the point: the skill stays with you when the phone is dead.