The one trick that covers most cases
To find any percentage of a number, start from 10% — just move the decimal point one place left. From there, everything else is quick addition or division:
- 10% of $84 → $8.40
- 20% → double the 10% figure → $16.80
- 5% → half the 10% figure → $4.20
- 15% → 10% + 5% → $12.60
This covers tips, most retail discounts, and rough sales tax estimates without reaching for a calculator.
Percentage change vs. percentage of
These are easy to mix up. "20% of 80" is 80 × 0.20 = 16. "What percentage did 80 increase to 100 by" is a different calculation: (100 − 80) ÷ 80 × 100 = 25%. Confusing the two is one of the most common percentage mistakes — always check which one the situation actually calls for.
When to skip mental math
Mental shortcuts break down with awkward numbers, multi-step calculations (like tax on a tip, or a discount on top of a discount), or when you need precision to the cent. That's when a calculator earns its keep.
Pick your calculation type
The Percentage Calculator handles "X% of Y," percentage change, and reverse percentage.
Enter your numbers
Get an exact result instantly, down to the decimal.
Double-check with the mental shortcut
A quick 10%-based estimate is a good sanity check on the exact answer.
Stacked discounts don't add up the way they look. "30% off, then an extra 10% off" is not 40% off total — it's 37%, because the second discount applies to the already-reduced price.
Skip the mental math
Free, instant, and precise to the decimal.