Guides · 2026-06-08
How to Calculate a Percentage (3 Everyday Cases)
Learn the three percentage calculations people need most — a percent of a number, what percent one number is of another, and percentage change — with examples.
Percentages come up everywhere — discounts, tips, taxes, grades, and growth rates — but most people only need to master three simple cases. Here's each one with a clear formula and example.
The one idea behind all of them
"Per cent" literally means "per hundred". A percentage is just a fraction of 100. To use a percentage in a calculation, convert it to a decimal by dividing by 100: 20% becomes 0.20, 7.5% becomes 0.075. That single step unlocks all three cases below.
Case 1: What is X% of Y?
Multiply Y by X as a decimal. For example, 15% of 200 is 0.15 × 200 = 30. This is how you find a discount amount, a tip, or a tax portion.
Case 2: X is what percent of Y?
Divide X by Y, then multiply by 100. For example, 30 out of 150 is 30 ÷ 150 × 100 = 20%. This turns a score, a share, or a portion into a percentage.
Case 3: Percentage increase or decrease
Subtract the old value from the new, divide by the old, and multiply by 100. Going from 80 to 100 is (100 − 80) ÷ 80 × 100 = 25% increase. A negative answer means a decrease. This is the formula behind price changes and growth rates.
A common mistake to avoid
A percentage increase and the matching decrease aren't symmetrical. If a $100 item rises 25% to $125, it must then fall 20% (not 25%) to get back to $100 — because the base changed. Always divide by the correct starting value.
Skip the math
The free Percentage Calculator handles all three cases instantly — just type the numbers and read the answer. Everything runs in your browser.