How to Calculate Percentages (The 3 Formulas You Actually Need)
2026-06-03 · 5 min read
Most percentage problems are one of three types. Learn the formula for each — and when to use it — with worked examples.
Percentages trip people up not because the maths is hard, but because there are three different questions that all sound similar. Once you can tell them apart, every percentage problem becomes a one-line calculation.
1. What is X% of a number?
This is the most common case: a discount, a tip, a tax. The formula is simply (percentage ÷ 100) × number. For example, 15% of 200 is (15 ÷ 100) × 200 = 30.
2. X is what percent of Y?
Use this when you have two raw numbers and want the ratio as a percentage — a test score, a completion rate, a market share. The formula is (X ÷ Y) × 100. So 30 out of 150 is (30 ÷ 150) × 100 = 20%.
3. Percentage increase or decrease
This measures change over time — a price rise, a salary bump, a drop in traffic. The formula is ((new − old) ÷ |old|) × 100. Going from 80 to 100 is a 25% increase; going from 100 to 80 is a 20% decrease. Note the result is not symmetric, which surprises a lot of people.
A common mistake: a 50% drop followed by a 50% rise does NOT return you to the start. 100 → 50 → 75. Always calculate change against the most recent value.
Do it instantly
Rather than reaching for a formula each time, the Percentage Calculator gives you all three calculators on one screen with live results. Type your numbers and read the answer.