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How to calculate a percentage

"Percentage" covers three different questions, and mixing them up is why the maths feels confusing. This guide lays out all three plainly, with the formula and a worked example for each — and a free Percentage Calculator that does them for you instantly.

The fastest way

Open the Percentage Calculator, pick the case you need, type your numbers, and read the answer. No formula to remember. The explanations below are for when you want to understand what it is doing.

Case 1 — What is X% of a number?

Formula: (percent ÷ 100) × number.
Example: 15% of 80 = (15 ÷ 100) × 80 = 12.

This is the one you use for a tip, a discount amount, or a share of a total.

Case 2 — What percent is X of Y?

Formula: (part ÷ whole) × 100.
Example: 18 correct out of 72 = (18 ÷ 72) × 100 = 25%.

Use it for a test score, a completion rate, or a slice of a budget.

Case 3 — Percent increase or decrease

Formula: ((new − old) ÷ old) × 100.
Example: a price rising from 50 to 65 = ((65 − 50) ÷ 50) × 100 = 30% increase. A negative result is a decrease.

This is the one for price changes, growth, and before/after comparisons.

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A note on privacy

The Percentage Calculator runs entirely in your browser — the figures you enter about money, grades, or plans are never uploaded or stored.

Open the Percentage Calculator →

Common questions

How do I find X% of a number?

Divide the percent by 100 and multiply by the number. For example, 15% of 80 is (15 ÷ 100) × 80 = 12. The percentage calculator does this instantly — type 15 and 80 and it shows 12.

How do I work out what percent one number is of another?

Divide the part by the whole and multiply by 100. If 18 of 72 questions were correct, that is (18 ÷ 72) × 100 = 25%. Enter the two numbers and the calculator returns the percentage.

How do I calculate a percentage increase or decrease?

Subtract the old value from the new, divide by the old value, and multiply by 100. From 50 to 65 is ((65 − 50) ÷ 50) × 100 = 30% increase. A negative result means a decrease.

Is the calculator free and private?

Yes. Every calculation runs in your browser with no sign-up and no limit, and nothing you type is sent to or stored on a server.