How to Calculate a Percentage (of a Number, Change & More)

By the Tool Goodie Team · Updated July 4, 2026 · 5 min read

To find a percentage of a number, divide the percentage by 100 and multiply by the number. For example, 20% of 150 is 0.20 × 150 = 30. That single idea — "percent means out of 100" — covers almost every percentage question you'll ever face. Below we walk through the three that come up most: a percent of a number, what percent one number is of another, and percentage change.

Key takeaways

Percent of a number: (percent ÷ 100) × number.
What percent of: (part ÷ whole) × 100.
Percentage change: ((new − old) ÷ old) × 100.
Skip the arithmetic with the percentage calculator.

1. Finding a percent of a number

This is the classic "what is 20% of 150?" question — the one you need for tips, tax, discounts, and test scores.

Amount = (percent ÷ 100) × number

Turn the percent into a decimal (divide by 100), then multiply:

20% of 150 → 0.20 × 150 = 30
8% sales tax on $45 → 0.08 × 45 = $3.60
15% tip on a $60 bill → 0.15 × 60 = $9

A handy trick: 10% of any number is just that number with the decimal point moved one place left. 10% of 150 is 15; so 20% is 30, 5% is 7.5, and you can build most percentages from there in your head.

2. What percent one number is of another

This answers "30 is what percent of 150?" — useful for scores, progress, and proportions.

Percent = (part ÷ whole) × 100

30 out of 150 → (30 ÷ 150) × 100 = 20%
A test score of 42 out of 50 → (42 ÷ 50) × 100 = 84%

3. Percentage increase and decrease

This is how you describe a change — a price rise, a pay raise, a drop in sales.

Change % = ((new − old) ÷ old) × 100

Price rises from 150 to 180 → (180 − 150) ÷ 150 × 100 = +20%
Price falls from 180 to 150 → (150 − 180) ÷ 180 × 100 = −16.7%

Notice the two aren't symmetric: going up 20% and then down 20% doesn't bring you back to the start, because the second percentage is taken from a different base. This trips a lot of people up with sales and stock prices.

Do it in one step: the free Percentage Calculator handles all three questions at once — type your numbers and read the answer instantly, no mental math required.

Everyday places percentages show up

Once you know these three formulas, a surprising amount of daily life becomes easy: working out a tip or sales tax, checking whether a discount is as good as it looks, reading a test score, comparing this month's numbers to last, or understanding an interest rate. The maths never changes — only the words around it.

Frequently asked questions

How do you find a percentage of a number?

Divide the percentage by 100 and multiply by the number. For example, 20% of 150 is 0.20 × 150 = 30.

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

Divide the part by the whole and multiply by 100. For example, 30 out of 150 is 30 ÷ 150 × 100 = 20%.

How do you calculate percentage increase?

Subtract the old value from the new value, divide by the old value, and multiply by 100. A rise from 150 to 180 is (180 − 150) ÷ 150 × 100 = 20% increase.