Last updated: September 26, 2026

Work out what a percentage of a number is, what percentage one number is of another, how much something changed by, or the original number before a percentage was applied. Choose the mode that matches your question below.

Percentage (%) Of this number

How to use the Percentage Calculator

Pick the tab that matches your question:

X% of Y – find a percentage of a number, for example 20% of 150.
X is what % of Y – find what percentage one number is of another, for example what percent 30 is of 150.
% change – find the percentage change between two values, and whether it's an increase or a decrease.
Increase by % – add a percentage on top of a number.
Decrease by % – take a percentage off a number.
Find the original – work backwards from a new value and a percentage to the number you started with.
% difference – compare two values as a percentage of their average, without treating either one as the "original".

What is a percentage?

A percentage is a way of expressing a number as a fraction of 100. "20%" means 20 out of every 100, or 0.20 as a decimal. Percentages let you compare amounts of different sizes on the same scale, which is why they show up in discounts, tax, exam results, interest rates and business growth figures alike.

How to calculate a percentage

To find a percentage of a number, divide the percentage by 100 to turn it into a decimal, then multiply by the number. To find what percentage one number is of another, divide the first number by the second and multiply by 100.

Percentage formulas

X% of Y = (X ÷ 100) × Y
X is what % of Y = (X ÷ Y) × 100
Percentage change = ((New − Original) ÷ Original) × 100
Percentage increase = Original × (1 + Percentage ÷ 100)
Percentage decrease = Original × (1 − Percentage ÷ 100)
Original from an increase = New value ÷ (1 + Percentage ÷ 100)
Original from a decrease = New value ÷ (1 − Percentage ÷ 100)
Percentage difference = |A − B| ÷ ((A + B) ÷ 2) × 100

Percentage examples

20% of 150. (20 ÷ 100) × 150 = 30.
30 is what % of 150? (30 ÷ 150) × 100 = 20%.
100 to 120. ((120 − 100) ÷ 100) × 100 = a 20% increase.
100 to 80. ((80 − 100) ÷ 100) × 100 = a 20% decrease.
Reverse example. A price is $120 after a 20% increase. Original = 120 ÷ 1.20 = $100.

Percentage increase vs percentage decrease

An increase adds a percentage of the original value on top of it, so the result is always larger than the original (for a positive percentage). A decrease removes a percentage from the original, so the result is smaller. The two use similar formulas, but a small mix-up between + and − in the formula gives a very different answer, so it's worth double-checking which one applies before you calculate.

Percentage change vs percentage difference

Percentage change compares a new value to an original value, so it matters which number came first, for example a price before and after a sale. Percentage difference compares two values as equals, using their average as the base, with no "before" or "after". Use percentage change when one number is clearly the starting point; use percentage difference when the two numbers are just two measurements being compared, such as two competing prices.

How to find the original number from a percentage

If you know a value after a percentage increase or decrease was applied, divide by (1 + percentage ÷ 100) for an increase, or by (1 − percentage ÷ 100) for a decrease, rather than adding or subtracting the percentage from the new value. For example, if $120 is 20% higher than the original, the original is 120 ÷ 1.20 = $100, not 120 − 20% of 120 (which would incorrectly give $96).

Common percentage calculation mistakes

Reversing a percentage by simple subtraction. $120 after a 20% increase does not come from a $96 original; you need to divide by 1.20, not multiply by 0.80.
Confusing percentage change with percentage difference. These use different formulas and different bases.
Working out a percentage on the wrong base number. "X is what % of Y" depends on which number is X and which is Y; swapping them gives a different answer.
Adding percentages that apply to different amounts. A 20% increase followed by a 20% decrease does not return you to the original number.

Real-world uses

Working out a discount or a sale price.
Adding tax to a price, similar to the UK VAT calculator.
Calculating a tip on a restaurant bill.
Turning exam marks into a percentage score.
Tracking business growth, such as checking your profit margin or markup after a price change.
Comparing payment provider costs, for example with the PayPal fee calculator.
Checking that a price change still lets a business break even.

This calculator gives estimates for general information only. It is not financial, tax or legal advice.

Frequently asked questions

How do I calculate a percentage?

Divide the percentage by 100 to get a decimal, then multiply by the number. For 20% of 150: 0.20 × 150 = 30.

What is 20% of 100?

20% of 100 is 20, since (20 ÷ 100) × 100 = 20.

How do I find what percentage one number is of another?

Divide the first number by the second, then multiply by 100. For example, 30 ÷ 150 × 100 = 20%.

How do I calculate percentage increase?

Multiply the original number by (1 + the percentage ÷ 100). A $100 price increased by 25% becomes 100 × 1.25 = $125.

How do I calculate percentage decrease?

Multiply the original number by (1 − the percentage ÷ 100). A $100 price decreased by 25% becomes 100 × 0.75 = $75.

What is the difference between percentage change and percentage difference?

Percentage change measures how much a value moved from a starting point to a new point. Percentage difference compares two values against their average, with neither treated as the starting point.

How do I calculate a percentage backwards?

Divide the new value by (1 + percentage ÷ 100) if it was an increase, or by (1 − percentage ÷ 100) if it was a decrease. Do not simply add or subtract the percentage from the new value.

Can a percentage be greater than 100%?

Yes. A percentage above 100% simply means more than the whole amount, for example "150% of 40" is 60, which is one and a half times 40.

How do I calculate a discount percentage?

Use the "X is what % of Y" idea on the amount saved: divide the discount amount by the original price and multiply by 100. The discount calculator does this directly and also works backwards from a sale price.

How do I calculate tax as a percentage?

Multiply the pre-tax price by the tax rate as a decimal to get the tax amount, then add it to the price for the total. See the UK VAT calculator for a worked example with UK rates.

How do I calculate exam marks as a percentage?

Divide the marks scored by the total marks available and multiply by 100. For 45 out of 60: (45 ÷ 60) × 100 = 75%.