Percentage Calculator
Pick the question you need answered — a percentage of a number, one number as a percentage of another, or the percentage change between two numbers — and enter the two values.
20% of 150
30
- Percentage (A)
- 20%
- Base value (B)
- 150
20% of 150 = (20 ÷ 100) × 150 = 30.
How to use this calculator
Choose which of the three questions you're asking from the dropdown, then enter the two values, A and B. For "What is A% of B?" enter the percentage as A and the base number as B. For "A is what percent of B?" enter the part as A and the whole as B. For "Percentage change from A to B" enter the starting value as A and the ending value as B — the sign of the result tells you whether it's an increase or a decrease.
How the calculation works
All three modes are elementary-algebra identities with no regional variation. "What is A% of B?" computes (A ÷ 100) × B. "A is what percent of B?" computes (A ÷ B) × 100, and is undefined when B is zero because division by zero has no value. "Percentage change from A to B" computes (B − A) ÷ |A| × 100 — the absolute value in the denominator means the formula still gives a sensible signed answer when the starting value A is negative, and the result is undefined when A is zero because there is no baseline to measure a change against. These three forms cover the overwhelming majority of everyday percent questions: discounts and tips are a "percent of" question, exam scores and survey shares are an "is what percent" question, and price rises, pay rises, and population growth are "percentage change" questions.
Worked example
What is 20% of 150? (20 ÷ 100) × 150 = 30. Second: 30 is what percent of 150? (30 ÷ 150) × 100 = 20%. Third: a price moves from 150 to 180 — percentage change = (180 − 150) ÷ 150 × 100 = +20% (an increase). If it instead moved from 150 to 120, the change is (120 − 150) ÷ 150 × 100 = −20% (a decrease). Notice the first two examples are inverses of each other — that's always true: if A% of B is C, then C is A% of B.
Frequently asked questions
What is the formula for finding a percentage of a number?
To find A% of B, divide A by 100 and multiply by B: (A ÷ 100) × B. For example, 15% of 80 = (15 ÷ 100) × 80 = 12. This is the same as multiplying B by the decimal form of the percentage (0.15 × 80 = 12).
How do I work out what percentage one number is of another?
Divide the part by the whole and multiply by 100: (part ÷ whole) × 100. For example, if you scored 42 out of 50 on a test, that's (42 ÷ 50) × 100 = 84%. This calculation is undefined if the whole is zero.
How do you calculate percentage increase or decrease?
Percentage change = (new value − old value) ÷ |old value| × 100. A positive result is an increase, a negative result is a decrease. For example, a salary rising from £30,000 to £33,000 is a (33,000 − 30,000) ÷ 30,000 × 100 = +10% increase. The formula is undefined when the old (starting) value is zero.
What is the difference between percentage change and percentage points?
Percentage change is a relative measure — it expresses the change as a proportion of the starting value. Percentage points are an absolute measure — the simple difference between two percentages. If an interest rate moves from 5% to 7%, that's a 2 percentage-point rise, but a (7 − 5) ÷ 5 × 100 = 40% relative increase. Both are correct; they answer different questions, and conflating them is a common source of misleading statistics in news reporting.
Can a percentage be greater than 100%?
Yes. Percentages greater than 100% simply mean the value being described is more than the base it's being compared to. For example, 250 is 250% of 100, and a value that grows from 10 to 35 has increased by 250%. There's nothing mathematically special about the 100% boundary.
Why is my percentage change calculation showing as undefined?
Percentage change is undefined when the starting value (A) is zero, because the formula divides by A and division by zero has no defined result. The same applies to "A is what percent of B" when B is zero. In both cases there is no baseline to express the other number as a proportion of.