Percentage Calculator

A percentage expresses one number as a fraction of 100, so 18% simply means 18 out of every 100. This calculator handles the three problems people most often mix up: finding X% of a number (multiply by X/100), finding what percent one number is of another (divide and multiply by 100), and finding the percent change between two values (the difference divided by the starting value). Switch the mode to match the question you are asking — the three formulas are not interchangeable.

18% of 250

Inputs

Mode

% of

This takes a slice of a whole

18% of 250 scales the second value by the percent. To go the other way — what percent one number is of another — switch to the 'X is what % of Y' mode.

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Uses your inputs above

45 18% of 250. Want to try a variation?

The math

Reviewed 2026

Formula

of: X·Y/100;  is: X/Y·100;  change: (Y−X)/X·100

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Example: how percentage is calculated

Step-by-step with default inputs

Suppose you put the default values into Percentage Calculator:

Mode

X% of Y

250

Plug those into the formula of: X·Y/100; is: X/Y·100; change: (Y−X)/X·100 and the result is:

18% of 250

With the defaults, 18% of 250 works out to 45: divide 18 by 100 to get 0.18, then multiply 0.18 × 250. You can sanity-check it in reverse with the second mode — 45 is 18% of 250 because 45 ÷ 250 × 100 = 18. For mental math, decompose the percentage: 10% of 250 is 25, 5% is 12.5, 2% is 5, and 1% is 2.5, so 18% = 25 + 12.5 + 5 + 2.5 = 45.

How to calculate percentage by hand

To find a% of b, divide a by 100 to convert it to a decimal: 18 ÷ 100 = 0.18.
Multiply that decimal by b: 0.18 × 250 = 45.
To find what percent a is of b, divide a by b and multiply by 100: 18 ÷ 250 × 100 = 7.2%.
To find the percent change from a to b, subtract a from b, divide by a, and multiply by 100 — from 40 to 50 that is 10 ÷ 40 × 100 = +25%.
Read the sign of a percent change: positive means an increase from the original value, negative means a decrease.

How does the percentage calculator work?

All three modes follow from the definition of percent — per hundred. The first mode converts the percentage to a decimal and multiplies: X% of Y = (X / 100) × Y. The second inverts it: A as a share of B is (A / B) × 100. The third, percent change, measures a move relative to where it started: (B − A) / A × 100, so the starting value A is always the denominator. That last detail causes most percentage mistakes — a change is relative to the original value, not the new one, which is why the change from 40 to 50 (+25%) is not the mirror image of the change from 50 to 40 (−20%). The calculator does plain arithmetic and rounds only for display.

Last reviewed July 2, 2026 · Editorial policy

Frequently asked questions

How do you find a percentage of a number?

Divide the percentage by 100 and multiply by the number: 18% of 250 is 0.18 × 250 = 45. A quick mental shortcut is to find 10% (move the decimal point one place left) and 1% (two places), then combine the pieces you need.

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

Divide the part by the whole and multiply by 100: 18 is 7.2% of 250 because 18 ÷ 250 = 0.072. Make sure the whole goes in the denominator — swapping the two numbers gives a completely different answer.

What is the difference between percent change and percentage points?

Percentage points measure the simple difference between two percentages, while percent change measures the relative move. Going from 10% to 15% is a rise of 5 percentage points, but it is a 50% increase, because (15 − 10) ÷ 10 × 100 = 50.

Why doesn't a 50% increase undo a 50% decrease?

Because each change is measured against a different starting value. Drop 100 by 50% and you have 50; raise 50 by 50% and you get 75, not 100 — the increase applies to the smaller base. Reversing a 50% decrease actually takes a 100% increase.

Is the percent change from A to B the same as from B to A?

No — percent change always divides by the starting value, so direction matters. Going from 40 to 50 is a +25% change (10 ÷ 40), but going from 50 to 40 is a −20% change (10 ÷ 50).

What is percent difference, and how is it different from percent change?

Percent difference compares two values without treating either as the start: divide the gap by the average of the two, so 40 versus 50 gives 10 ÷ 45 × 100 ≈ 22.2%. Percent change (this calculator's third mode) divides by the starting value instead, giving 25% for the same pair.

Guides

How do I calculate a percentage?How to calculate a percentage: to find X% of a number multiply by X/100; to find what percent one number is of another, divide and multiply by 100.How do I calculate percentage change?How to calculate percentage change: subtract the old value from the new, divide by the old value, and multiply by 100 — positive is an increase, negative a decrease.