How do I calculate a percentage of a number?

Pick “X% of Y”, enter the percentage and the value. For 20% of 150 you get 30.

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

Pick “X is what % of Y”, enter the part and the whole. 30 out of 150 is 20%.

What does percentage change mean?

It compares a new value to an old one, relative to the old one. Going from 150 to 180 is a 20% increase; going from 180 to 150 is a 16.67% decrease.

Why is my percentage above 100%?

When the part is larger than the whole, the percentage is above 100%. That is expected, not an error.

Results are shown rounded to two decimals. The calculation behind them uses full precision.

Can I share a calculation?

Yes. Use Share to copy a link that reopens the calculator with the same mode and numbers.

Percentage calculator

Three common percentage questions in one place. Pick a mode, type two numbers, and the answer updates as you go. The field labels show which number is the percentage, part, whole, old value, or new value.

Written by Kenneth Schrøder, Founder of WiseCalcs

Updated August 17, 2026·Sources verified August 17, 2026

Use it to find X% of a number, to see what percent one number is of another, or to measure the percentage change from one value to the next.

What do you want to work out?

Percentage

Value

Result

The result updates as you type. Use the buttons at the top to switch questions, and check the labels carefully because the same two numbers mean something different in each mode.

How does it work?

1. What is x% of y. 2. x is what percent of y. 3. Percentage change from x to y.

Percentage formulas

\begin{aligned} &\textbf{1.}\quad y \cdot \tfrac{x}{100}\\[2pt] &\textbf{2.}\quad \tfrac{x}{y} \cdot 100\\[2pt] &\textbf{3.}\quad \tfrac{y-x}{x} \cdot 100 \end{aligned}

x: First value: the percent, the part, or the starting value.
y: Second value: the base, the whole, or the new value.

20% of 150 = 30. 30 is 20% of 150. From 150 to 180 is a 20% increase.

Method & sources

Inputs are treated as plain numbers without units or currency. Percentage change uses the first value as the baseline.

Sources

Where this method comes from — use these references to understand the formula, assumptions, and limits.

Percents — key concepts and calculations — OpenStax, Rice University, verified 2026-08-17

How we calculate

Inputs are treated as plain numbers without units or currency.
Percentage change uses the first value as the baseline.

Rounding

Results are rounded to two decimals for display. The calculation itself uses full precision.

What this calculator does

Percentages answer three everyday questions, and this calculator handles all of them. You can take a percentage of a number, express one number as a percentage of another, or measure how much a value went up or down in percentage terms.

Choose the right mode

Use X% of Y when you know the rate and want the amount, such as 15% of a bill.
Use X is what % of Y when you know a part and a whole, such as 18 correct answers out of 24.
Use Change X → Y when you know an old and new value and want the relative increase or decrease.

How to use it

Choose the question you want to answer at the top.
Type the two numbers. The labels change to match the mode.
Read the result and the bar below it.

A worked example

Say an item priced at 150 is marked up to 180. Switch to “Change X → Y”, enter 150 and 180, and you get +20%. Switch to “X% of Y”, enter 20 and 150, and you get 30 — the size of that increase in the same currency.

Why increases and decreases are not symmetrical

Percentage change always uses the starting value as its baseline. Moving from 150 to 180 is an increase of 30 on a base of 150, so the increase is 20%. Moving back from 180 to 150 is a decrease of the same 30 on a larger base of 180, so the decrease is only 16.67%. A 20% rise followed by a 20% fall therefore does not return to the starting value.

Percentage points and a zero baseline

A rate moving from 20% to 25% rises by 5 percentage points, but the relative increase is 25%. Percentage change from a starting value of 0 is undefined because the formula would divide by zero. For a focused old-to-new comparison, use the percent change calculator; for a price reduction, use the discount calculator; and use the fraction calculator when the part and whole are easier to enter as exact fractions.

Common mistakes

Mixing up the part and the whole. The whole is the number you compare against.
Reading a percentage change as a final value. A +20% change on 150 is an increase of 30, ending at 180.
Expecting percentages to stay under 100%. A part larger than the whole is over 100%, which is correct.

When it's useful

Tips, discounts, test scores, price changes, and quick sanity checks on numbers you see day to day.

FAQ

How do I calculate a percentage of a number?: Pick “X% of Y”, enter the percentage and the value. For 20% of 150 you get 30.
How do I find what percent one number is of another?: Pick “X is what % of Y”, enter the part and the whole. 30 out of 150 is 20%.
What does percentage change mean?: It compares a new value to an old one, relative to the old one. Going from 150 to 180 is a 20% increase; going from 180 to 150 is a 16.67% decrease.
Why is my percentage above 100%?: When the part is larger than the whole, the percentage is above 100%. That is expected, not an error.
Are results rounded?: Results are shown rounded to two decimals. The calculation behind them uses full precision.
Can I share a calculation?: Yes. Use Share to copy a link that reopens the calculator with the same mode and numbers.

Related calculators

Embed this calculator

Add this calculator to your own site. The snippet includes the calculator iframe and a small attribution link:

<iframe src="https://wisecalcs.com/embed/en/percentage-calculator" width="100%" height="520" style="border:0" loading="lazy"></iframe>
<p>Calculator from <a href="https://wisecalcs.com/en/math/percentage-calculator" rel="nofollow noopener">WiseCalcs</a></p>