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Percentage calculator

Need help with a percentage? Choose one of three common questions, enter two numbers, and get an answer straight away. The labels tell you whether each number is the percentage, part, whole, old value, or new value.

Written by Kenneth Schrøder, Founder of WiseCalcs

Updated ·Sources verified

Use this percentage calculator to find X% of a number, find what percent one number is of another, or compare an old value with a new one.

What do you want to work out?
%

Result

30

Your answer changes as you type. In the increase-or-decrease method, the heading says whether the value went up or down, so you do not have to rely on the plus or minus sign alone. Check the field labels when you change method: the same two numbers can mean different things.

How does it work?

1 finds X% of Y. 2 finds how many percent X is of Y. 3 finds the percentage change from old X to new Y. First find new minus old, then compare that difference with the size of the old value. In the formula, that size is written |x|. From −100 to −80, the number rises by 20; compared with 100, that is +20%. If the values cross zero, such as −50 to 50, which gives +200%, the result is still a percentage but does not describe ordinary growth. A starting value of 0 cannot be used.

How the percentage calculations work

1.y⋅x1002.xy⋅1003.y−x∣x∣⋅100\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}{\lvert x \rvert} \cdot 100 \end{aligned}
x
The first number: the percentage in option 1, the part in option 2, or the old value in option 3. In option 3, |x| means the size of the old value without its plus or minus sign.
y
The second number: the number you take a percentage of, 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

Enter plain numbers; the calculator does not add units or currency. The first two methods keep plus and minus signs. Only the whole in the second method cannot be 0. For percentage change, first find the difference between the new and old value. Then compare it with the size of the old value. In formulas, that size is written |x|. The old value cannot be 0.

Sources

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

How we calculate

  • Enter plain numbers; the calculator does not add units or currency.
  • The first two methods keep plus and minus signs. Only the whole in the second method cannot be 0.
  • For percentage change, first find the difference between the new and old value. Then compare it with the size of the old value. In formulas, that size is written |x|. The old value cannot be 0.

Limitations

  • Calculators can use different rules for a negative starting value. Here, −100 to −80 rises by 20. Compared with 100, the size of the old value, that is +20%. Check the rule before comparing results.
  • If the values cross 0, the calculator can still find a percentage, but it is not a normal measure of growth. To find the amount changed, calculate new − old.

Rounding

The answer is shown with up to two decimal places. A value exactly halfway is rounded away from zero. The calculation itself keeps all available decimals.

What this percentage calculator helps with

Percentages turn a comparison into something easier to read. This calculator answers three common questions: how much a percentage of a number is, what share one number is of another, and how much a value has changed from before to after.

Choose the question that matches your task

  • Choose X% of Y when you know the percentage and want the amount. For example, 15% of a bill.
  • Choose X is what % of Y when you know the part and the whole. For example, 18 correct answers out of 24.
  • Choose Increase or decrease X → Y when you have an old value and a new value and want to compare them in percent.

How to use it

  1. Pick the question at the top.
  2. Enter the two numbers. The field labels change with the method.
  3. Read the answer. The bar underneath is a quick visual aid; the number is the answer to use.

How the percentage calculations work

For X% of Y, multiply Y by X and divide by 100. To find what percent X is of Y, divide X by Y and multiply by 100. For a change from old to new, first find new minus old. Then divide by the size of the old value and multiply by 100. The formula box shows the three versions.

Example: a price goes up, then down

Imagine something costs 150 and then costs 180. In “Increase or decrease X → Y”, enter 150 and 180. The answer is “Percentage increase +20%”. Going back from 180 to 150 gives “Percentage decrease −16.67%”. The amount changes by 30 both times, but the starting number is different. You can also choose “X% of Y”: 20% of 150 is 30.

Percentage points

A rate moving from 20% to 25% goes up by 5 percentage points. Compared with 20%, that is a 25% increase. A percentage change cannot start at 0 because you would divide by zero.

Negative numbers and changes across zero

Negative numbers work too. From −100 to −80, the number rises by 20. Compare that rise with 100, the size of the old value, and you get +20%. Other calculators may use a different rule. If the numbers cross 0, the result is still a percentage, but it is not a normal measure of growth. Need the amount changed? Calculate new − old yourself.

For a before-and-after calculation with more examples, use the percent change calculator. For a lower price, use the discount calculator. If your numbers are fractions, try the fraction calculator.

Common mistakes

  • Mixing up the part and the whole. The whole is the number you compare the part with.
  • Treating a percentage change as the final number. A +20% change from 150 means an increase of 30, ending at 180.
  • Expecting a 20% rise and a 20% fall to cancel out. They use different starting values.
  • Using the rule about more than 100% for positive numbers when one of the numbers is negative.

When it is useful

Use it for discounts, tips, test scores, price changes, game statistics, and quick checks in homework or everyday life.

FAQ

How do I find a percentage of a number?
Choose “X% of Y”, enter the percentage and the number. For example, 20% of 150 is 30.
How do I find what percent one number is of another?
Choose “X is what % of Y”. Enter the part first and the whole second. For example, 30 out of 150 is 20%.
What does percentage change mean?
It compares a new value with an old value and shows the rise or fall as a percentage of the starting value.
Can I enter zero or negative numbers?
Yes, with two limits: the whole in “X is what % of Y” cannot be 0, and the old value in the change method cannot be 0. A zero percentage or value in “X% of Y” gives 0, and the new value may be 0 in the change method. The first two methods keep signs: −20% of 150 is −30, and 10 is −10% of −100. For change, −100 to −80 rises by 20; compared with 100, that is +20%.
Why can my answer be more than 100%?
With positive numbers in “X is what % of Y”, the answer is above 100% when the part is bigger than the whole. With negative numbers, read the sign and the ratio rather than using that rule.
What is the difference between percent and percentage points?
From 20% to 25% is an increase of 5 percentage points. Compared with 20%, it is a 25% relative increase.
Can I share a calculation?
Yes. Choose Share to copy a link that opens the calculator again with the same method and numbers.

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