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Betterbit Tools

Percentage Calculator

All four percentage calculations update as you type, with the formula behind each result — no button needed.

  • Fewer clicks, keyboard friendly

All four calculations update as you type — no button needed.

Your numbers are saved in this browser

Percent of a number

10
Result

(20 ÷ 100) × 50 = 10

What percent is X of Y

20%
Percent

10 ÷ 50 × 100 = 20%

Find the whole from a part and percent

40
Result

10 ÷ (25 ÷ 100) = 40

Percent increase or decrease

+50%
Change
+25
Difference

(75 − 50) ÷ |50| × 100 = +50%

How to use

  1. Enter a percent and a number in the first box to find the percent of that number.
  2. Enter two numbers in the second box to see what percent one is of the other.
  3. Enter a part and its percent in the third box to find the whole number.
  4. Enter a starting and ending value in the fourth box to see the percent increase or decrease.
  5. Read the worked formula under each result to see exactly how it was calculated.

The percentage formula behind each of the four calculators

A percentage is just a fraction of 100, so every box on this page reduces to the same core formula: percent ÷ 100 × base = part. The first calculator applies it directly — multiply a percent by a number. The second calculator rearranges it to solve for the percent: part ÷ whole × 100. The third solves for the whole instead: part ÷ (percent ÷ 100). The fourth, percent change, compares two values with (new − old) ÷ |old| × 100. Using the magnitude of the starting value keeps the direction intuitive for negative values too: moving from −50 to −25 is a 50% increase.

Why a percentage point and a percent are not the same thing

If a rate moves from 20% to 25%, that is a 5 percentage-point increase, but a 25% increase in the rate itself, because (25 − 20) ÷ 20 × 100 = 25. News headlines often blur this distinction, especially for interest rates, tax rates and survey results, so when you read "up 5%" versus "up 5 percentage points," the underlying numbers can differ by several times over. The fourth calculator on this page computes the second kind — the relative percent change — which is almost always what "percentage increase" or "percentage decrease" means in everyday use.

Rounding and negative numbers

All four calculators accept negative numbers and decimals — useful for comparing losses, temperature changes or any value that can go below zero. Calculations are rounded to six decimal places to remove tiny floating-point errors such as 29.999999999999996; results are then displayed to up to four decimal places for readability. Dividing by zero has no mathematical answer, so the "what percent" and "find the whole" calculators show a plain-language notice instead of a raw division-by-zero error when the denominator would be zero.

Frequently asked questions

How do you calculate a percentage of a number?

Divide the percent by 100, then multiply by the number: percent ÷ 100 × number. For example, 20% of 50 is (20 ÷ 100) × 50 = 10, which is exactly what the first calculator on this page computes as you type.

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

Divide the part by the whole, then multiply by 100: part ÷ whole × 100. For example, 10 is (10 ÷ 50) × 100 = 20% of 50 — the second calculator above shows this live for any two numbers you enter.

How do you find the original number from a percentage?

Divide the known part by the percent expressed as a decimal: part ÷ (percent ÷ 100). For example, if 10 is 25% of a number, the whole is 10 ÷ (25 ÷ 100) = 40 — that's the third calculator's formula.

What is the formula for percentage increase or decrease?

Subtract the old value from the new value, divide by the magnitude of the old value, then multiply by 100: (new − old) ÷ |old| × 100. Going from 50 to 75 is a (75 − 50) ÷ 50 × 100 = 50% increase; going from 100 to 80 is a −20% change, a decrease.

Does a percentage increase followed by the same percentage decrease return the original value?

No. Increasing 100 by 20% gives 120, but decreasing 120 by 20% gives 96, not 100 — because the second 20% is taken from a larger base. To exactly undo a percent change you need to divide by (1 + percent/100) rather than apply the opposite percent.

Does this percentage calculator send my numbers anywhere?

No. Every calculation runs with plain JavaScript in your browser; the numbers you type are stored only in this browser's local storage so they're still there next time, and are never sent to a server.

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Updated 2026-10-16