How is percentage change calculated?

As (new value minus old value) divided by the absolute value of the old value, times 100, so a rise from 100 to 125 is +25% and a drop from 100 to 75 is -25%.

Can I enter negative numbers?

Yes, all three rows accept negative numbers, except that the whole in the second row and the starting value in the third row can't be zero, since dividing by zero has no defined percentage answer.

Why does the whole or starting value show an error at zero?

Both the what-percent-of-what row and the percentage-change row divide by that number, and dividing by zero is undefined, so the tool shows an error instead of Infinity or NaN.

Can I calculate the whole from a part and a percentage?

Not directly as a fourth row, but you can get there with the first row: if you know the part is a given percent of an unknown whole, treat it as solving percent-of-value in reverse using the math evaluator tool.

Percentage Calculator

Percent of a value, what-percent-of, and percentage change — all in one, locally.

Local, Runs entirely in your browser, your data never leaves your device.

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Three percentage questions, answered side by side

Most percentage confusion comes from mixing up which of three related questions you're actually asking: what a percent of a number is, what percent one number is of another, or how much a value changed. This tool gives each question its own row with two input fields, so you can compare them and see immediately which formula applies to your numbers. Calculations happen locally the moment you edit a field, with no page reload.

How to use the Percentage Calculator

In the first row, enter a percent and a value to see what that percent of the value equals.
In the second row, enter a part and a whole to see what percent the part is of the whole.
In the third row, enter a starting value and an ending value to see the percentage change.
Watch each row's result update immediately as you edit either number in that row.

Results are rounded to six decimal places to avoid floating-point noise like 33.33000000000001, while staying precise enough for financial or engineering figures.

Percentage change uses the absolute value of the starting number as the base, so a change from -50 to 50 still reports a sensible percentage instead of a sign-flipped one caused by dividing by a negative starting value.

Frequently asked questions

How is percentage change calculated?: As (new value minus old value) divided by the absolute value of the old value, times 100, so a rise from 100 to 125 is +25% and a drop from 100 to 75 is -25%.
Can I enter negative numbers?: Yes, all three rows accept negative numbers, except that the whole in the second row and the starting value in the third row can't be zero, since dividing by zero has no defined percentage answer.
Why does the whole or starting value show an error at zero?: Both the what-percent-of-what row and the percentage-change row divide by that number, and dividing by zero is undefined, so the tool shows an error instead of Infinity or NaN.
Can I calculate the whole from a part and a percentage?: Not directly as a fourth row, but you can get there with the first row: if you know the part is a given percent of an unknown whole, treat it as solving percent-of-value in reverse using the math evaluator tool.