Percentage Calculator

How it works

This calculator covers four everyday percentage questions with a dedicated formula for each.

Percent of a number. To find X% of a value, multiply the value by the percentage and divide by 100:

result = value × percent / 100

What percent is X of Y. To find what percentage a part represents of a whole, divide the part by the whole and multiply by 100:

percent = part / whole × 100

The whole can't be zero — dividing by zero is undefined, so this mode rejects a zero "whole" with an error instead of showing a meaningless result.

Percentage change. To find the percentage change between a starting value and an ending value:

percent = (to − from) / |from| × 100

Note the absolute value in the denominator. With the plain formula (to − from) / from, a negative starting value flips the sign of the result. For example, going from -100 to -50 is actually an improvement — the number moved up by 50, closer to zero — but the plain formula reports it as -50%, which reads as a decrease. Dividing by |from| instead keeps the result's sign tied to the real direction of movement: positive always means "went up," negative always means "went down," no matter the sign of the starting value. The starting value can't be zero, since a change from zero has no defined percentage.

Increase or decrease a number by a percentage. To add or subtract a percentage from a value:

result = value × (1 ± percent / 100)

The adjustment itself (the amount added or subtracted) is value × percent / 100, with the sign set by whether you're increasing or decreasing.

Examples

Percent of a number. A $250 jacket is on sale for 20% off. The discount is 250 × 20 / 100 = $50, so the jacket now costs $200 (calculated separately using the "Increase or decrease" mode below).

What percent is X of Y. You answered 45 questions correctly out of 180 on a test. That's (45 / 180) × 100 = 25%.

Percentage change (regular case). A company's revenue went from $80,000 to $100,000. The change is (100,000 − 80,000) / 80,000 × 100 = 25%, an increase.

Percentage change (negative starting value). A business's cash balance moved from -$100 to -$50 (it borrowed less than before). The change is (-50 − (-100)) / |-100| × 100 = 50%, an increase — because the balance genuinely improved, even though both numbers are negative.

Increase or decrease by a percentage. A $200 price increases by 15%: the adjustment is 200 × 15 / 100 = $30, so the new price is $230. Decreasing the same $200 by 15% instead gives an adjustment of -$30 and a new price of $170.

Things to watch for

• Zero isn't a valid "whole" or starting value. "What percent is X of Y" and "percentage change" both divide by one of your inputs; if that input is zero, the result is mathematically undefined and the calculator shows an error instead of a number.
• A percentage change and a percentage point difference are not the same thing. If a discount rate moves from 10% to 15%, that's a 5 percentage point increase, but it's a 50% increase relative to the original rate. Read result summaries carefully when comparing two percentages.
• Rounding is only applied for display. The calculator computes with full precision internally and rounds the number you see; if you copy a result into a spreadsheet for further calculations, small rounding differences compared to a manual calculation are expected and normal.
• Negative percentages and percentages over 100% are valid inputs for "percent of a number" and "percentage change" — a -20% "percent of" calculation or a 250% increase are both legitimate results, not errors.
• Everything runs in your browser. No number you enter is sent to a server; the calculation happens locally on your device.