Percentage Calculator
Work out a percentage, a percent change, or what percent one number is of another.
20% of 80
- Plus the base
- 96
- Minus from the base
- 64
Enter a percentage and a base value.
How the percentage calculator works
This tool covers the three percentage questions that come up most often, each from a single, exact formula. Pick the calculation from the selector and fill in the two values it needs.
- What is X% of Y uses
result = Y × X / 100. This is the everyday “percent of” used for tips, discounts, tax, and commissions. - X is what percent of Y uses
result = X / Y × 100. It turns a part and a whole into a percentage. The whole is a divisor, so it must not be zero. - Percent change from A to B uses
result = (B − A) / |A| × 100. It measures growth or decline relative to the starting value. The starting value A is a divisor, so it must not be zero.
Everything is computed at full precision and only rounded for display, so repeated calculations do not drift. Negative inputs and percentages above 100 are all valid; the only restrictions are the two divisors, which the calculator guards and explains rather than showing a broken result.
Worked example
Say a 80 item is on sale and you want to reason about it three ways.
| Question | Formula | Result |
|---|---|---|
| What is 20% of 80? | 80 × 20 / 100 | 16 |
| 16 is what percent of 80? | 16 / 80 × 100 | 20% |
| Percent change from 30 to 45? | (45 − 30) / 30 × 100 | +50% |
The first row says a 20% discount on the 80 item is worth 16, leaving 64 to pay. The second row is the same relationship read backwards: a 16 saving is 20% of the original 80. The third row, an unrelated example, shows that a value rising from 30 to 45 has grown by 50%, because the gain of 15 is half of the starting 30.
For a fall, the sign flips: going from 50 to 40 is (40 − 50) / 50 × 100, which is −10 / 50 × 100, or −20%. A drop of 10 from a base of 50 is a 20% decrease.
How to use it
- Choose the calculation from the selector: percent of a value, a part as a percent of a whole, or the change between two values.
- Enter the two numbers the chosen mode asks for. The result updates as you type.
- For discounts and tips, use “what is X% of Y”, then read the plus and minus rows to see the total with the amount added or removed.
- For scores and shares, use “X is what percent of Y”, for example marks out of a total or one category against a budget.
- For growth and decline, use “percent change from A to B”, such as this month against last month or a before and after price.
Limitations
This is a general arithmetic tool, not financial, tax, or statistical advice. It treats each calculation as a single exact relationship and does not compound percentages over several periods, weight averages, or convert between percent change and percentage points for you. Stacking percentages also does not commute: a 10% rise followed by a 10% fall does not return to the original value, because the second percentage is taken of a different base. When an answer feeds a real financial or academic decision, confirm the figures and the convention being used in that context.
Frequently asked questions
How do I work out X percent of a number?
Multiply the number by the percentage and divide by 100. For 20% of 80, that is 80 times 20 divided by 100, which equals 16. The same rule covers tips, discounts, and tax: a 15% tip on a 40 bill is 40 times 15 divided by 100, or 6.
How do I find what percentage one number is of another?
Divide the part by the whole and multiply by 100. If 16 is the part and 80 is the whole, then 16 divided by 80 is 0.2, and times 100 gives 20%. The whole must not be zero, because dividing by zero has no defined answer, so the calculator asks for a non-zero whole.
What is the difference between a percentage point and a percent change?
A percentage point is a flat difference between two percentages: going from 30% to 45% is a rise of 15 percentage points. A percent change measures that rise relative to the starting value: from 30 to 45 is a 50% increase, because the gain of 15 is half of the original 30. This tool reports percent change, not percentage points.
Why does the calculator use the absolute value of the starting number for percent change?
Using the magnitude of the start keeps the sign intuitive: any rise reads as positive and any fall as negative, even when the starting value is negative. Without it, a move from -50 to -40 (a clear improvement) could appear negative. The formula here is (to minus from) divided by the absolute value of from, times 100.
Can I enter negative numbers or values above 100%?
Yes. Negatives are allowed everywhere except as a zero divisor, so you can model a drop, a refund, or a balance that swings below zero. Percentages above 100 are fine too: 150% of 200 is 300, and a part larger than its whole simply reads above 100%.