The page has three separate calculators, one under the other. Each has two boxes and no button. A result appears as soon as both boxes have a number, and it changes as you type. Results of 1 or more show two decimal places, like 30.00. Smaller results show up to six decimal places, so tiny answers do not turn into 0.00.
| Calculator | Boxes | What it does | Example |
|---|---|---|---|
| What is X% of Y? | Percentage, of Number | Percentage ÷ 100 × Number | 15 and 200 → 30.00 |
| X is what % of Y? | Number, of Total | Number ÷ Total × 100 | 45 and 60 → 75.00% |
| Percentage change from X to Y | From, To | (To − From) ÷ From × 100 | 100 and 150 → +50.00% |
If you clear a box, the result of that calculator disappears.
A service charge is 18% of a bill of 1,250. Type 18 in Percentage and 1250 in of Number.
A useful shortcut: X% of Y is the same as Y% of X. 8% of 50 is hard in your head, but 50% of 8 is easy. Both give 4.00.
You scored 45 out of 60 in a test. Type 45 in Number and 60 in of Total.
The answer can be more than 100%. If this year's sales are 150 and last year's were 120, then 150 is 125.00% of 120.
Rent goes up from 1,200 to 1,500. Type 1200 in From and 1500 in To.
A fall shows a minus sign. From 250 to 245 gives −2.00%. Always put the old value in From. The change is measured against where you started.
A percentage change depends on the starting value. So the same amount up and down gives different percentages:
This is the most common percentage mistake. A shirt costs 80 after 20% off. What was the original price?
The 20% was taken from the original price, not from 80. You can check the answer with this page. In the first calculator, 20% of 100 is 20.00, and 100 − 20 = 80. In the third calculator, 100 to 80 shows −20.00%.
The same rule works for tax that is already included in a price. If 1,130 includes 13% VAT, the price before VAT is 1,130 ÷ 1.13 = 1,000. Taking 13% off 1,130 gives 983.10, which is wrong.
When the numbers are themselves percentages, there are two ways to describe a change. A loan rate goes from 5% to 8%:
Both are true, but they answer different questions. News reports sometimes write "rose 3%" when they mean 3 points. If you read that a rate went from 3% to 5%, it is 2 points or +66.67%.
Divide the sale price by what was left after the discount. If a price is 80 after 20% off, 80% of the original is left, so the original is 80 divided by 0.80, which is 100. Adding 20% to 80 gives 96, which is wrong because the 20% was taken from the original price.
Use the third calculator. Put the old value in From and the new value in To. It subtracts From from To, divides by From and multiplies by 100. From 1,200 to 1,500 gives +25.00%. From 250 to 245 gives −2.00%.
Percentage points are the simple difference between two percentages. Percent is the change compared with the starting value. A rate going from 5% to 8% is up 3 percentage points, but it is up 60%, because 3 is 60% of 5.
Each change is measured from a different starting value. 100 plus 10% is 110. Then 10% of 110 is 11, so the fall takes you to 99. The same logic means 100 to 150 is +50%, but 150 back to 100 is only −33.33%.
You cannot divide by zero, so there is no percentage to show. In the third calculator, a change from 0, such as 0 to 50, shows a note instead of a result. In the second calculator, the same note appears when of Total is 0. Change the zero to a real starting value.