All four cases: a percentage of a number, what percentage one is of another, increases and discounts. Free, nothing to install, no sign-up.
This calculator handles the four percentage cases that actually come up: working out a percentage of a number, finding what percentage one number is of another, increasing an amount by a percentage and decreasing it. Pick the case with the buttons at the top, enter the two numbers and you get the answer. It runs inside your browser and downloads as a PDF.
Per cent means "out of a hundred". 20% is twenty out of every hundred, that is 20 divided by 100, which is 0.20. That is why every percentage works the same way: divide the percentage by a hundred and multiply by the number. Once that line makes sense, all four cases fall out of it and there is no formula to memorise.
20% of 80: 20 ÷ 100 × 80 = 16. What percentage 16 is of 80: divide the part by the total and multiply by a hundred, 16 ÷ 80 × 100 = 20%. Increase 80 by 20%: 80 × 1.20 = 96. Decrease 80 by 20%: 80 × 0.80 = 64. Notice that increases and decreases are a single multiplication, not two steps: adding 20% to 80 and multiplying 80 by 1.20 are the same thing.
Put 100 up by 20% and you get 120; take 20% off that 120 and you get 96, not 100. That is not the calculator getting it wrong: the increase was worked out on 100 and the decrease on 120, which is a bigger number, so the 20% is worth more. This is why a 20% discount on an already increased price does not put you back where you started, and why something that goes up 50% and then down 50% ends up costing 25% less than it did.
For a percentage of a number, sanity-check the size: 10% is moving the decimal point one place, 50% is half, and 25% is half of half. If you ask for 20% of 80 and get something bigger than 40, you have entered something wrong. For increases and decreases, run it backwards by dividing: 96 ÷ 1.20 has to give you the 80 you started with.
Because the two percentages are worked out on different numbers. The increase is based on the starting number and the decrease on the already-increased number, which is larger. To undo a 20% increase you divide by 1.20 — you do not take 20% off.
Divide the part by the total and multiply by a hundred. If you got 16 questions right out of 80: 16 ÷ 80 × 100 = 20%. The order matters — the total goes on the bottom. If your answer is over a hundred, you have almost certainly swapped them.
Lean on 10%, which is just moving the decimal point one place left. 10% of 240 is 24, so 20% is 48 and 30% is 72. For 15%, take the 10% and add half of it again. It is quick and it lets you check a price tag at a glance.
No. The calculator runs entirely inside your browser: what you type never leaves your device and is not stored anywhere. No account, no email address.
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