Taking 20% off a price
Twenty per cent of 80 is 16, so the price paid is 64. The quick way in your head is to take a tenth (8), double it (16) and subtract. A discount stacked on a discount is not the sum of the two: 20% off and then a further 10% off leaves 72% of the original price, not 70%.
The same base at common percentages
| -20.00% | -16 | 64 | 96 |
| 1.00% | 0.8 | 80.8 | 79.2 |
| 2.00% | 1.6 | 81.6 | 78.4 |
| 5.00% | 4 | 84 | 76 |
| 10.00% | 8 | 88 | 72 |
| 12.50% | 10 | 90 | 70 |
| 15.00% | 12 | 92 | 68 |
| 20.00% | 16 | 96 | 64 |
| 25.00% | 20 | 100 | 60 |
| 30.00% | 24 | 104 | 56 |
Results are provided as-is, with no warranty of accuracy. The method and its sources are published below so you can check the working.
Two discounts do not add up. 20% off then 10% off is 28% off in total, because the second one applies to the already reduced price.
Adding a percentage and then taking the same percentage off does not return you to the start. 80 plus 20% is 96; 96 less 20% is 76.80.
The saving and the price paid always add back to the original figure, which is the quickest way to check a till receipt.
How it works
The formula
Every percentage question is the same four quantities:
base the number the percentage is measured against
percent the rate
amount base x percent / 100
after base + amount
Each mode fixes two of them and the other two follow:
A% of B base = B, percent = A
A is what % of B base = B, amount = A, percent = amount / base x 100
From A to B base = A, amount = B - A, percent = amount / base x 100
A increased by B% base = A, percent = B
A decreased by B% base = A, percent = -B
after = base + amount, in every mode, including the ones where the amount
is negative.
When base = 0 the percentage is undefined and the tool says so rather than
dividing.
What it assumes
- Percentage change is measured against where you started, never where you ended. From 40 to 50 is a 25% rise; the same 10 over 50 would be 20%, and the two are different answers to different questions.
- With a negative starting value the sign follows the textbook formula rather than the direction of travel. From -40 to -50 reads as a 25% increase, because the size of the figure grew by a quarter.
- A base of zero has no percentage, so the answer is a dash. An infinite figure on a page somebody is checking a bill against is worse than no figure at all.
- Arithmetic is decimal, not binary floating point. 10% of 1.15 comes out as 0.115 rather than 0.11499999999999999, which is what the same sum gives in a double.
- Figures are carried to ten decimal places and displayed to two. The sentence beside the answer rounds to four, because a repeating decimal written out in full is unreadable in prose.
- Nothing is rounded to a currency. The tool does not know whether the numbers are pounds, kilograms or exam marks, so it will not silently turn 0.435 into 0.44.
Common questions
How do I get the original price back from a discounted one?
Divide; do not add the percentage back. A price of 80 after 20% off was 80 / 0.8 = 100, not 80 + 20% = 96. It is the same trap as removing VAT, which is why the two operations are separate rather than one field with a sign on it.
How do I combine two discounts?
Multiply what is left rather than adding the percentages. 20% off and then a further 10% off leaves 0.8 × 0.9 = 0.72, which is 28% off, not 30%. Retailers apply them in sequence for exactly this reason, and the order does not change the result.
Sources
Method written and checked by Tessalor on Jul 30, 2026.
The full method, worked example and every assumption behind this figure are on Percentage Calculator.