Percentage Calculator
Four percentage calculations in one place — with the formula shown, so you can check the answer or explain it to someone else.
Runs entirely in your browser. Nothing you type here is uploaded, stored or logged.
How to use this calculator
Pick the question you're actually asking — that's the part people get wrong far more often than the arithmetic. The four tabs cover almost every percentage problem that comes up in shopping, coursework, invoices and reporting.
“What is 15% of 240?”
The % of a number tab. Use it for tips, sales tax, commission, VAT,
a discount amount, or scoring a test out of a total. The formula is
(percent ÷ 100) × number.
“36 is what percent of 240?”
The X is what % of Y tab. This is the one for marks out of a total,
what share of a budget a line item takes, or what proportion of a target you've hit.
The formula is (part ÷ whole) × 100.
“It went from 50 to 65 — by how much?”
The increase / decrease tab, which gives percentage change. The
formula is ((new − old) ÷ old) × 100. Note the denominator: change is
always measured against where you started, which is why the order of your two numbers
matters.
“I paid $68 after 15% off — what was the original price?”
The reverse % tab. You know a result and the percentage it represents,
and you want the original. The formula is amount ÷ (percent ÷ 100).
The three mistakes worth knowing about
Adding a discount back on. A 20% discount followed by a 20% increase does not return you to the original price. Take 20% off 100 and you have 80; add 20% to 80 and you get 96. The percentages are calculated against different bases.
Stacking percentages by adding them. Two successive 10% discounts are not 20% off — they're 19%, because the second is applied to the already-reduced price (0.9 × 0.9 = 0.81).
Confusing percentage points with percent. A poll moving from 40% to 44% is a four percentage point move, or a 10% relative increase. Headlines pick whichever sounds bigger; now you can check.
Quick mental shortcuts
- 10% — move the decimal one place left. 10% of 84 is 8.4.
- 5% — take 10%, then halve it.
- 15% — 10% plus 5%. Handy for tipping.
- 20% — 10% doubled, or divide by 5.
- Flip it — x% of y always equals y% of x. 4% of 75 is awkward; 75% of 4 is obviously 3.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100, then multiply by the number. For 15% of 240: 15 ÷ 100 = 0.15, and 0.15 × 240 = 36. The shortcut most people use in their head is to find 10% by moving the decimal point one place left (24), halve it for 5% (12), and add them together.
What is the difference between percentage change and percentage points?
They are not the same, and mixing them up is a common way to be misled. If an interest rate goes from 2% to 3%, that is a rise of one percentage point but a 50% increase. Use percentage points when comparing two percentages, and percentage change when comparing two amounts.
How do I work backwards from a discounted price?
Use the reverse mode. If you paid $68 after a 15% discount, you paid 85% of the original, so the original was 68 ÷ 0.85 = $80. The mistake to avoid is adding 15% back to $68, which gives $78.20 — wrong, because the discount was taken off the larger number.
Why does a 50% drop need a 100% rise to recover?
Because each percentage is measured against a different starting number. Falling from 100 to 50 is a 50% drop measured against 100. Climbing back from 50 to 100 is a 50-point move measured against 50 — which is 100%. This is why investment losses hurt more than equivalent-looking gains help.
Is my data sent anywhere?
No. The calculation runs in JavaScript inside your browser. Nothing you type is transmitted to a server or stored.