Percentage Calculator at a glance
- What it does
- Calculate percentages, percentage increase and decrease, percentage difference and reverse percentages, with the formulas explained.
- Where it runs
- Entirely in your browser — no data is uploaded
- Works offline
- Yes, once the page has loaded
- Cost
- Free, with no account and no usage limit
- Category
- General Calculators
How to use the calculator
- Pick the calculation that matches your question.
- Enter the numbers.
- Read the result, along with the formula used so you can check it.
The formulas
| Question | Formula | Example |
|---|---|---|
| What is 15% of 80? | 80 × 0.15 | 12 |
| 12 is what % of 80? | 12 ÷ 80 × 100 | 15% |
| Increase from 80 to 92 | (92 − 80) ÷ 80 × 100 | +15% |
| Decrease from 92 to 80 | (80 − 92) ÷ 92 × 100 | −13.04% |
| Add 20% to 80 | 80 × 1.20 | 96 |
| Remove 20% from 96 | 96 ÷ 1.20 | 80 |
The fifth and sixth rows are the pair people get wrong. Adding 20% and then subtracting 20% does not return you to where you started, because the second percentage is applied to a larger number.
The mistakes that cost money
Percentage of what? An increase from 80 to 92 is +15%, but the decrease from 92 back to 80 is −13.04%. The base differs, so the two are not symmetrical. A price cut of 50% needs a 100% rise to undo it.
Percentages do not add. Two successive 10% increases give 21%, not 20%, because the second applies to the already-increased amount. Three successive 10% cuts leave 72.9%, not 70%.
Percentage points versus percent. If an interest rate goes from 4% to 6%, that is a rise of 2 percentage points and an increase of 50%. Both are correct and they describe very different things; conflating them is a standard trick in misleading statistics.
Reversing a discount. To find the original price of an item reduced by 30%, divide by 0.70 - do not add 30%. A £70 item at 30% off was £100, not £91.
Working backwards from a tax-inclusive price
A frequent real-world need: you have a price including VAT or sales tax and want the amount before tax.
Price excluding 20% VAT = inclusive price ÷ 1.20
VAT amount = inclusive price − (inclusive price ÷ 1.20)
£120 inclusive → £100 net + £20 VATSubtracting 20% from £120 gives £96, which is wrong by £4. The same reasoning applies to any inclusive figure - a gross salary, a commission-inclusive total, a marked-up price.
Reading percentages critically
Relative versus absolute risk. "Doubles your risk" means little without the starting point. A rise from 1 in 100,000 to 2 in 100,000 is a 100% relative increase and a negligible absolute one. Health reporting frequently uses the relative figure because it sounds larger.
Percentages of small numbers. "Sales up 300%" is unimpressive if it means four units instead of one. Always ask for the base.
Averages of percentages. Averaging percentage figures without weighting them by size gives a wrong answer. A 50% success rate on 10 cases and 90% on 1,000 cases is not 70% overall.
When percentages compound
Repeated percentage changes multiply rather than add, and the gap widens quickly.
| Change | Naive sum | Actual |
|---|---|---|
| +10% twice | +20% | +21% |
| +10% then −10% | 0% | −1% |
| −50% then +50% | 0% | −25% |
| +5% a year for 10 years | +50% | +62.9% |
| −20% three times | −60% | −48.8% |
The second and third rows are the ones worth remembering. An increase followed by an equal-sized decrease never returns you to the start, because the decrease applies to a larger base. This is why an investment that falls 50% needs a 100% gain to recover, and why "we cut costs 20% three times" does not mean costs were eliminated.
Margin and markup are not the same thing
A distinction that costs small businesses real money, because the two words are used interchangeably and mean different things.
Markup is the increase over your cost. Margin is the profit as a proportion of the selling price.
Cost £60, sold at £100
Markup = (100 − 60) / 60 = 66.7%
Margin = (100 − 60) / 100 = 40%Applying a 40% markup when you meant a 40% margin gives a selling price of £84 rather than £100 — a shortfall of 16% of revenue on every unit. To convert, divide by one minus the margin: for a 40% margin, the selling price is cost ÷ 0.60.
The same confusion appears in discounting. Offering "40% off" reduces revenue by 40% of the selling price, which may exceed your entire margin.
Frequently asked questions
Because each percentage applies to a different base. Adding 20% to 100 gives 120; removing 20% of 120 removes 24, leaving 96. To reverse an increase, divide rather than subtract.
A move from 4% to 6% is 2 percentage points, and also a 50% increase. Percentage points describe the arithmetic difference; percent describes the proportional change.
Divide by one minus the discount. At 30% off, divide the sale price by 0.70.
Divide by 1 plus the rate. For 20% VAT, divide the inclusive price by 1.20 to get the net amount.
Nothing you enter here leaves your browser
Percentage Calculator does its work in JavaScript running on your own device. The page loads once, and after that there is no upload step and no server involved — which matters here because your figures are yours, and nobody needs a record of them.
You can verify this rather than taking our word for it: load the page, disconnect from the internet, and the tool keeps working. Our privacy policy sets out what is and is not collected, and this guide explains why the distinction matters.