How to use the percentage calculator
The calculator has four boxes, one for each of the percentage questions people ask most often. Type into any box and the answer updates straight away. You can use whole numbers, decimals and negative numbers.
- What is X% of Y? Use this to find a share of an amount, such as a 15% tip on a bill of 80, or the 6.5% fee on a sale.
- X is what percent of Y? Use this when you know a part and the whole and want the percentage, such as 12 correct answers out of 48 questions.
- Percentage change. Enter a starting value and a new value to see how much it went up or down in percent, for example a price that moved from 80 to 100.
- Increase or decrease by X%. Choose increase or decrease, then enter a number and a percentage, for example to add 20% tax to a price or to take 20% off it.
The formulas behind each box
All four questions come from one idea: a percentage is a number out of 100. Once you know that, each formula is short.
- X% of Y = X ÷ 100 × Y. For example, 15% of 80 = 0.15 × 80 = 12.
- X as a percentage of Y = X ÷ Y × 100. For example, 12 ÷ 48 × 100 = 25%.
- Percentage change = (new value − old value) ÷ old value × 100. From 80 to 100 the change is 20 ÷ 80 × 100 = a 25% increase. From 100 to 80 it is −20 ÷ 100 × 100 = a 20% decrease.
- Increase by X% = Y × (1 + X ÷ 100), and decrease by X% = Y × (1 − X ÷ 100). So 50 increased by 20% is 60, and 50 decreased by 20% is 40.
Why a 25% rise and a 20% fall can be the same change
The last two examples show something that confuses many people. Going from 80 to 100 is a 25% increase, but going back from 100 to 80 is only a 20% decrease. The difference is the same 20 in both cases, but a percentage change is always measured against the starting value, and the starting value is different each time. This is also why a price that rises by 10% and then falls by 10% ends up lower than where it started: 100 becomes 110, and 10% of 110 is 11, so it falls to 99.
Percentage points vs percent
When a rate itself changes, be careful with your words. If an interest rate goes from 4% to 5%, it has risen by one percentage point, but by 25% in relative terms, because 1 is a quarter of 4. News reports and business documents often mix these up. If you are comparing two percentages, say which one you mean.
Everyday uses
- Shopping: work out a sale price, or check whether "save 30" or "25% off" is the better deal.
- Work and business: compare monthly sales, set a markup, or see what share of revenue goes on fees. Sellers can go further with our sale and discount calculator.
- Study: turn test scores into percentages, or check homework answers.
- Personal finance: see how much a bill went up, or what share of your income goes on rent.
Results are shown with up to four decimal places. For money, round to two decimal places in the way your bank or tax office requires.
Want the full explanation? Read our guide: How to calculate percentage increase and decrease.
More guides on this topic:
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. For example, 15% of 80 is 0.15 × 80 = 12. The first box of the calculator does this for you.
How do I work out percentage increase?
Subtract the old value from the new value, divide by the old value and multiply by 100. From 80 to 100: (100 − 80) ÷ 80 × 100 = 25%. If the answer is negative, it is a decrease.
Why can't the starting value be zero for percentage change?
Percentage change divides by the starting value, and you cannot divide by zero. Going from 0 to any other number cannot be expressed as a percentage, so the calculator asks for a non-zero starting value.
How do I reverse a percentage, for example find the price before tax?
Divide by (1 + rate ÷ 100). If a price is 120 including 20% tax, the price before tax is 120 ÷ 1.2 = 100. Subtracting 20% from 120 would give 96, which is wrong.
Is my data stored?
No. Every calculation runs in your browser and nothing you type is sent to our server.