About the Percentage Calculator
Percentages turn up everywhere: a 25% sale, a 17% sales tax, an 8% pay rise, a test score of 42 out of 50, a price that went from Rs 1,200 to Rs 1,450. The maths is simple once you know which formula to use, but it is easy to mix them up when you are in a hurry. This calculator puts the four most common percentage questions side by side, so you just pick the line that matches your question and fill in the blanks.
Every answer appears as you type and comes with the working underneath, so you can see exactly how it was calculated and reuse the method on paper or in a spreadsheet.
How to use it
- Find the line that matches your question.
- Type your two numbers into that line.
- Read the answer on the right, and the working underneath it.
- Use Clear all to start again.
The four formulas
X% of Y
Y × X ÷ 100. 15% of 80 = 80 × 15 ÷ 100 = 12. Use it for discounts, tips and tax.
X is what % of Y
X ÷ Y × 100. 42 out of 50 = 42 ÷ 50 × 100 = 84%. Use it for marks and shares of a total.
Percentage change
(New − Old) ÷ Old × 100. 50 → 65 is a 30% increase. Use it for price changes and growth.
Add or subtract %
Y × (1 ± X ÷ 100). 2,500 + 18% = 2,950. Use it for prices with tax or after a discount.
Common mistakes to avoid
Reversing a percentage. If a price including 18% tax is 2,950, the price before tax is 2,950 ÷ 1.18 = 2,500 — not 2,950 minus 18%, which gives 2,419. Divide by (1 + rate) to undo an increase.
Stacking discounts. A 20% discount followed by an extra 10% is not 30% off. The second discount applies to the already-reduced price, so the total saving is 28%.
Percent versus percentage points. When an interest rate moves from 10% to 12%, it has risen by 2 percentage points, but by 20% in relative terms. News reports and loan offers mix these up often, so it is worth checking which one is meant.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 15% of 80 is 80 × 15 ÷ 100 = 12.
How is percentage change calculated?
Subtract the old value from the new value, divide by the old value and multiply by 100. Going from 50 to 65 is (65 − 50) ÷ 50 × 100 = a 30% increase.
Why doesn't a 20% rise followed by a 20% fall get me back to the start?
Because the second percentage is taken from a bigger number. 100 plus 20% is 120, and 120 minus 20% is 96, not 100.
What is the difference between percent and percentage points?
Percentage points measure the gap between two percentages. A rate rising from 10% to 12% has gone up 2 percentage points, which is a 20% increase in the rate itself.