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How to Calculate Percentages, Discounts and Tax
Easy formulas and worked examples for discounts, tax, marks and price changes.
By the ToolWorlds team · Updated 2026-10-29
The three formulas
- X% of Y: Y x X / 100.
- What percent is X of Y: X / Y x 100.
- Percentage change: (new - old) / old x 100.
Worked examples
- Discount: 15% off a 4,000 item saves 600, so you pay 3,400.
- Tax: 17% added to 5,000 is 850, so the total is 5,850.
- Marks: 412 out of 500 is 82.4%.
- Price change: from 80 to 100 is a 25% increase.
Finding the original price
If an item costs 3,400 after a 15% discount, divide by 0.85 to get the original price of 4,000. Many people subtract 15% from the sale price by mistake, which gives the wrong answer.
Common mistakes
Percentage change is always measured from the old value. A 20% rise followed by a 20% fall does not return you to the start: 100 becomes 120 and then 96. Also, a move from 10% to 12% is 2 percentage points but a 20% increase.
Type your own numbers into the Percentage Calculator and the answers update instantly. For loans, see our guide on how EMI is calculated.
Try it now: Percentage Calculator – Calculate percent of a number, what percent one number is of another, and percentage change.
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