About the Percentage converter
Percentages come up constantly, in tips, discounts, tax, salary rises, and exam marks, and the same word can mean slightly different calculations. This tool covers the four questions that trip most people up: what is X% of Y, what percent is X of Y, what is X after a percentage increase, and what is X after a percentage decrease.
The core formula
X% of Y means (X ÷ 100) × Y. So 15% of 80 is (15 ÷ 100) × 80 = 12. That single formula covers most everyday cases from tipping to sales tax.
Percentage change
The change from an old value to a new value, expressed as a percentage, is ((new − old) ÷ old) × 100. If a subscription goes from £10 to £13, that is ((13 − 10) ÷ 10) × 100 = 30% up. If it goes from £13 to £10, that is a 23% drop, not 30%, because the base changed.
Why increases and decreases don't cancel
A 20% rise followed by a 20% cut does not bring you back to the start. £100 rises by 20% to £120, then a 20% cut on £120 is £24, leaving £96. The cut is applied to a bigger base than the rise was, so you finish 4% down. Something to remember when investments recover from a fall.
Frequently asked questions
- How do I calculate a tip?
- Multiply the bill by 0.10 for 10%, 0.15 for 15%, or 0.20 for 20%. A quick trick: 10% is the bill divided by 10, and 20% is that number doubled.
- How do I work out my discount?
- For a 25% off sale on a £60 item, work out 25% of 60 (which is 15) and subtract, giving £45. Or multiply the price by (1 − 0.25) = 0.75 in one step.