Percentage calculator
Three modes for the three percentage questions you actually ask. Updates as you type.
(20 ÷ 100) × 250 = 50
The three flavours of percentage math
Most percentage questions reduce to one of three patterns: what is X% of Y, X is what % of Y, or what's the % change from X to Y. They sound similar but they need different formulas, and confusing them is where most percentage mistakes come from. Each tab here owns one flavour so you don't have to remember which formula to use.
The formulas
- % of:(a ÷ 100) × b — "15% of $80" → 12
- is what %:(a ÷ b) × 100 — "12 is what % of 80" → 15%
- % change:((b − a) ÷ |a|) × 100 — "from $80 to $100" → +25%
The trap with percent change
A 50% loss followed by a 50% gain doesn't put you back where you started. If you start at $100, lose 50%, you're at $50. A 50% gain on $50 only takes you to $75 — you needed a 100% gain to recover. This is why percent-change figures over multiple periods are easy to misread; for compounded growth, look at multiplicative ratios instead of additive percentages.
Tipping, sales tax, and discounts
For tipping a meal, use the tip calculator — it adds the tip on top and splits the bill in one move. For a sales discount, multiply by (1 − discount%). For sales tax on top of a price, multiply by (1 + tax%). Stacked discounts (e.g. 20% off, then another 10% off the discounted price) compound multiplicatively, not additively.