Percentage Calculator
The three problems everyone actually needs.
The formulas behind the three rows
Row 1 — "What is X% of Y" is multiplication: X/100 × Y. It answers discounts (15% off 200), tax, tips and commission. Row 2 — "X is what % of Y" is division: X/Y × 100 — the "I scored 45 of 300" or "we used 45 of the 300GB quota" question. Row 3 — percentage change is (new − old)/old × 100, and the sign matters: negative means a decrease. Note change is always relative to the starting value, which is why a 50% drop needs a 100% rise to recover.
The reverse trap worth remembering: applying +10% then −10% does not return to the start — 100 → 110 → 99. Percentages multiply, they don't add.
FAQ
How do I add a percentage to a number?
Use row 1, then add the result — or multiply by (1 + X/100) directly: 200 × 1.15 = 230 for a 15% increase.
What does percentage change of negative numbers mean?
It gets ambiguous (growth from −50 to −40 is +20% in magnitude terms). For such cases report absolute change alongside the percentage.