Percentage Calculator
Five everyday percentage questions, answered live as you type — a share of a number, a ratio as a percent, percentage change, and increase or decrease. No formulas to remember. Runs in your browser.
The formulas, in plain terms
"Percent" means per hundred, so a percentage is just a fraction with 100 on the bottom — 15% is 15/100 = 0.15. Every calculation here is one of four moves:
Share of a number
X% of Y = (X ÷ 100) × Y. 15% of 200 is 0.15 × 200 = 30. This is the one behind tips, taxes, and discounts.
Ratio as a percent
X is (X ÷ Y) × 100 percent of Y. 45 out of 180 is 45/180 × 100 = 25% — test scores, completion rates, market share.
Percentage change
Change = ((new − old) ÷ old) × 100. From 80 to 100 is (20/80) × 100 = +25%. Note it's asymmetric: a +25% rise then a −25% fall does not return you to the start, because the base changed.
Increase & decrease
Y ± X% = Y × (1 ± X/100). 50 increased by 20% is 50 × 1.2 = 60; decreased by 20% is 50 × 0.8 = 40.
Privacy
Nothing you type is sent anywhere — the arithmetic runs on your device.