A percentage calculator takes the mental arithmetic out of three common problems: finding a percentage of a number, expressing one number as a percentage of another, and calculating how much something has increased or decreased. All three run instantly in your browser — no data is sent anywhere.
This finds a portion of a number — for example, working out a 20% discount or a service charge. The formula is (percentage ÷ 100) × number. So 20% of 150 is (20 ÷ 100) × 150 = 30. This is the mode most people reach for when calculating a tip, a discount, or a tax on a known amount.
This expresses one number as a percentage of another — useful for things like exam scores, survey results, or working out what proportion of a budget has been spent. The formula is (part ÷ whole) × 100. So 45 out of 180 is (45 ÷ 180) × 100 = 25%.
This measures how much a value has increased or decreased, as a percentage of the original value. The formula is ((new value − original value) ÷ original value) × 100. Going from 80 to 100 is an increase of ((100 − 80) ÷ 80) × 100 = 25%. A negative result means a decrease. This is the mode to use for tracking changes in price, weight, salary, or any other measurable quantity over time.
These are easy to mix up. If an interest rate moves from 5% to 7%, that's a rise of 2 percentage points — but a percentage change of 40% (since 2 is 40% of the original 5). Percentage points measure the raw difference between two percentages; percentage change measures that difference relative to the starting value. This calculator's "Percentage change" mode always gives the relative figure, not the raw point difference.