Percentage Calculator

A percentage is just a way of expressing a number as a fraction of 100 — useful shorthand for comparing amounts that would otherwise be hard to compare directly. This calculator covers the three questions people most commonly need a percentage for, in one tool instead of three.

% of a numberanswers questions like "what is 20% of 50?" — handy for discounts, tips, sales tax, and commissions. Is what % flips that around: given two amounts, it tells you what percentage one is of the other — useful for test scores, budget shares, or completion rates. % change measures how much a value grew or shrank between two points — the calculation behind price changes, population growth, and year-over-year statistics.

Every mode uses the standard, unambiguous percentage formula for that calculation — there's no rounding trick or approximation involved. The one thing to watch for: percentage change is a relative measure, so a change from a very small starting number can produce a very large percentage even if the actual difference is small.

A worked example

Take all three modes with fresh numbers. % of a number: 15% of 80 is (15 ÷ 100) × 80 = 12. Is what %: 45 is what percent of 180? (45 ÷ 180) × 100 = 25%. % change: going from 250 to 300 is ((300 − 250) ÷ 250) × 100 = 20%, an increase. Each mode applies the same underlying idea — a ratio expressed out of 100 — to a different question.

A common misconception

A common mistake is assuming percentage changes are additive — that a 10% increase followed by a 10% decrease returns a value to where it started. It doesn't: increasing 100 by 10% gives 110, and decreasing 110 by 10% gives 99, not 100, because the second percentage is calculated on a different (larger) base than the first. Percentage changes always apply to whatever the current value is at that moment, not to some fixed original number, which is why chaining several percentage changes together rarely produces the result intuition suggests.

Where percentages can mislead

Percentage change is a relative measure, so it can make a small absolute difference look dramatic, or a large one look modest, depending entirely on the size of the starting value. A figure going from 1 to 2 is a 100% increase — technically correct, but a change of exactly one unit — while a figure going from 1,000,000 to 1,900,000 is a 90% increase representing a far larger real-world shift. Always check the underlying values behind a percentage change before treating the percentage alone as the full picture, especially in news or statistics where only the percentage tends to get reported.

How to use these results

Use "% of a number" whenever you know a rate and a base and want the resulting amount (discounts, tips, tax, commissions); use "Is what %" when you have two amounts and want the relationship between them (test scores, budget shares, completion rates); use "% change" when you're comparing a value at two points in time or two states (price changes, growth rates, before-and-after comparisons). Picking the mode that matches your actual question, rather than reverse-engineering one mode's formula into another's, is the fastest way to get a trustworthy number.

A percentage compared to a ratio

A percentage is really just a ratio with the denominator fixed at 100 — "20%," "1:5," and "0.2" all describe the exact same relationship between two quantities, expressed three different ways. Percentages are useful specifically because comparing two percentages is more intuitive than comparing two ratios with different denominators — a 3-in-8 chance versus a 5-in-16 chance takes a moment to work out, while 37.5% versus 31.25% doesn't. Converting a ratio or fraction to a percentage is exactly what "Is what %" mode does — take the ratio, multiply by 100, and the comparison becomes immediate.