Percentage Calculator

Three questions, one tool

Percentage confusion usually comes down to one of three questions: what's X% of a number, what percent is one number of another, or what's the percent change between two values. This tool covers all three so you don't have to remember which formula goes with which question.

The three formulas, worked out

Using the default 20 and 150 above: 20% of 150 is 30 (150 × 0.20). Asking what percent 20 is of 150 gives about 13.3% (20 ÷ 150). And the percent change from 20 to 150 is +650% — a much bigger number, because change is measured against the smaller starting value, not the larger one. Mixing up those last two is the single most common percentage mistake people make.

Percentage points vs. percent

One more common trip-up: if an interest rate moves from 5% to 7%, that's a 2 percentage-point increase, but a 40% relative increase (2 ÷ 5). Both descriptions are technically correct — they're just answering different questions — so it's worth checking which one a headline or a bill is actually using before reacting to it.

Where each question actually comes up

"What's X% of Y" is the one behind tips, sales tax, and commission — you know the base amount and a rate, and need the resulting figure. "What percent is A of B" shows up when you're grading a test (24 out of 30 correct is what percent?) or checking how much of a budget got spent. "Percent change" is the one people reach for least often but need most in finance and reporting — a stock that goes from $40 to $52 is up 30%, a metric that drops from 200 to 150 is down 25%. Knowing which of the three questions you're actually asking, before reaching for a calculator, avoids a lot of the mixed-up-formula mistakes below.

A quick mental-math shortcut

For a fast estimate without any tool: 10% of a number is just moving the decimal point one place left (10% of 150 is 15), and every other round percentage builds off that — 20% is double the 10% figure, 5% is half of it, 15% is 10% plus half of that. It won't replace this calculator for anything precise, but it's a useful sanity check for catching an obviously wrong number before you trust it.