Percentage Calculator
Three everyday percentage problems, one tool. Work out what a percent of a number is (15% of 240), what percent one number is of another (30 of 45), or the percent change between two values (200 → 250). No formulas to memorize — just pick a mode and type.
Choose a Mode
Which Percentage Method Do You Actually Need?
Most "percentage" questions are really one of three different operations. Picking the wrong mode is the #1 reason people get a number that looks plausible but is wrong. The table below maps the wording of a question to the right tool:
Three modes, three questions
| The question sounds like… | Formula | Example |
|---|
Notice the third mode is anchored to the OLD value — that is why the same 50-point swing reads as +25% one way and −20% the other.
How to Calculate a Percentage Step by Step
- Decide the mode. Are you taking a slice of something (mode 1), expressing a ratio (mode 2), or measuring a before/after change (mode 3)?
- Convert the percent to a decimal by dividing by 100 — 15% becomes 0.15. Multiplying by the whole gives the part.
- For "what percent", divide the part by the whole first, then ×100. Keep the order right: small ÷ big, not big ÷ small.
- For change, subtract first (new − old), then divide by the old value — never the new one. The sign tells you increase vs decrease.
- Sanity-check the magnitude. A 66.67% result means the part is most of the whole; a 25% change means a quarter shift. If your answer feels off, re-check which mode you used.
Worked Example: Black Friday, 30% Off, Then 8% Tax
A $79 jacket is marked 30% off, then your state adds 8% sales tax. Two of the three modes do the work:
Frequently Asked Questions
How do I find a percentage of a number?
Multiply by the decimal form: 15% of 240 = 240 × 0.15 = 36.
What percent is one number of another?
Divide part by whole, ×100. 30 of 45 = 66.67%.
How do I calculate percent change?
(New − Old) ÷ Old × 100. 200 → 250 = +25%.
Percent vs percentage point — what's the difference?
A percent is a ratio out of 100; a percentage point is the arithmetic gap between two percents. 3% → 4% is +1 point but a +33.3% relative rise.
Why doesn't a gain and loss of the same % cancel?
The base changes. A 25% drop from 250 is 187.50; recovering needs a 33% gain.