One percentage calculator, every common question
The percentage calculator solves the four percentage problems that show up in real life: "what is X% of Y", "X is what percent of Y", "what is the percentage change from one number to another", and "add or subtract a percent from a number". Most online tools only handle the first one. We built all four into a single page so a tip, a discount, a grade or a price rise can all be worked out without hopping between tools.
The percentage formula in one sentence
Percent literally means "per hundred". So percent = (part ÷ whole) × 100, and everything else is a rearrangement. If you have the percent and want the part, multiply: part = (percent ÷ 100) × whole. If you have both numbers and want the percentage change between them: (new − old) ÷ |old| × 100. The calculator picks the right rearrangement for whichever mode you're on, and shows the formula under the result so the maths never feels like magic.
When you'd use each mode
- X% of Y — tip on a bill, sales tax on a price, commission on a sale, exam weight (e.g. "this is worth 30% of 200 points").
- X is what % of Y — grade out of total, test score percentages, market share, "how much of the budget did we use", proportions.
- Percent change — price rise or fall, salary bump, weight change, growth or shrinkage between two snapshots.
- Add or subtract a percent — final price after sales tax (add), price after a discount (subtract), final salary after a raise.
Practical tips that save time
For mental maths, remember 10% (just move the decimal one place) and 1% (move it two). A 15% tip is "10% + half of that". A 30% discount is "10% × 3". A 20% tip is "double the 10%". Pair these tricks with this calculator when you need exact figures.
Edge cases the tool handles cleanly
Empty fields produce a 0 result rather than NaN. Percent change from 0 is mathematically undefined (you can't divide by zero), so the calculator says so explicitly instead of showing infinity. Negative numbers work in every mode — useful for things like "how much did revenue fall by".
Use with the other calculators
For shopping, the percent off calculator is purpose-built for discounts and tax. For grades, the GPA calculator handles credit-weighted averages. For tax rates, the federal income tax percentage calculator gives effective vs marginal rates.