Percentages show up everywhere — sales discounts, tips, tax, interest rates, grade scores, body-composition stats — and the four questions people actually ask are always the same. A percent calculator answers all of them without you needing to remember which formula goes where. Pick a tab above, type two numbers, and you get the answer plus the formula it used, so you can double-check the logic yourself. Everything runs privately in your browser; no data leaves your device.
The four modes cover the complete toolkit. What is X% of Y? handles discounts, tips, and commission ("20% off $250"). X is what percent of Y? converts any fraction to a percentage ("I scored 45 out of 200 — that's 22.5%"). Percent change compares an old and new value ("rent went from $1,000 to $1,200 — a 20% increase"). Increase/decrease by % applies a markup or discount directly ("$100 plus 15%"). Results show up to four decimal places only when they matter, so you get clean numbers, not 13-digit calculator noise.
The trickiest concept is percent change asymmetry: percentages are relative, so a 20% rise followed by a 20% fall does not return you to the start. Raise $100 by 20% to $120, then cut $120 by 20% and you land at $96, because the second cut is taken from a bigger base. Investors and shoppers get burned by this constantly — always compute change against the original value, and when a "limited-time" discount is followed by a restocking markup, the final price is almost never what you expect.
How to use the Percent Calculator
- Pick the question you're asking using the tabs — of, part-of, change, or increase/decrease.
- Enter the two numbers — they can be decimals, negatives, or large values.
- Click Calculate to see the result, the exact formula applied, and a plain-English reading.
- Switch tabs freely — your inputs stay put while you explore the same numbers from different angles.
Quick percentage rules of thumb
- 10% of anything: move the decimal one place left ($40 of $400). Scale up for 20%, 30%, etc.
- Percent change formula: ((new − old) ÷ old) × 100 — always divide by the starting value.
- Percent change and percentage points differ: 40% → 50% is a 25% rise, but only 10 percentage points.
- A 50% discount followed by 50% back up lands below the original — the base moved.
Where percentages hide in daily life
Loan interest, pay raises, tax brackets, investment returns, sale stickers, macros in nutrition, test scores, and battery life are all percentages wearing costumes. The same four formulas unlock all of them, which is why getting comfortable with one calculator pays off constantly. If you need money-specific versions with monthly schedules and amortization, try the Mortgage Calculator or the Tip Calculator — both live in the Everyday and Finance sections.
Frequently asked questions
How do you calculate X percent of a number?
Convert the percentage to a decimal and multiply: 20% of 250 = 0.20 × 250 = 50. Use the first tab of this calculator — enter the percentage and the number and it does the conversion for you.
How do you find what percent one number is of another?
Divide the part by the whole and multiply by 100: (45 ÷ 200) × 100 = 22.5%. Use the second tab here — enter the part and the whole to get the answer instantly.
What is the difference between percent change and percentage points?
Percent change is relative: going from 40% to 50% is a 25% increase. Percentage points are absolute: the same move is 10 percentage points. Mixing them up is one of the most common math mistakes in finance and statistics reporting.
Why doesn't a 20% increase followed by a 20% decrease return to the original?
Because the second percentage applies to a bigger base. Increase $100 by 20% to get $120, then decrease $120 by 20% and you land at $96 — you need a 16.67% decrease to get back to $100. Try the change tab to see this asymmetry.
All calculations are estimates for personal use. This tool is not financial advice.