Everyday math tool
Four percentage problems, answered instantly as you type: percent of a number, one number as a percent of another, percentage increase or decrease, and discount sale prices.
Quick reference
| Problem | Method | Example |
|---|---|---|
| X% of Y | (X ÷ 100) × Y | 15% of 200 = 30 |
| X is what % of Y | X ÷ Y × 100 | 45 of 60 = 75% |
| Percent change | (new − old) ÷ old × 100 | 40 → 50 = +25% |
| Price after discount | price × (1 − discount ÷ 100) | $60 at 25% off = $45 |
| Add X% (tip, tax, markup) | amount × (1 + X ÷ 100) | $80 + 10% = $88 |
| Reverse: find original | final ÷ (1 ± X ÷ 100) | $88 was $80 before +10% |
FAQ
Percent means "per hundred". 15% is simply the fraction 15/100, or 0.15 as a decimal. Every percentage problem reduces to multiplying or dividing by that decimal — which is why all four modes above are one-step conversions.
Subtract the old value from the new one, divide by the old value, and multiply by 100. Positive results are increases; negative results are decreases. Always divide by the original value, never the new one.
Because each percentage applies to a different base. A 20% rise then 20% fall leaves you at 96% of the start (1.2 × 0.8 = 0.96). To undo a 25% increase you only need a 20% decrease: 1.25 × 0.8 = 1.0.
Multiply by (1 + rate/100). A 10% tip on $80 is 80 × 1.10 = $88. To strip a known tax or markup back out, divide by (1 + rate/100) instead of subtracting the percentage — a common mistake that understates the original amount.
If a rate rises from 4% to 6%, it rose 2 percentage points — but 50% in relative terms (2 ÷ 4). News headlines often mix these up; the distinction matters a lot in finance and statistics.
No. Everything runs locally in your browser and nothing is transmitted or saved.
Disclaimer
Results are provided for convenience and education. Verify critical figures independently before using them in contracts, taxes, or financial decisions.