Common percentage, increase, decrease and tip math.
Updated
Quick start
Pick a calculation mode, type the numbers, copy the result. Six common percent problems, one tool.
% of value, what percent, % change, increase, decrease, or reverse '% of what'. Each tab keeps its own inputs.
Results update live. Empty or zero inputs that would divide-by-zero show '—' instead of misleading zeroes.
Hit Copy on the result tile to grab the formatted number for your spreadsheet, email or invoice.
In-depth guide
Six independent calculators for the percentage problems people actually run into: discounts, tips, sales tax, year-over-year growth, markup, and reverse calculations. Each tab works in isolation so you can switch without losing your inputs.
| Mode | Formula | Use it for |
|---|---|---|
| % of value | (X ÷ 100) × Y | Discounts, taxes, tips, percentages of a budget |
| What percent | (X ÷ Y) × 100 | Quiz scores, test coverage, batch yields |
| % change | ((Y − X) ÷ |X|) × 100 | Year-over-year revenue, weight change, stock moves |
| Increase by % | X × (1 + Y/100) | Markup, price-after-tax, growth projection |
| Decrease by % | X × (1 − Y/100) | Sale price, discount, depreciation |
| % of what | (X × 100) ÷ Y | Reverse calculations: 'tip was $9, that was 18% of what?' |
No. Every formula runs in JavaScript in your browser. Try it in airplane mode — it works identically with no connection.
(new − old) ÷ |old| × 100. A change from 100 to 125 is +25%. A change from 100 to 75 is −25%. We divide by the absolute value of the old number so the sign of the result reflects direction even when old is negative.
Most percentage formulas have a division-by-zero edge case (e.g. 'what % of 0' is undefined). When that happens, the result shows '—' instead of a misleading 0.
Up to four decimal places. JavaScript numbers are IEEE-754 doubles, so values with very large or very small magnitudes can show floating-point artifacts (e.g. 0.1 + 0.2 = 0.30000000000000004). For accounting-grade math use a dedicated decimal library.
Not in this UI — switch tabs to keep each calculation separate. Copy the result you need, paste it into the next mode's input.
Increase X by Y% multiplies: 100 + 20% → 120. 'Is what %' divides: 25 is what % of 200 → 12.5%. The first answers 'how much bigger should this be?'; the second answers 'how much of the whole is this?'.
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