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?' |
When to reach for it. Use it for everyday percentage-of, percentage-change and reverse-percentage problems where doing the arithmetic by hand invites slips.
When something else is better. It won’t reason about which base is the right one for your problem — that’s a modelling choice you still own.
The pitfall to watch. The biggest trap is a percentage of a percentage: a 10% rise then a 10% fall does not return you to the start. Mind the moving base.
Everything runs on your device. The values you enter are processed locally in this browser tab — EpitomeTool does not send your input to a server, store it, or log it. That means you can use the tool offline once the page has loaded, and refreshing the tab wipes the slate.
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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