Percentage Calculator
This calculator puts the three percentage problems people actually run into side by side: finding X% of a value, working out what percent one number is of another, and measuring the percent change between two values. Each card recomputes on every keystroke, so you can sanity-check a discount, a tip, a tax line, or a year-over-year growth figure without opening a spreadsheet. Results are rounded to six decimal places to suppress the floating-point noise (like 32.999999999) that raw binary arithmetic can produce. Everything runs client-side in your browser — nothing is sent or logged, and the page keeps working offline once loaded. The percent-change card uses the standard (new − old) ÷ old formula and labels the result as an increase or a decrease automatically, so the sign can't be misread. Because that formula divides by the starting value, a change from zero is mathematically undefined, and the tool hides the result instead of showing a misleading number.
What is X% of a value?
X is what percent of Y?
Percent change from X to Y
How to use
- Find the card that matches your question: “What is X% of a value?”, “X is what percent of Y?”, or “Percent change from X to Y”.
- Type both numbers — decimals and negative values are accepted in every field.
- The result appears under the card and updates instantly as you type.
- For a discounted price, combine cards: compute X% of the original in the first card, subtract it, then verify the deal with the percent-change card.
Frequently asked questions
- How is percent change calculated?
- As (new − old) ÷ old × 100 — the standard convention for price changes and growth rates. The tool shows the magnitude and labels it Increase or Decrease, so a change from 80 to 60 reads as “Decrease: 25%”.
- What's the difference between percent change and percentage points?
- If a rate moves from 40% to 50%, that is a rise of 10 percentage points but a 25% relative increase. This tool computes the relative change; for percentage points, simply subtract the two raw values.
- How do I find the original price before a discount?
- Divide instead of multiplying: an item costing 80 after 20% off originally cost 80 ÷ 0.80 = 100. Adding 20% back onto 80 gives 96 — a classic mistake, because the 20% was taken from the larger original number.
- What if the base value is zero?
- “X is what percent of Y” and percent change both divide by the base, and dividing by zero is undefined — there is no meaningful percentage “from 0 to 50”. Those cards keep their result hidden until the base is non-zero.
- Is anything stored or sent anywhere?
- No. The math runs entirely in your browser with plain JavaScript; the numbers you type are never uploaded, saved, or used for analytics, and the page works offline once it has loaded.
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