Percentage Calculator — Free, Five Operations in One
Calculate X% of Y, what percent one number is of another, percent change, and adding or subtracting a percent. Every answer shows the formula breakdown. Runs 100% in your browser.
Calculation
e.g. What is 18% of 250 ?
Five Operations in One Tool
Switch between "X% of Y", "X is what % of Y", "% change from X to Y", "add X%", and "subtract X%" with one click. No re-learning the layout.
Shows the Math
Every answer comes with a one-line breakdown — the exact formula and substitution. Educational for students; receipt-ready for invoices and refunds.
Five Real-World Presets
Sales tax, item discount, restaurant tip, price change, and "what fraction is that" — one click loads the canonical scenario.
100% Client-Side
Calculations stay in your browser. Salary changes, sale prices, exam scores — none of it ever reaches our servers.
Percentage calculator: percent of, percent change, and percent off
A percentage calculator turns a percent into a real number for one of five operations: X% of Y, X is what % of Y, percent change between two values, and adding or subtracting a percent. Enter your numbers, pick a mode, and every answer shows the exact formula and substitution. It runs 100% in your browser, free.
How to use the percentage calculator
- Pick a Calculation mode: "What is X% of Y?", "X is what % of Y?", "% change from X to Y", "Add X% to Y", or "Subtract X% from Y".
- Type your numbers into the two fields. The percent field shows a % suffix; both accept decimals like 8.875.
- Read the large Result, the plain-English sentence, and the Breakdown line that shows the substituted formula.
- Need a quick start? Click a preset chip — Sales tax 8.875%, 20% off $80, Tip 18% on $42 — to load the mode and inputs in one tap.
- Switching modes clears the fields, so each calculation starts clean with no stale numbers.
What is a percentage and how does the math work?
A percent is a fraction out of 100. The symbol % is shorthand for the number 0.01, so "18%" literally means 0.18. That single fact powers all five operations: to find X% of Y you multiply Y by (X ÷ 100); to express X as a % of Y you divide and multiply by 100. The NIST Guide to the SI (SP 811, §7.10.2) makes the rule explicit: "the symbol % represents simply a number," and a space belongs between the number and the symbol (0.25 % , not 0.25%).
Percent change is a relative measure: the difference between a new and old value, divided by the old value. Per Wikipedia's relative-change definition, the formula is ((V₂ − V₁) / V₁) × 100%. This calculator divides by |old| (the absolute value) so the sign still tracks direction when the starting value is negative. Results round to four decimals, then drop trailing zeros — integers display whole, fractions keep up to two decimals via toLocaleString for locale-aware grouping.
Five percentage formulas, side by side
| Question | Formula | Worked example |
|---|---|---|
| What is X% of Y? | (X / 100) × Y | 18% of 250 = 45 |
| X is what % of Y? | (X / Y) × 100 | 45 is 18% of 250 |
| % change from X to Y | ((Y − X) / |X|) × 100 | 200 → 230 is +15% |
| Add X% to Y | Y × (1 + X / 100) | $100 + 8.875% tax = $108.88 |
| Subtract X% from Y | Y × (1 − X / 100) | $80 − 20% off = $64 |
Worked examples: input → output
Add X% to Y · sales tax
$100 + 8.875% = 100 × (1 + 8.875/100) = $108.88
% change from X to Y · price rise
200 → 230 = ((230 − 200) / 200) × 100 = +15%
Reverse percentage · find the original
$108.88 tax-inclusive ÷ 1.08875 = $100 pre-tax (divide, do not subtract the rate)
Edge case · a 50% drop then a 50% rise does not break even
$100 down 50% is $50; up 50% from there is only $75, not $100. Percent changes are not symmetric because each one applies to a different base. Recovering a 50% loss needs a 100% gain. The same trap hits stacked discounts: 20% off then 10% off is 0.80 × 0.90 = 0.72, an effective 28% off — never 30%.
Six everyday percentage scenarios
1. Sales tax
Use "Add X% to Y". US tax rates range from 0% (Oregon, Delaware) to 9.5%+ in some California cities. Add to subtotal for the final price.
2. Sale discounts
Use "Subtract X% from Y". 20% off $80 = $64. Stacked discounts do not add: 20% then 10% is 0.8 × 0.9 = 0.72, so the effective discount is 28%.
3. Salary raises
Use "% change from X to Y" to confirm an announced raise. A 4% raise on $80,000 = $83,200. Compare to inflation for the real-wage change.
4. Exam scores
Use "X is what % of Y". 87 out of 100 = 87%. 38 out of 45 = 84.4%. Convert any raw score to a percentage for grade lookup.
5. Tipping
Use "What is X% of Y" with 18 or 20 as the rate and the subtotal. For bill-splitting across a party, see the tip calculator.
6. Stock / index returns
Use "% change from X to Y" with start and end values. This is a simple return, not annualised — for multi-year growth use the compound interest calculator.
Four percentage mistakes even experts make
1. Percent vs percentage points
A rate rising from 40% to 44% is a 4 percentage-point increase but a 10% relative increase ((44-40)/40 × 100). Mixing them is the #1 reporting error in financial news.
2. Stacked discounts do not add
20% off + extra 10% off = 28% off, NOT 30%. Multiply the survival factors (0.80 × 0.90 = 0.72) and subtract from 1.
3. Asymmetric reversals
A 50% drop then a 50% rise does NOT return to the original. $100 → $50 → $75 (not $100). Going back requires a 100% rise.
4. Reverse tax-inclusive math
To find the pre-tax price from a tax-inclusive total, DIVIDE by 1 + (rate/100), do not subtract the rate. $108.88 ÷ 1.08875 = $100, not $108.88 × 0.91125.
Why this calculator never returns Infinity
Two of the five modes divide by a number you enter: "X is what % of Y" divides by Y, and "% change from X to Y" divides by X. If that divisor is zero, plain JavaScript would return Infinity or NaN. This tool guards both cases explicitly and shows a readable error instead — because the answer is genuinely undefined, not infinite. Growth from a base of zero (a product's first sale, a startup's first revenue) has no meaningful percent; report the absolute change instead.
The percent-change mode also divides by Math.abs(from), not the raw value. That subtle choice keeps the sign of the result pointing the right way when your starting number is negative — for example a debt moving from −200 to −150 reads as a real reduction, not a backwards one. Most quick web calculators skip this and flip the sign on negative bases.
Runs 100% in your browser
Your numbers never leave your device. Every calculation is plain JavaScript arithmetic done locally — no uploads, nothing leaves your device — so salary figures, sale prices, and exam scores stay private and the tool works offline once the page loads. I tested all five modes against hand-worked answers, including the zero-divisor guards, negative starting values, and the 8.875% NYC tax preset, and confirmed the breakdown line matches the displayed result every time.
Frequently asked questions
Is this percentage calculator free?
Yes — 100% free with no signup and no usage limit. All five operations run in your browser, so there is nothing to buy or install.
Are my numbers sent to a server?
No. Calculations run entirely in your browser via JavaScript. The tool makes no network requests and ships no analytics tied to your inputs, so it also works offline.
What is the difference between "percent off" and "percent change"?
"Percent off" is a one-way discount (20% off $80 = $64). "Percent change" is two-directional and can be positive or negative; $80 to $64 is a -20% change. They are two views of the same math.
How is a percent change different from a percentage point?
A percentage point is the plain gap between two percentages; a percent change is relative. A rate going 40% to 44% rises 4 percentage points but 10% relatively ((44-40)/40 × 100).
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Last updated: June 2, 2026 · Runs 100% in your browser — no uploads, nothing leaves your device.
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