Percentage Calculator
Solve the three everyday percentage questions: what is X% of Y, X is what percent of Y, and the percentage change from one number to another.
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Disclaimer: This free tool is provided “as is,” without warranties of any kind, and is for general informational purposes only — not professional, legal, financial, medical, tax, or engineering advice. Results may contain errors; verify anything important independently and use at your own risk. We accept no liability for any loss or damage arising from its use. See our Terms of Use for details.
Step-by-Step Guide
Pick a mode tab, enter the two numbers, and read the result plus a sentence describing it. The Copy button puts the sentence on your clipboard.
'15% of 80' ➔ 12; from 80 to 100 ➔ a 25% increase (Δ 20).
Who it's for
Shoppers, students, finance and spreadsheet users, and anyone checking discounts or growth.
Core Features
- Mode A: what is X% of Y.
- Mode B: X is what percent of Y.
- Mode C: percentage increase or decrease from A to B, with the absolute delta.
- Plain-English result sentence you can copy in one click.
🛡️ No tracking — your inputs, keys, and details never leave this client sandbox.
How do I calculate a percentage of a number?
Use the "X% of Y" mode: enter the percent and the value, and it multiplies them — for example 15% of 80 is 12. The result comes with a plain-English sentence you can copy.
How do I find what percent one number is of another?
Use the "X is what % of Y" mode: enter the part and the total and it returns the percentage — for example 12 is 15% of 80. Handy for working out test scores, tips, or shares of a total.
How do I calculate percentage increase or decrease?
Use the "% Change" mode: enter the original and new values and it shows the percent change and whether it's an increase or decrease, plus the raw difference — for example from 80 to 100 is a 25% increase.
Does it work for any numbers, including decimals?
Yes. All three modes accept decimals and negative numbers and compute instantly in your browser. Dividing by zero is handled gracefully (it shows a dash rather than an error).
What is the percentage change formula?
Percentage change = ((new value − original value) ÷ original value) × 100. If the result is positive, it is an increase; if negative, a decrease. For example, from 80 to 100: ((100 − 80) ÷ 80) × 100 = 25% increase. From 100 to 80: ((80 − 100) ÷ 100) × 100 = −20% decrease. Note that a 25% increase followed by a 20% decrease does not return to the original — because the percentage is applied to different bases each time.
Three percentage calculations that look similar but use different formulas
Percentages appear in discounts, test scores, salary negotiations, financial reports, and nutrition labels. Despite looking like a single concept, three structurally distinct questions get labeled as 'percentage problems,' each requiring a different formula. Mixing them up — which is extremely common — produces confidently wrong answers.
Myth vs. reality: percentage increase and percentage decrease are not symmetric
Myth: if something increases 50% and then decreases 50%, you're back where you started. Reality: a $100 item that increases 50% goes to $150. A 50% decrease from $150 gives $75 — not $100. The percentage operations apply to different bases: the increase uses $100 as the base, the decrease uses $150. This asymmetry is why '30% off followed by 30% back on' does not return to the original price. It also explains why recovering from a 50% stock market loss requires a 100% gain — the base shrinks on the way down and must fully double to recover.
Myth vs. reality: 'percentage points' and 'percentage change' mean different things
If a political party's polling share rises from 40% to 45%, it rose by 5 percentage points (absolute difference) but by 12.5% (relative change: 5 ÷ 40). These are genuinely different measures and both appear in news coverage, often without labeling which is which. Percentage point difference is: new − old. Percentage change is: (new − old) ÷ old × 100. This calculator computes percentage change (the relative version). When comparing two numbers described as percentages themselves — poll numbers, interest rates, tax rates — the absolute difference in percentage points is often the more meaningful figure.
The three formulas, stated plainly
Mode 1 (what is X% of Y): result = (X ÷ 100) × Y. Use this for discounts, tips, and proportional allocations. Mode 2 (X is what percent of Y): percent = (X ÷ Y) × 100. Use this for test scores, market share, and composition ratios. Mode 3 (percentage change from A to B): change = ((B − A) ÷ A) × 100. Use this for growth rates, price changes, and year-over-year comparisons — always with A as the original (base) value, not the new one.