Percentage Calculator
Five modes: percent-of, is-what-percent, percent-change, increased-by, decreased-by.
published
- [FREE]
- [NO_SIGNUP]
- [NO_UPLOAD]
A percentage calculator handles the five most common percentage questions: what is X% of Y, X is what % of Y, percent change from X to Y, X increased by Y%, X decreased by Y%. This tool runs entirely in your browser.
The five modes
| Mode | Formula | Example |
|---|---|---|
| What is X% of Y? | (X / 100) × Y | 25% of 200 = 50 |
| X is what % of Y? | (X / Y) × 100 | 50 is what % of 200? 25% |
| Percent change | ((Y − X) / X) × 100 | 100 → 120 = +20% |
| Increased by | X × (1 + Y/100) | 100 + 25% = 125 |
| Decreased by | X × (1 − Y/100) | 100 − 25% = 75 |
Common use cases
- Tip / sales tax: “what is X% of Y” — 18% tip on a $42 meal.
- Test scores: “X is what % of Y” — 38 out of 50 questions correct.
- Growth rates: “percent change” — last month 100 sales, this month 137.
- Inflation-adjusted price: “increased by” — $1000 + 8% inflation.
- Sale price: “decreased by” — $80 shirt at 30% off.
Privacy and security
A static HTML page with a small JavaScript bundle. All arithmetic runs in your browser tab. No upload, no analytics on input. The Network tab in DevTools confirms it.
Related tools
- CIDR Calculator — for the network-engineering equivalent.
- Hash Generator — different kind of math, same privacy posture.
Frequently asked questions
What does "What is X% of Y?" mean?
Multiply X by Y then divide by 100. 25% of 200 is 50. Used for tips, sales tax, discounts, and any fraction of a known total.
What does "X is what % of Y?" mean?
Divide X by Y then multiply by 100. 50 is what % of 200? Answer: 25%. Used when you want to express a part as a fraction of a known whole.
What does "From X to Y, % change" mean?
Subtract X from Y, divide by X, multiply by 100. From 100 to 120 is a +20% change. From 100 to 80 is -20%. Use for growth rates, deltas, and before/after comparisons.
Why does "From 0 to Y" give no result?
Percent change is undefined when the starting value is zero — there is no fraction of zero to express. The tool returns em-dash for that case.
Is this calculator running in my browser?
Yes. Pure JS, no library, no backend. Open DevTools → Network and you will see zero requests when you compute.
How precise is the result?
Computed in JavaScript double-precision floats then rounded to 4 decimal places for display. Trailing zeros are trimmed. For exact decimal arithmetic (financial, legal), use a decimal library in your code; this tool is for everyday math.
Can I share my calculation?
Yes. Copy share link encodes the mode and both inputs into the URL fragment. Recipients see the same view.
Is the percentage calculator really free?
Yes. No signup, no account, no ads. Source on the project repository.