Percentage Calculator — quick guide
1) Core idea
Percent means “per hundred.” Use it to answer: X% of Y, X is what % of Y, X is Y% of what, percentage difference, and percentage change.
2) How this tool works
- X% of Y = (X/100) × Y
- X is what % of Y = (X/Y) × 100
- X is Y% of what = X ÷ (Y/100)
- Difference% = |A − B| ÷ ((A + B)/2) × 100
- Change% = (Final − Initial)/|Initial| × 100
3) Sanity checks
- 50% of 200 = 100
- 200 is 100% of 200; 50 is 25% of 200
- From 100 to 150 is +50%; 150 to 100 is −33.33%
4) Shortcuts that help
- 10%: move decimal one left; 5%: half of 10%; 1%: divide by 100
- Percent → decimal: ÷ 100; decimal → percent: × 100
- Use averages for difference% to avoid “which is the base?” debates
5) Common pitfalls
- Division by zero when Y or Initial is 0
- Confusing percentage points (e.g., 10%→12% is +2 pp, not +2%)
- Rounding too early—this tool rounds for display only
6) Micro-examples
- 25% of 200 = 50
- 50 is what % of 200? 25%
- Change from 120→96 = −20%
7) Mini-FAQ
- Over 100%? Yes, e.g., 150 is 150% of 100.
- Negative values? Supported; sign carries through formulas.
- Precision? Results use JS double precision; rounding is display-only.
8) Action tip
Use “what percent” to compare progress or completion rates; then apply “change%” to track trends over time.