Three Answers From Two Numbers
Enter X and Y once, and the calculator solves all three classic percentage questions simultaneously — no mode switching:
| Question | Formula | X=20, Y=80 |
|---|---|---|
| What is X% of Y? | (X ÷ 100) × Y | 16 |
| X is what percent of Y? | (X ÷ Y) × 100 | 25% |
| Percent change from X to Y | ((Y − X) ÷ X) × 100 | +300% |
Results appear as you type, rounded to two decimals, with increases shown as positive and decreases as negative.
Worked Example: A Price Rise
Your rent goes from $1,400 to $1,540. Enter X = 1400, Y = 1540: the change row shows +10% ((1540 − 1400) ÷ 1400 × 100). Cross-check with the first row: 10% of 1400 is 140 — exactly the increase. And the middle row tells you the old rent is 90.91% of the new one. Three views of the same numbers, and each answers a different real question.
The Asymmetry That Trips Everyone Up
Percent change is always measured against the starting value, which makes it direction-dependent. From 80 to 100 is +25%, but from 100 back to 80 is −20% — same distance, different percentages, because the base changed. The extreme case: a stock that drops 50% needs a +100% gain to recover. Whenever a headline says "up X% then down X%", the end point is below the start.
Everyday Shortcuts
- Tips: use row one — 18% of a $64 bill: enter 18 and 64, read $11.52.
- 10% mentally: shift the decimal (10% of 84 = 8.4); 5% is half that; 20% is double.
- The swap trick: X% of Y equals Y% of X. 4% of 75 feels awkward; 75% of 4 is obviously 3.
- Discount stacking: −20% then −10% is not −30% — it's ×0.8 ×0.9 = ×0.72, i.e. −28%.
- Points vs. percent: a rate going from 4% to 5% rises 1 percentage point but 25 percent.
All calculation happens live in your browser — nothing is sent to a server.