How it works
- X% of Y: (X ÷ 100) × Y. For tips, discounts, sales tax.
- X is what % of Y: (X ÷ Y) × 100. For ratios, market share, completion %.
- Increase/decrease X by Y%: X + (X × Y/100) and X − (X × Y/100). For salary raises, price changes.
FAQ
How do I work out a 20% tip on $54? Use the "X% of Y" mode: X = 20, Y = 54. Answer: $10.80. Or just multiply 54 × 0.2 in your head — it is the same operation.
My salary went from 50k to 60k. What is the increase? Use the "X is what % of Y" mode with X = 10 (the increase) and Y = 50 (the original). Answer: 20%. Going the other way (60 → 50) would be a 16.67% decrease — percentages are not symmetric.
How is percentage change calculated? ((new − old) ÷ old) × 100. The "old" value is always the denominator. That is why a 50% increase followed by a 50% decrease does not return to the original — you end up at 75% of the start.