Three everyday percent tasks
The percent calculations people mix up, split by purpose:
- Discount — enter a price and discount % to get the sale price and amount saved.
- Ratio — part ÷ whole to find "what percent" (scores, shares).
- Change — old vs new value to find the percent increase or decrease.
Example: 30% off $80
| Value | |
|---|---|
| Original | $80 |
| Discount (30%) | −$24 |
| You pay | $56 |
Where people get it wrong
- Increase and decrease aren't symmetric. 100 up 20% is 120, but 120 down 20% is 96 — not back to the start.
- "% more" vs "percentage points" differ. Going from 3% to 5% is a 2-point rise, but about a 67% increase.
- Stacked discounts: 30% off then an extra 10% is not 40% off — it's an effective 37% (0.7 × 0.9 = 0.63).
This is for quick percent math. For loan repayment, compound interest, or FX use the Finance Calculator; for body mass index, the BMI Calculator.