Formulas
| To find | Formula | Example |
|---|---|---|
| B% of A | A × B ÷ 100 | 15% of 80 = 12 |
| A as a % of B | A ÷ B × 100 | 18 is 75% of 24 |
| Percent change | (new − old) ÷ old × 100 | 80 → 100 is a 25% increase |
| Add B% | A × (1 + B ÷ 100) | $40 + 8.5% tax = $43.40 |
| Subtract B% | A × (1 − B ÷ 100) | $60 − 25% = $45 |
Common mistakes
- Up then down isn't back to the start. 100 up 10% is 110, and 110 down 10% is 99, because the second step works on the larger number.
- Stacked discounts multiply. 20% off and then an extra 10% off is 1 − 0.8 × 0.9 = 28% off, not 30%.
- Percent vs. percentage points. A rate going from 2% to 3% rose by 1 percentage point, which is a 50% increase.
FAQ
How do I work out a tip?
Use “Add a percentage”: enter the bill and the tip rate (for example 18 or 20) to see the total, with the tip amount in brackets.
How do I find the discount percentage?
Put the original price and the sale price into “Percent increase or decrease”. A drop from $45 to $36 is a 20% decrease, so it's 20% off.
Are my numbers saved?
No. Everything is calculated in your browser and nothing is sent to a server.