Three everyday percentage calculations
- X% of a number: multiply the number by the percentage and divide by 100. 15% of 200 = 30.
- What percent: divide the part by the whole and multiply by 100. 45 out of 50 = 90%.
- Percentage change: subtract the old value from the new, divide by the old value, and multiply by 100. From 80 to 100 is a 25% increase.
Worked examples
| Question | Working | Answer |
|---|---|---|
| 15% of 2,500 | 2,500 × 15 ÷ 100 | 375 |
| 18 out of 24 as a percentage | 18 ÷ 24 × 100 | 75% |
| Salary from 4,000 to 4,600 | (4,600 − 4,000) ÷ 4,000 × 100 | +15% |
| Price from 1,200 to 900 | (900 − 1,200) ÷ 1,200 × 100 | −25% |
Reverse percentages: finding the original number
If a price is 120 after a 20% increase, the original is 120 ÷ 1.20 = 100 — not 120 − 20% = 96. In general: original = new value ÷ (1 + increase), or new value ÷ (1 − decrease) after a decrease. The same idea finds a price before VAT: a total of 115 with 15% VAT is 115 ÷ 1.15 = 100 before tax.
Frequently Asked Questions
Why is a 25% increase followed by a 25% decrease not back to the start?
Because each percentage applies to a different base. 100 increased by 25% is 125, and 125 decreased by 25% is 93.75, not 100.
What is the difference between a percentage and percentage points?
If a rate moves from 10% to 12%, it rose by 2 percentage points, but by 20% relative to its old value. The calculator's change mode gives the relative (percentage) change.
Can a percentage be more than 100%?
Yes. If the part is bigger than the whole, or a value more than doubles, the percentage passes 100%. For example, a rise from 50 to 150 is a 200% increase.
How do I add a percentage to a number?
Multiply by (1 + the percentage). 250 plus 12% = 250 × 1.12 = 280. The calculator shows this as “number + percentage” under the result.