How to calculate percentages: the 4 everyday cases
Almost every everyday percentage question comes down to four cases. Here is each formula with a real example.
1. X% of a number
X% of Y = Y × X ÷ 100
20% of 150 = 150 × 20 ÷ 100 = 30. Tip: 10% means divide by 10, 1% means divide by 100.
2. What percent is one value of another?
Percent = X ÷ Y × 100
You got 34 out of 40: 34 ÷ 40 × 100 = 85%.
3. Percentage increase or decrease
Change = (new − old) ÷ old × 100
Rent goes from 800 to 840: (840 − 800) ÷ 800 × 100 = +5%. A price drops from 50 to 40: −20%.
Percentages don't cancel out: a 20% rise followed by a 20% fall does not bring you back. 100 → 120 → 96.
4. Finding the original value
After a 5% rise a price is 840. The original was 840 ÷ 1.05 = 800 (not 840 − 5%). It is the same idea as removing VAT.
Summary
| I want… | Formula | Example |
|---|---|---|
| X% of Y | Y × X ÷ 100 | 20% of 150 = 30 |
| X as % of Y | X ÷ Y × 100 | 34 of 40 = 85% |
| Change | (B − A) ÷ A × 100 | 800 → 840 = +5% |
| Value before a rise | B ÷ (1 + X/100) | 840 ÷ 1.05 = 800 |
Tools for this
The percentage calculator does all of these live. For sales, the discount calculator also handles stacked discounts (30% then 20% off is 44% off, not 50%). For proportions, the rule of three is often quicker.
Frequently asked questions
How do I work out 20% of a price?
Multiply the price by 20 and divide by 100. 20% of 150 is 30.
How do I calculate a percentage increase?
(new value − old value) ÷ old value × 100. From 800 to 840 is a 5% increase.
Does a 20% decrease cancel a 20% increase?
No. 100 plus 20% is 120, and 120 minus 20% is 96.
This information is for guidance only and does not replace professional advice.