How to Calculate Percentages: Percent Of, Increase and Decrease
Updated October 22, 2026
Percentages show up in discounts, taxes, grades, interest and statistics. There are only a few formulas to learn, and this guide shows each with an example.
The main formulas
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 15% of 200 = 200 × 15 ÷ 100 = 30 |
| X is what percent of Y? | X ÷ Y × 100 | 30 of 120 = 30 ÷ 120 × 100 = 25% |
| Percentage change | (new − old) ÷ old × 100 | 80 to 100 = 20 ÷ 80 × 100 = +25% |
| Add X% | Y × (1 + X ÷ 100) | $50 plus 13% tax = 50 × 1.13 = $56.50 |
| Subtract X% | Y × (1 − X ÷ 100) | $85 less 20% = 85 × 0.80 = $68 |
Finding the original amount
If you know the final amount after a percentage was applied, divide by the matching factor.
- A jacket costs $68 after 20% off: 68 ÷ 0.80 = $85 before the discount.
- A total of $113 including 13% tax: 113 ÷ 1.13 = $100 before tax.
- A bill of $66 after a 10% increase: 66 ÷ 1.10 = $60 before the increase.
Mental maths shortcuts
- 10%: move the decimal point one place left (10% of 85 is 8.5).
- 1%: move it two places left (1% of 85 is 0.85).
- 5% is half of 10%. 15% is 10% plus 5%. 20% is double 10%.
- 25% is a quarter, 50% is half, and 75% is three quarters.
- To tip 18%: take 10%, then add 5% and three lots of 1%.
Common mistakes
- Taking a percentage of the wrong number. A 20% discount comes off the original price, not the sale price.
- Adding and subtracting percentages directly. A 50% rise followed by a 50% fall leaves you 25% below where you started (100 to 150 to 75).
- Mixing up percent and percentage points. A rate rising from 4% to 5% is one percentage point, but a 25% increase.
- Swapping the order in a percentage change. The old value is always the one you divide by.
In Excel or Google Sheets
To find 15% of A1 use =A1*15%. For the percentage change from A1 to B1 use =(B1-A1)/A1 and format the cell as a percentage. For a share of the total use =A1/SUM(A1:A10).
Frequently asked questions
How do I calculate 20% of a number quickly?
Find 10% by moving the decimal point one place left, then double it. For 20% of 340, 10% is 34, so 20% is 68.
Why is a 50% increase followed by a 50% decrease not back to the start?
The second percentage is taken from a bigger number. 100 up 50% is 150, and 50% of 150 is 75. To undo a 50% increase you need a decrease of about 33.3%.