Fast everyday math
How to calculate percentages, discounts, and percentage change
Use clear formulas, quick examples, and instant calculations for the percentage questions people ask most often.
How do you calculate X% of Y?
Use this when you need to turn a percentage into a concrete amount.
Why this formula works
Formula: Result = (percent / 100) * value.
We divide the percentage by 100 to turn it into a decimal, then multiply by the base value. That gives the amount represented by the percentage.
Quick steps
- Convert the percentage into a decimal by dividing it by 100.
- Multiply that decimal by the full value.
- Read the answer as the amount represented by the percentage.
Common examples
| Calculation | Answer |
|---|---|
| 15% of 80 | 12 |
| 20% of 50 | 10 |
| 7.5% of 240 | 18 |
What percentage of Y is X?
Use this when you want to compare a part against a whole.
Why this formula works
Formula: Percentage = (part / whole) * 100.
We divide the part by the whole to find the share, then multiply by 100 to express that share as a percentage.
Quick steps
- Divide the part by the reference value.
- Multiply the result by 100.
- Read the answer as the percentage share of the whole.
Common examples
| Comparison | Answer |
|---|---|
| 15 out of 60 | 25% |
| 18 out of 120 | 15% |
| 45 out of 90 | 50% |
How do you calculate percentage change?
Use this when you need to measure an increase, a decrease, or no change.
Why this formula works
Formula: Change = ((next - original) / original) * 100.
We compare the difference between the new value and the original value against the original value itself. A positive answer means an increase, a negative answer means a decrease, and zero means no change.
Quick steps
- Subtract the original value from the new value.
- Divide that difference by the original value.
- Multiply by 100 to convert the change into a percentage.
Common examples
| Change | Answer |
|---|---|
| 50 to 60 | 20% increase |
| 120 to 90 | 25% decrease |
| 80 to 80 | 0% change |
How do you calculate the final price after a discount?
Use this when you need both the savings amount and the final sale price.
Why this formula works
Formula: Final price = original price - ((discount / 100) * original price).
We first calculate the savings amount as a percentage of the original price, then subtract that savings amount from the original price to get the final price.
Quick steps
- Multiply the original price by the discount percentage divided by 100.
- Read that result as the savings amount.
- Subtract the savings from the original price to get the final price.
Common examples
| Discount | Savings | Final price |
|---|---|---|
| 10% off 50 | 5 | 45 |
| 15% off 50 | 7.5 | 42.5 |
| 20% off 50 | 10 | 40 |
Percentage formulas at a glance
These are the exact calculations used by the four percentage tools on this page.
| Question | Formula | Main result |
|---|---|---|
| How do you calculate X% of Y? | (X / 100) * Y | Percentage value |
| What percentage of Y is X? | (X / Y) * 100 | Percent share |
| How do you calculate percentage change? | ((next - original) / original) * 100 | Increase, decrease, or no change |
| How do you calculate the final price after a discount? | price * (1 - discount / 100) | Savings and final price |