How to Calculate Percentages
Percentage calculations come up constantly — discounts, tax, tips, grade scores, data analysis, business metrics. The math is simple, but doing it in your head with real numbers is where mistakes happen.
The four percentage problems
Most percentage questions fall into one of four categories:
1. What is X% of Y?
Formula: (X / 100) x Y
Example: What is 15% of 200? → (15 / 100) x 200 = 30
Use for: calculating tips, discounts, tax amounts, commissions.
2. X is what percent of Y?
Formula: (X / Y) x 100
Example: 45 is what percent of 180? → (45 / 180) x 100 = 25%
Use for: test scores, conversion rates, budget breakdowns.
3. Percentage change (increase or decrease)
Formula: ((New - Old) / Old) x 100
Example: Price went from 80 to 100 → ((100 - 80) / 80) x 100 = 25% increase
Use for: price changes, growth rates, performance comparisons.
4. Reverse percentage
Formula: Final / (1 + percentage/100) for increases, or Final / (1 - percentage/100) for decreases
Example: After a 20% increase, the price is 120. Original? → 120 / 1.20 = 100
Use for: finding original prices before tax or markup.
How to use the calculator
- Choose your calculation type — select from the four modes above.
- Enter your numbers — type your values into the fields.
- Read the result — it updates instantly as you type, no submit button needed.
Tips
- Percentage of vs. percentage off — 20% of 50 is 10. But 20% off 50 means 50 - 10 = 40. Make sure you know which one you need.
- Percentage points vs. percent — if a rate goes from 10% to 15%, that is a 5 percentage point increase but a 50% increase in the rate itself. These are different things.
- Chain percentages carefully — a 50% increase followed by a 50% decrease does not get you back to the original. 100 → 150 → 75. Always calculate from the new base.
- Works offline — once the page loads, all calculations run in your browser with no internet needed.
Frequently Asked Questions
How do I calculate what percent one number is of another?
Divide the part by the whole and multiply by 100. For example, 30 is what percent of 120? (30 / 120) x 100 = 25%.
How do I calculate percentage increase or decrease?
Subtract the old value from the new value, divide by the old value, and multiply by 100. If a price goes from 80 to 100, the increase is ((100 - 80) / 80) x 100 = 25%.
What is a reverse percentage?
A reverse percentage finds the original number before a percentage was applied. If an item costs 120 after a 20% markup, the original price is 120 / 1.20 = 100.
Why do I get slightly different results in different calculators?
Rounding differences. Some tools round to 2 decimal places, others to more. The underlying math is the same. For most purposes, 2 decimal places is sufficient.