Percentage Calculator
Work out a percentage of a number, turn a part into a percent of a whole, or measure how much a value has gone up or down — all in one place.
20% of 150
30
Formula
X% of Y = (X ÷ 100) × Y · % change = ((New − Old) ÷ |Old|) × 100
Worked example
What is 20% of 150?
- Turn 20% into a decimal: 20 ÷ 100 = 0.20
- Multiply by the whole: 0.20 × 150 = 30
Answer: 30
How it works
Percent literally means "per hundred", so any percentage is just a fraction with 100 on the bottom. Converting it to a decimal first keeps the arithmetic simple on paper or in your head.
For a percent change, the old value is the reference point you compare against. Subtract old from new, divide by the old value, then multiply by 100 to get the change as a percentage.
If you only need a quick increase or decrease, multiply the original by (1 + percent/100) for a markup or (1 − percent/100) for a markdown.
Common mistakes
- Adding two percentages from different totals — they only add up if both are calculated from the same base.
- Dividing by the new value instead of the old value when measuring percent change.
- Forgetting to convert the percent to a decimal before multiplying (using 20 instead of 0.20).
FAQ
- How do I find what percent one number is of another?
- Divide the part by the whole and multiply by 100. Example: 30 of 150 is 30 ÷ 150 × 100 = 20%.
- Can the percent change be negative?
- Yes. A negative result means the new value is smaller than the old one — that's a decrease.
- What does 100% mean?
- 100% of a number is the number itself. 200% is double, and 50% is half.
- Can I use decimals like 12.5%?
- Yes. Decimal percentages work the same way — just divide by 100 (12.5 ÷ 100 = 0.125).
- Why does percent change need an absolute value?
- Using the absolute value of the old number keeps the sign of the result tied to the direction of change rather than the sign of the starting point.