Source-linked deep guide
Percentage Calculator: compare any part to the whole
A percentage tells you how much of something you have when the full amount is scaled to 100. The percentage calculator uses that idea for discounts, grades, tips, tax, progress, and percentage change.
Picture a 100-space scoreboard. If 30 spaces are filled, the board is 30% full. The number 30 is the part. The full board is the whole.
Core Formula
Divide the part by the whole first. Then multiply by 100 so the answer is written as a percent.
| Term | Meaning | Simple example |
|---|---|---|
| Part | The amount you are comparing | $30 |
| Whole | The full amount or starting base | $150 |
| × 100 | Converts the ratio to a percent | 0.20 becomes 20% |
Step-by-step example
Find what percentage $30 is of $150.
- Start with the formula.
- Put the part over the whole: 30 ÷ 150.
- Divide: 30 ÷ 150 = 0.20.
- Multiply by 100: 0.20 × 100 = 20.
- So, $30 is 20% of $150.
Percentage change formula
Use this when a value grows or drops from a starting amount.
Real-life examples
School score
You answer 18 questions correctly out of 20.
18 ÷ 20 × 100 = 90%
Your score is 90%.
Shopping discount
A $80 jacket is 25% off.
80 × 0.25 = 20
The discount is $20, so the new price is $60.
Common mistakes and edge cases
- Do not forget to divide by 100. 20% is 0.20, not 20.
- Do not swap part and whole. 30 out of 150 is 20%, but 150 out of 30 is 500%.
- Do not divide by zero. If the whole is 0, the calculator shows a warning.
- Percentages over 100% are valid. They mean the part is bigger than the whole.
Sources and accuracy notes
The formula follows the standard definition of percent as a value measured per hundred. See Cuemath: Percentages and Percentage overview.
CalculTools blocks division by zero and rounds long decimals for readability. The shown formula stays visible so you can check the answer by hand.