Percentage calculator for tips, test scores and percent change
Five calculators in one: percent of a number, what percent X is of Y, percent change, adding or subtracting a percent, and finding the whole from a part.
The short answer
A percentage is part ÷ whole × 100. If you scored 42 out of 50, that is 84%. To find a percent of a number, multiply by the percent and divide by 100: 20% of 85 is 17. Percent change is (new − old) ÷ old × 100, so going from 64 to 80 is a 25% increase.
How it is calculated
What % X is of Y = X ÷ Y × 100
X% of Y = Y × X ÷ 100
Percent change = (new − old) ÷ old × 100
Add X% = Y × (1 + X ÷ 100); subtract X% = Y × (1 − X ÷ 100)
Whole from a part = part ÷ percent × 100
Worked example
Dinner comes to $64.50 and you want to leave an 18% tip. Later, your kid brings home a quiz marked 42 out of 50.
- Tip: $64.50 × 18 ÷ 100 = $11.61
- Bill with tip: $64.50 × 1.18 = $76.11
- Quiz score: 42 ÷ 50 × 100 = 84%
Leave $11.61 for a total of $76.11; the quiz score is 84%.
Quick reference table
| Percent | of 20 | of 50 | of 85 | of 100 | of 250 |
|---|---|---|---|---|---|
| 5% | 1 | 2.5 | 4.25 | 5 | 12.5 |
| 10% | 2 | 5 | 8.5 | 10 | 25 |
| 15% | 3 | 7.5 | 12.75 | 15 | 37.5 |
| 20% | 4 | 10 | 17 | 20 | 50 |
| 25% | 5 | 12.5 | 21.25 | 25 | 62.5 |
| 30% | 6 | 15 | 25.5 | 30 | 75 |
| 50% | 10 | 25 | 42.5 | 50 | 125 |
| 75% | 15 | 37.5 | 63.75 | 75 | 187.5 |
Special cases and common mistakes
- Up 10% then down 10% is not a wash. 100 becomes 110, then 99, because the cut is taken from the bigger number.
- Percentage points are not percent. A rate going from 40% to 50% rose 10 percentage points, which is a 25% increase.
- Percent change depends on the starting point. 80 to 100 is +25%, but 100 to 80 is −20%. Always divide by the old value.
- Quick tip math. 10% is the bill with the decimal moved one place left. Double it for 20%, or add half of it for 15%. On $64.50: 10% is $6.45, 20% is $12.90.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percent and divide by 100. 20% of 85 is 17, and 15% of 240 is 36.
How do I calculate my test score percentage?
Divide the points you got by the points possible, then multiply by 100. 42 out of 50 is 84%, and 27 out of 30 is 90%.
How do I calculate percent increase between two numbers?
Subtract the old number from the new one, divide by the old number and multiply by 100. From 64 to 80: 16 ÷ 64 × 100 = 25%.
How much should I tip on $80?
15% is $12.00, 18% is $14.40, and 20% is $16.00. Use the add-a-percent mode to see the bill with the tip included.
45 is 60% of what number?
75. Divide the part by the percent and multiply by 100: 45 ÷ 60 × 100.
How do I calculate a percentage in Excel?
Type =part/whole, for example =42/50, and format the cell as a percentage. For percent change use =(new-old)/old.