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About this percentage calculator
Percentages show up everywhere in school and daily American life: a 20% tip at a restaurant, a 15% grade penalty, a 30% off sale, or the percent your savings account grew. Yet the same three questions cover nearly every case, and this calculator answers all of them.
Use the first mode to take a percent of a number — tips, discounts, tax. The second finds what share one number is of another — a test score of 43 out of 50, or what portion of your income goes to rent. The third measures growth between two values — price changes, grade improvements, population statistics.
Everything computes as you type, with the exact wording of each answer shown under the result so you can double-check that you picked the right mode.
The three percentage formulas
Every percentage problem is one of these three. Memorize them and you can solve any percent question without a calculator:
1) Percent of a number: multiply the number by the percent and divide by 100. 15% of 200 = 200 × 15 ÷ 100 = 30.
2) One number as a percent of another: divide the part by the whole and multiply by 100. 30 of 200 = 30 ÷ 200 × 100 = 15%.
3) Percent change: subtract the old value from the new one, divide by the old value's absolute size, multiply by 100. From 40 to 50 = (50 − 40) ÷ 40 × 100 = +25%.
X is what % of Y = X ÷ Y × 100
% change = (Y − X) ÷ |X| × 100
Handy percentage references
| Percent | of 100 | of 200 | of 500 |
|---|---|---|---|
| 5% | 5 | 10 | 25 |
| 10% | 10 | 20 | 50 |
| 15% | 15 | 30 | 75 |
| 18% | 18 | 36 | 90 |
| 20% | 20 | 40 | 100 |
| 25% | 25 | 50 | 125 |
| 30% | 30 | 60 | 150 |
| 50% | 50 | 100 | 250 |
How to calculate percentages
- Pick the question type: Choose whether you need a percent of a number, a share of a total, or the change between two values.
- Enter both numbers: The field labels update to match your chosen mode, so you always know which number goes where.
- Read the answer instantly: The result updates as you type, with a plain-English sentence confirming exactly what was calculated.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percent, then divide by 100. For example, 20% of 150 is 150 × 20 ÷ 100 = 30. A quick mental shortcut: 10% is the number with the decimal moved one place left, so 10% of 150 is 15, and 20% is double that.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100. If you scored 43 points out of 50, the math is 43 ÷ 50 × 100 = 86%. This is exactly how teachers convert rubric points into a percentage grade.
How do I calculate percent increase or decrease?
Subtract the old value from the new one, divide by the absolute old value, and multiply by 100. A price moving from $80 to $100 rose by (100 − 80) ÷ 80 = +25%. Note that a 25% increase followed by a 25% decrease does not return to the start — decreases are computed on a smaller base.
Why does percent change divide by the old value?
Percent change always expresses the difference relative to where you started. Going from 40 to 50 adds 10 units on a base of 40, which is 25%. The calculator uses the absolute value of the base so that negative starting numbers (like a debt) still produce a meaningful direction.