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Calculate percent of a number, percentage change, reverse percentages, percent off, fractions, decimals, and grade percentages.
To find a percent of a number, divide the percent by 100 and multiply by the number. For example, 20% of 150 is 0.20 times 150, which equals 30.
This is the calculation to use for tips, tax estimates, commissions, grade weights, discounts, and any situation where a known percentage applies to a known base number.
Subtract the original value from the new value, divide by the original value, and multiply by 100. A positive result is an increase, while a negative result is a decrease.
Use percentage change when one value is clearly the starting point. For example, if a bill rises from 80 to 100, the increase is 25% because the change of 20 is measured against the original 80.
Many percentage mistakes happen because the wrong formula is used for the question. Pick the card that matches the wording of the problem before entering numbers.
| Question wording | Use this calculator card | Example |
|---|---|---|
| What is X% of Y? | Percent of number | 15% of 240 = 36 |
| X is what percent of Y? | Part to whole | 18 out of 60 = 30% |
| How much did this rise or fall? | Percentage change | 50 to 65 = 30% increase |
| What was the original before the discount? | Reverse percent | 80 after 20% off = 100 original |
Percent difference compares two numbers by dividing their absolute difference by their average, then multiplying by 100. It is useful when neither value is clearly the starting value.
For example, comparing two estimates from different contractors may call for percent difference because neither estimate is the original. Comparing last month's bill with this month's bill usually calls for percentage change because last month is the starting value.
To add a percentage, multiply the starting number by 1 plus the percentage as a decimal. To subtract a percentage, multiply by 1 minus the percentage as a decimal. For example, adding 15% to 200 gives 230, while subtracting 15% from 200 gives 170.
This is useful for estimating a markup, a sales tax, a raise, a fee, a reduction, or a price cut. If multiple percentages apply one after another, calculate them in sequence instead of adding the percentages together.
Reverse percentage questions work backward from the final value. If a price is 80 after a 20% discount, the sale price is 80% of the original. Divide 80 by 0.80 to get the original price of 100.
The same idea works for increases. If a value is 120 after a 20% increase, the final value is 120% of the original, so divide 120 by 1.20 to get 100.
Divide the top number by the bottom number, then multiply by 100. The same formula works for grades: points earned divided by points possible equals the score as a decimal, and multiplying by 100 converts it to a percentage.
Common mistakes include using the new value instead of the original value for percentage change, adding stacked discounts together, forgetting to move the decimal two places, or comparing values with different units. A 10-point change is not always a 10% change; the base number matters.
For financial, tax, grade, or business decisions, treat this calculator as a quick arithmetic helper. Confirm policy-specific rounding rules with the organization that owns the final number.
For a fuller walkthrough with examples, read the percentage calculator guide. It explains percent of a number, percentage change, reverse percentages, discounts, grades, fractions, and decimals in plain language.