Percentage guide
Everyday percentage math without the headache.
Percentages show up in discounts, price changes, taxes, tips, budgets, grades, and comparisons. The trick is knowing which percentage question you are asking.
Percent of a number
Use this for discounts, tips, and tax estimates
To find a percent of a number, convert the percent to a decimal and multiply. For example, 15 percent of 80 is 0.15 times 80. This is the same basic idea behind estimating a tip, a sales tax amount, a discount, or a commission.
When checking a sale price, calculate the discount amount first, then subtract it from the original price. When estimating tax or tip, calculate the percent amount and add it to the starting amount.
Percentage change
Use this for increases and decreases
Percentage change compares the difference between an old number and a new number against the old number. If a price moves from 50 to 60, the increase is 10, and 10 divided by 50 is 20 percent.
The starting number matters. A move from 50 to 60 and a move from 60 to 50 are both 10 points, but they are not the same percentage change because the base number is different.
What percent is one number of another?
Use this for comparisons
To find what percent one number is of another, divide the part by the whole and multiply by 100. If 25 out of 200 people chose an option, 25 divided by 200 is 0.125, which is 12.5 percent.
This kind of percentage is useful for survey results, budgets, completion progress, split costs, and understanding how large one value is compared with another.
Common mistakes
Use the right starting number
Percentage change depends on the original number. If a price rises from 80 to 100, the increase is 25 percent because the 20-point change is compared with 80. If it falls from 100 to 80, the decrease is 20 percent because the same 20-point change is compared with 100.
That is why two changes that look equal in dollars can be different in percentage terms. Always identify the starting number before calculating increase or decrease.
Before relying on the number
Check whether the result should be added or subtracted
A percentage result is often only one step. A sales tax estimate is added to the price. A discount is subtracted. A tip is usually added after the bill subtotal, and a percentage change describes movement between two numbers rather than a final price by itself.
Use the percentage calculator to get the math right, then read the real-world context to decide what the number means.