Use percentages without overthinking them.

Last updated: August 27, 2026

Percent of a number

Use percent of a number when the question sounds like "What is 15 percent of 80?" Convert the percent to a decimal or divide by 100, then multiply by the number.

This is useful for sales tax, discounts, tips, commissions, fees, and budget categories.

Discount example

If a $60 item is 25% off, the discount is $60 x 25 / 100, or $15. The sale price is $60 - $15, or $45. If sales tax applies after the discount, calculate tax from the discounted price, not the original price, unless local rules or store policy say otherwise.

Percentage change

Use percentage change when a number moves from an old value to a new value. Subtract the old number from the new number, divide by the old number, and multiply by 100.

A positive result is an increase. A negative result is a decrease. This is useful for price changes, raises, rent changes, traffic changes, and before-and-after comparisons.

What percent one number is of another

Use this when the question sounds like "24 is what percent of 80?" Divide the part by the whole and multiply by 100.

This pattern is useful for test scores, progress toward a goal, expense categories, or comparing a partial amount against a total.

Common mistakes

1. Using the new number as the denominator when calculating percentage change.
2. Forgetting that a 50% decrease followed by a 50% increase does not return to the original number.
3. Adding percentages from different bases as if they share the same denominator.
4. Rounding too early and creating a noticeably different final answer.

The percentage calculator helps avoid these mistakes by separating the three common percentage questions into clear inputs.