What Is a Percentage Calculator?
A percentage calculator solves the three percent problems people run into every day: finding a percentage of a number, figuring out what percentage one number is of another, and measuring percent change between two values. "Percent" literally means "per hundred," so 20% is just another way of saying 20 out of every 100, or the fraction 20/100, or the decimal 0.20. Once you see percentages as hundredths, the formulas become easy to remember and check by hand.
This page stacks all three calculators together so you can solve whichever version of the problem you have without hunting for the right tool. Each one updates instantly as you type, and each guards against division by zero so you never get a confusing error or a "NaN" result.
The Three Percentage Formulas
1. What is X% of Y?
Result = (X ÷ 100) × Y
Convert the percentage to a decimal by dividing by 100, then multiply by the number. Worked example: what is 20% of 150? Divide 20 by 100 to get 0.20, then multiply: 0.20 × 150 = 30. This is the formula behind tips, sales tax, and commission calculations.
2. X is what percent of Y?
Result = (X ÷ Y) × 100
Divide the part by the whole, then multiply by 100 to turn the decimal back into a percent. Worked example: 30 is what percent of 150? Divide 30 by 150 to get 0.20, then multiply by 100 = 20%. Use this for test scores, completion rates, and any "what share" question.
3. Percent change from Old to New
Result = ((New − Old) ÷ Old) × 100
Subtract the old value from the new value, divide by the old value, and multiply by 100. A positive result is an increase; a negative result is a decrease. Worked example: a price rises from $50 to $65. Subtract: 65 − 50 = 15. Divide: 15 ÷ 50 = 0.30. Multiply by 100 = 30% increase. If the price had fallen from $65 to $50, the same formula gives −23.08%, a 23.08% decrease.
Real-World Uses
Discounts: A 30% off sale on a $120 jacket means the discount is 0.30 × 120 = $36, so you pay $84. To go straight to the sale price, multiply by (1 − 0.30): $120 × 0.70 = $84.
Tips: A 20% tip on an $84.50 restaurant bill is 0.20 × 84.50 = $16.90. (See our tip calculator for a dedicated tool.)
Grades: Scoring 42 out of 50 on a test is (42 ÷ 50) × 100 = 84%.
Pay raises and price changes: A raise from $22 to $24 per hour is (24 − 22) ÷ 22 × 100 = 9.09% increase. A stock dropping from $200 to $170 is a −15% change, or a 15% decrease.
Investments and savings: If a $5,000 investment grows to $5,750, the return is (5,750 − 5,000) ÷ 5,000 × 100 = 15%. A savings account advertising 4% annual interest on $2,000 earns 0.04 × 2,000 = $80 per year. Percent change is the language of finance — stock moves, inflation, and interest rates are all quoted this way.
Health and fitness: Losing 12 pounds from a starting weight of 180 is (12 ÷ 180) × 100 = 6.67% of body weight. Nutrition labels use percentages of daily values the same way: if a snack has 9g of fat against a 78g daily value, that's (9 ÷ 78) × 100 = 11.5% of your daily fat.
Percentage Shortcuts for Mental Math
A few tricks make everyday percentages fast without a calculator. To find 10% of any number, move the decimal point one place to the left: 10% of $84.50 is $8.45. From there you can build other percentages: 20% is double the 10% figure ($16.90), and 5% is half of it ($4.23). Combining them, 15% is 10% plus 5% ($12.68 on $84.50). For a quick discount estimate, 25% is just one quarter of the price — halve it, then halve it again: 25% of $120 is $60, then $30. And 50% is simply half. These shortcuts won't replace exact math for bills and taxes, but they're perfect for sanity-checking a result: if your 20% tip on an $84.50 bill isn't close to $17, something was typed wrong.
How to Use This Calculator
- Pick the question you're asking. "What is X% of Y?", "X is what percent of Y?", or "percent change from old to new."
- Type your numbers. Each calculator updates the instant you type — no button to press.
- Read the result. Percent change also tells you whether the result is an increase or a decrease.
- Watch for the dash. If a calculation would divide by zero, the result shows "—" instead of an error.
Frequently Asked Questions
How do I find X percent of a number?
Divide the percentage by 100 to get a decimal, then multiply by the number. For example, 20% of 150 is 0.20 × 150 = 30.
How do I figure out what percent one number is of another?
Divide the part by the whole and multiply by 100. For example, 30 is what percent of 150? (30 ÷ 150) × 100 = 20%.
What happens if I divide by zero in a percentage calculation?
Division by zero is undefined in mathematics, so the calculator shows a dash instead of a result. If you see this, check that the "whole" or "old value" field is not zero.
What is the difference between percent change and percentage points?
They measure different things. Going from 50% to 60% is a 10 percentage point change, but the percent change is (60 − 50) ÷ 50 × 100 = 20%. Percent change is always relative to the starting value.
How do discounts work with percentages?
Multiply the original price by (1 − discount). A 25% discount on $80 is $80 × (1 − 0.25) = $80 × 0.75 = $60. The discount itself is $80 × 0.25 = $20.