How to Calculate a Percentage (4 Everyday Formulas)
Four formulas cover almost every percentage question: a percent of a number, what percent one number is of another, percent change, and discounts.
The short version
- X% of Y = Y × X ÷ 100. 15% of $80 is $12.
- X is what percent of Y = X ÷ Y × 100. 18 of 24 is 75%.
- Percent change = (new − old) ÷ old × 100. $80 to $92 is a 15% increase.
- Sale price = price × (1 − discount). 25% off $120 is $90.
- A 20% rise followed by a 20% drop leaves you 4% lower, not back where you started.
Percent means “per hundred,” so every percentage is just a fraction with 100 on the bottom. Whether you’re working out a tip, a raise, a discount, or a change in price, the question almost always fits one of four formulas.
1. What is X% of a number?
Multiply the number by the percentage, then divide by 100. 15% of $80 is $80 × 15 ÷ 100 = $12. Moving the decimal works too: 15% is 0.15, and 0.15 × $80 = $12.
Mental math for tips
Find 10% by moving the decimal one place left — 10% of $64 is $6.40. Double it for 20% ($12.80), or add half of it for 15% ($9.60).
2. X is what percent of Y?
Divide the part by the whole, then multiply by 100. If you answered 18 of 24 questions correctly, that’s 18 ÷ 24 = 0.75, or 75%. The same formula tells you what share of your income goes to rent or how much of a paycheck goes to taxes.
3. Percent change between two numbers
Subtract the old value from the new one, divide by the old value, and multiply by 100. A bill that rises from $80 to $92 went up ($92 − $80) ÷ $80 = 15%. A negative result is a decrease.
Always divide by the starting value
Going from $92 back down to $80 is a 13% decrease, not 15%, because the starting point is now $92. That’s also why a 20% gain followed by a 20% loss leaves you at 96% of where you began.
Percent change is how raises, price increases, and inflation are measured. A raise only increases your buying power if its percent change beats the inflation rate over the same period.
4. Discounts and sale prices
Multiply the price by (1 − the discount). 25% off $120 is $120 × 0.75 = $90, a $30 saving. Stacked discounts multiply rather than add: 20% off and then an extra 10% off is $100 × 0.80 × 0.90 = $72 — a 28% total discount, not 30%.
Adding a percentage works the same way in reverse. A $90 purchase with 8% sales tax costs $90 × 1.08 = $97.20 at the register.
The four formulas at a glance
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 15% of $80 = $12 |
| X is what % of Y? | X ÷ Y × 100 | 18 of 24 = 75% |
| Percent change | (new − old) ÷ old × 100 | $80 → $92 = +15% |
| Price after discount | price × (1 − discount) | 25% off $120 = $90 |
Check your math
Enter your numbers to answer all four questions at once, with the formula shown under each result.
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Frequently Asked Questions
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. 30% of 250 is 250 × 30 ÷ 100 = 75.
How do I calculate percent increase?
Subtract the original value from the new value, divide by the original, and multiply by 100. From 50 to 65 is (65 − 50) ÷ 50 × 100 = 30%.
What is the difference between percent and percentage points?
Percentage points measure the simple difference between two percentages; percent measures the relative change. A rate rising from 4% to 5% is up 1 percentage point, but 25 percent.
How do I find the original price before a discount?
Divide the sale price by (1 − the discount). A $60 item that was 25% off originally cost $60 ÷ 0.75 = $80.
This article is for general education and is not financial, tax, or legal advice. Figures reflect published IRS and SSA amounts as of the date above; verify current limits with the linked sources or a qualified professional before acting.
About the author
David Miles is the founder of FigureMoney and builds independent, source-backed personal-finance tools across the Modern Site Builders network. Every calculator and guide cites the IRS, SSA, or primary research behind its numbers.
Calculators in this guide
Percentage Calculator
Four quick percentage calculators: what is X% of Y, X is what percent of Y, percent change between two numbers, and percent off a price.
Tip Calculator
Calculate the tip and total for any bill at 15%, 18%, 20% or a custom percentage, tip on the pre-tax amount, and split the check — with optional rounding.
Sales Tax Calculator
Enter a purchase amount and your state to see the sales tax and total, using 2026 average combined state and local rates.
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