How to Calculate a Percentage (3 Ways, With Examples)

What a percentage really means

A percentage is just a fraction out of 100. "25%" means 25 out of every 100. Percentages show up everywhere — discounts, sales tax, exam scores, interest rates, and price changes — but they trip people up because the answer depends on what you treat as the base. Learn three core calculations and you'll handle almost every everyday case.

1. Finding X% of a number

This is the most common one — a discount, a tip, or a tax amount.

Result = number × X ÷ 100

Example: 18% of 1,200 = 1,200 × 18 ÷ 100 = 216.

A quick shortcut: 10% of any number is just that number with the decimal moved one place left (10% of 1,200 = 120), and you can scale from there — 5% is half of that, 20% is double.

2. What percent one number is of another

Use this for scores, completion rates, or "what share is this of the total?"

Percent = part ÷ whole × 100

Example: you scored 45 out of 180. That's 45 ÷ 180 × 100 = 25%.

3. Percentage increase or decrease

This is where most mistakes happen, because the base is the old value, not the new one.

Change % = (new − old) ÷ old × 100

Example: a price rises from 200 to 250. Change = (250 − 200) ÷ 200 × 100 = +25%. If it later falls from 250 back to 200, that's (200 − 250) ÷ 250 × 100 = −20% — not −25%, because the base changed.

Common mistakes to avoid

• Wrong base for change. Always divide by the original value. A 25% rise followed by a 25% fall does not return you to the start.
• Percentage points vs percent. Moving from 20% to 25% is "5 percentage points," but as a relative change it's a 25% increase. Keep the two ideas separate.
• Stacking discounts. A 20% discount and then a further 10% off is not 30% off — it's 0.8 × 0.9 = 0.72, i.e. 28% off, because the second discount applies to an already-reduced price.
• Reverse percentages. If a price includes 10% tax, you don't subtract 10% to get the pre-tax price — you divide by 1.10.

Where you'll use this

• Shopping: work out the real price after a discount, or compare "20% off" against "buy one get one."
• Tax: add a VAT/GST rate to a net price, or back it out of a gross total.
• Grades and KPIs: turn "correct out of total" into a clean percentage.
• Investing: measure how far a price moved versus your entry point.

Conclusion

Three formulas cover the vast majority of percentage questions: a percent of a number, what percent one number is of another, and percentage change. Try the Percentage Calculator above — pick a mode, enter two numbers, and get the answer instantly. For money problems, see the Interest Calculator and Loan Calculator.