How to Calculate Percentages: A Complete Guide with Examples
Percentages come up everywhere — shopping discounts, tip calculations, exam grades, investment returns, statistics. Here's how to handle every common type of percentage calculation.
Type 1: What is X% of Y?
This is the most common percentage question. The formula is simple:
Result = Y × (X / 100)
Example: What is 15% of 200? → 200 × 0.15 = 30
Use this for: tips, discounts, tax calculations, commissions.
Type 2: X is What Percent of Y?
Percentage = (X / Y) × 100
Example: 45 is what percent of 180? → (45 / 180) × 100 = 25%
Use this for: test scores, completion rates, proportions.
Type 3: Percentage Change
Change = [(New − Old) / |Old|] × 100
Example: Price went from $80 to $100 → [(100 − 80) / 80] × 100 = 25% increase
Use this for: price changes, growth rates, year-over-year comparisons.
Quick Mental Math Tricks
- 10% of anything: Move the decimal one place left. 10% of 250 = 25.
- 5%: Find 10%, then halve it. 5% of 250 = 12.50.
- 15%: Find 10% + 5%. 15% of 250 = 25 + 12.50 = 37.50.
- 25%: Divide by 4. 25% of 250 = 62.50.
- X% of Y = Y% of X. So 8% of 50 = 50% of 8 = 4. Use whichever is easier.
Practice with our percentage calculator to check your work instantly.