how to calculate a percentage

Percentages are everywhere: sale prices, interest rates, exam scores, nutrition labels, and business reports. The good news is that once you understand one core formula, the rest becomes straightforward.

Quick percentage formula

Use this when you already know the part and the whole and need to convert to a percent. For example, if you got 45 questions correct out of 60, your score is:

(45 ÷ 60) × 100 = 75%

How to find X% of a number

When the question is “What is 15% of 80?”, use this version:

Example: What is 15% of 80?

• Convert 15% to decimal: 15 ÷ 100 = 0.15
• Multiply: 0.15 × 80 = 12

Answer: 15% of 80 is 12.

How to calculate what percent one number is of another

This is common for tests, conversion rates, and completion percentages.

Example: 18 is what percent of 45?

• 18 ÷ 45 = 0.4
• 0.4 × 100 = 40%

Answer: 18 is 40% of 45.

How to calculate percentage increase or decrease

When values change over time, use the original value as the base.

Example: Increase

A price rises from 50 to 65.

• Difference = 65 − 50 = 15
• 15 ÷ 50 = 0.3
• 0.3 × 100 = 30%

Result: 30% increase.

Example: Decrease

Traffic drops from 2,000 visits to 1,500 visits.

• Difference = 1,500 − 2,000 = -500
• -500 ÷ 2,000 = -0.25
• -0.25 × 100 = -25%

Result: 25% decrease.

Everyday percentage shortcuts

• 10% is easy: move the decimal one place left. (10% of 250 = 25)
• 5% is half of 10%. (5% of 250 = 12.5)
• 1% is moving decimal two places left. (1% of 250 = 2.5)
• 15% can be 10% + 5%.
• 25% is one-fourth of a number.
• 50% is half.

Common use cases

1) Discounts

If a $120 item is 25% off:

• Discount = 25% of 120 = 30
• Sale price = 120 − 30 = 90

2) Tips

If your bill is $48 and you want to tip 18%:

• Tip = 0.18 × 48 = 8.64
• Total = 48 + 8.64 = 56.64

3) Grades

If you score 42 out of 50:

• 42 ÷ 50 = 0.84
• 0.84 × 100 = 84%

Percentage points vs percent change

These are not the same:

• If interest goes from 4% to 6%, that is a 2 percentage point increase.
• Relative to 4%, moving to 6% is a 50% increase.

Use percentage points when comparing two percentages directly. Use percent change when measuring relative growth or decline.

Mistakes to avoid

• Using the wrong base (whole) in the formula.
• Forgetting to divide by 100 when converting percent to decimal.
• Confusing percentage points with percent change.
• Rounding too early in multi-step calculations.
• Trying to compute percent change when old value is zero (undefined in standard form).

Final takeaway

If you remember only one thing, remember this: percentages are just ratios out of 100. Convert to decimal, multiply for “of,” and divide first when finding “what percent.” Use the calculator above to check your work and build intuition quickly.