how to calculate the percentage of a number

What does “percentage of a number” mean?

A percentage is simply a part out of 100. So when you ask for 20% of 50, you are asking, “What is 20 parts out of 100, scaled to 50?”

The core formula is:

Percentage of a number = (Percentage ÷ 100) × Number

Step-by-step method

1. Write the percentage as a fraction of 100 (or convert it to a decimal).
2. Multiply by the number you want a percentage of.
3. Simplify the result.

Example 1: Find 25% of 80

25% = 25 ÷ 100 = 0.25
0.25 × 80 = 20

Answer: 25% of 80 is 20.

Example 2: Find 12% of 350

12% = 0.12
0.12 × 350 = 42

Answer: 12% of 350 is 42.

Quick mental math tricks for percentages

• 10% is easy: move decimal one place left.
  10% of 470 = 47.
• 5% is half of 10%.
  5% of 470 = 23.5.
• 1% is moving decimal two places left.
  1% of 470 = 4.7.
• 15% = 10% + 5%.
  15% of 470 = 47 + 23.5 = 70.5.

Real-life uses

1) Shopping discounts

If a $120 jacket is on sale for 30% off:

Discount = 0.30 × 120 = 36. Final price = 120 - 36 = $84.

2) Tips at restaurants

Want to leave 18% tip on a $65 bill?

Tip = 0.18 × 65 = $11.70.

3) Interest and finance

If you earn 4% annual interest on $2,500:

Interest = 0.04 × 2,500 = $100.

Common mistakes to avoid

• Forgetting to divide the percent by 100.
• Confusing “% of” with “percent increase/decrease.”
• Subtracting the percentage directly from the number without calculating the percentage amount first.
• Typing commas incorrectly in calculators (use standard number format when possible).

Related percentage formulas

How to find what percent one number is of another

Percent = (Part ÷ Whole) × 100

Example: What percent is 30 of 120?

(30 ÷ 120) × 100 = 25%.

How to calculate percentage increase

Percentage increase = ((New - Old) ÷ Old) × 100

If price rises from 80 to 100:

((100 - 80) ÷ 80) × 100 = 25% increase.

How to calculate percentage decrease

Percentage decrease = ((Old - New) ÷ Old) × 100

If price drops from 80 to 60:

((80 - 60) ÷ 80) × 100 = 25% decrease.

Practice problems

• Find 35% of 240
• Find 7.5% of 920
• A $75 item is discounted by 20%. What is the final price?
• What percent is 18 of 72?

Answers: 84, 69, $60, 25%.

Final takeaway

To calculate the percentage of a number, remember one reliable rule: convert the percent to a decimal and multiply. With this method, you can quickly solve discounts, tips, taxes, exam scores, budget planning, and many everyday calculations.