how to calculate percentage of a number

How to calculate percentage of a number (simple formula)

The fastest way to calculate a percentage of a number is: (percentage ÷ 100) × number.

Example: What is 20% of 150?
20 ÷ 100 = 0.20
0.20 × 150 = 30
So, 20% of 150 is 30.

Step-by-step method you can always use

1. Take the percentage value (like 15%, 30%, or 85%).
2. Divide it by 100 to convert it into a decimal.
3. Multiply that decimal by the total number.

This method works for finance, school math, taxes, discounts, tips, commissions, and performance reports.

Real-life percentage examples

1) Discount in shopping

A jacket costs $80 and is on sale for 25% off.
Discount amount = 25% of 80 = (25 ÷ 100) × 80 = 20.
Final price = 80 - 20 = $60.

2) Tip at a restaurant

Your bill is $48 and you want to leave a 15% tip.
Tip = (15 ÷ 100) × 48 = 7.2.
Tip amount is $7.20.

3) Exam score

You got 42 points out of 50. What percentage is that?
Percentage = (42 ÷ 50) × 100 = 84%.

Common percentage formulas

• Percentage of a number: (P ÷ 100) × N
• What percent is A of B: (A ÷ B) × 100
• Percentage increase: ((New - Old) ÷ Old) × 100
• Percentage decrease: ((Old - New) ÷ Old) × 100

How to do percentage increase and decrease

Percentage increase

If your salary goes from $2,000 to $2,300:
Increase = 2,300 - 2,000 = 300
Increase % = (300 ÷ 2,000) × 100 = 15%

Percentage decrease

If a product drops from $120 to $90:
Decrease = 120 - 90 = 30
Decrease % = (30 ÷ 120) × 100 = 25%

Fast mental math tricks for percentages

• 10% = move decimal one place left (10% of 90 is 9).
• 1% = move decimal two places left (1% of 90 is 0.9).
• 5% = half of 10% (5% of 90 is 4.5).
• 15% = 10% + 5%.
• 25% = divide by 4.
• 50% = divide by 2.

Common mistakes to avoid

• Forgetting to divide by 100 before multiplying.
• Mixing up part and whole when finding “what percent”.
• Subtracting discount incorrectly from original price.
• Using the new value instead of old value for percentage change calculations.

Practice problems

• What is 35% of 240?
• 72 is what percent of 96?
• A price increases from 50 to 62. What is the percentage increase?
• A score drops from 88 to 66. What is the percentage decrease?

Try solving them manually, then check with the calculator above for instant verification.

Final takeaway

Once you remember one idea—percent means “out of 100”—all percentage problems become easier. Use the formula (percentage ÷ 100) × number for direct calculations, and use (part ÷ whole) × 100 when converting values into a percentage. With a little practice, you can solve most percentage questions in seconds.