how do you calculate the percentage of a number

Quick answer: the formula you need

To calculate the percentage of a number, use this formula:

(Percentage ÷ 100) × Number = Result

Example: What is 20% of 150?

(20 ÷ 100) × 150 = 0.2 × 150 = 30

So, 20% of 150 is 30.

Step-by-step: how do you calculate the percentage of a number?

Step 1: Turn the percentage into a decimal

Divide the percentage by 100. This moves the decimal point two places to the left.

• 5% becomes 0.05
• 12% becomes 0.12
• 250% becomes 2.5

Step 2: Multiply by the number

Take that decimal and multiply it by your base number.

• 12% of 80 = 0.12 × 80 = 9.6
• 5% of 360 = 0.05 × 360 = 18

Step 3: Check if your result makes sense

A quick reasonableness check can prevent mistakes:

• If the percentage is less than 100%, the result should be less than the original number.
• If it is exactly 100%, the result should equal the original number.
• If it is more than 100%, the result should be larger than the original number.

Common real-life examples

Sales tax

If an item costs $50 and sales tax is 8%, tax amount is 8% of 50:

0.08 × 50 = 4. Final cost is $54.

Tip calculation

Restaurant bill is $72 and you tip 18%:

0.18 × 72 = 12.96. Tip is $12.96.

Discounts

A jacket is $120 with a 25% discount:

Discount amount: 0.25 × 120 = 30

Sale price: 120 - 30 = 90

How to calculate percentage without a calculator

You can do many percentage calculations mentally by breaking them into easy chunks.

Use 10% as your anchor

10% is easy: just move the decimal one place left.

• 10% of 450 = 45
• 10% of 68 = 6.8

Build other percentages from 10% and 5%

• 5% is half of 10%
• 15% = 10% + 5%
• 25% = one quarter of the number
• 50% = half the number

Example: 15% of 80

• 10% of 80 = 8
• 5% of 80 = 4
• 15% of 80 = 8 + 4 = 12

Reverse percentage: finding the original number

Sometimes you know the result after a percentage change and need the starting value.

If a price after a 20% discount is $80, then $80 is 80% of the original.

Formula:

Original = Final ÷ (1 - discount rate)

Original = 80 ÷ 0.8 = 100

So the original price was $100.

What percent is one number of another?

This is a different but related question. Use:

(Part ÷ Whole) × 100 = Percentage

Example: 30 is what percent of 120?

(30 ÷ 120) × 100 = 25%

Frequent mistakes to avoid

• Forgetting to divide by 100 before multiplying.
• Mixing up part and whole in “what percent” calculations.
• Subtracting discount twice when finding a sale price.
• Rounding too early, which can create small errors in money calculations.

Practice problems

Try these quickly:

• What is 35% of 90? (Answer: 31.5)
• 18 is what percent of 60? (Answer: 30%)
• Increase 200 by 12% (Answer: 224)
• Decrease 150 by 20% (Answer: 120)

Final takeaway

If you remember one rule, remember this: percentage means “per 100.” Convert the percent to a decimal, multiply by the number, and you have your answer. For shopping, budgeting, grades, statistics, and finance, this skill is one of the most useful pieces of everyday math.