how do you calculate percent of a number

The simple formula for percent of a number

If you are asking, “How do you calculate percent of a number?”, the core formula is very short:

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

That means you convert the percent into a decimal, then multiply.

• 10% becomes 0.10
• 25% becomes 0.25
• 150% becomes 1.50

Step-by-step example

Example: What is 30% of 250?

• Step 1: Convert 30% to decimal: 30 ÷ 100 = 0.30
• Step 2: Multiply by 250: 0.30 × 250 = 75
• Answer: 30% of 250 is 75

Shortcut version

You can also write it directly as:

(30 ÷ 100) × 250 = 75

Real-world uses of percentage calculations

Percent math appears everywhere in daily life and business:

• Shopping: Find a discount (e.g., 20% off).
• Taxes: Add sales tax (e.g., 7.5%).
• Tips: Calculate 15%, 18%, or 20% tip.
• Finance: Calculate interest rates and investment returns.
• Grades: Convert points earned into percentages.

How to find what percent one number is of another

This is the reverse type of problem. Use:

Percent = (Part ÷ Whole) × 100

Example: 18 is what percent of 24?

• 18 ÷ 24 = 0.75
• 0.75 × 100 = 75
• Answer: 18 is 75% of 24

How to calculate percent increase or decrease

Use this formula to compare old and new values:

Percent change = ((New − Old) ÷ Old) × 100

Example: Price goes from $50 to $65

• Change = 65 − 50 = 15
• 15 ÷ 50 = 0.30
• 0.30 × 100 = 30%
• Answer: 30% increase

Example: Value drops from 200 to 150

• Change = 150 − 200 = −50
• −50 ÷ 200 = −0.25
• −0.25 × 100 = −25%
• Answer: 25% decrease

Common percentage mistakes to avoid

• Forgetting to divide by 100: 12% is 0.12, not 12.
• Swapping part and whole: Use the correct denominator in ratio problems.
• Using the new value as the base in percent change: percent change should usually be based on the original value.
• Rounding too early: Keep a few extra decimals until the final step.

Fast mental math tricks for percentages

Find 10%, then scale

10% of a number is just moving the decimal one place left. From there:

• 5% = half of 10%
• 20% = double 10%
• 15% = 10% + 5%

Use fractions for common percents

• 50% = 1/2
• 25% = 1/4
• 75% = 3/4
• 20% = 1/5

Quick FAQ

How do I calculate 15% of 80?

(15 ÷ 100) × 80 = 0.15 × 80 = 12.

How do I add a percentage to a number?

Multiply by (1 + percent as decimal). Example: add 8% to 200 → 200 × 1.08 = 216.

How do I subtract a percentage from a number?

Multiply by (1 − percent as decimal). Example: subtract 30% from 90 → 90 × 0.70 = 63.

Bottom line

To calculate percent of a number, use (percent ÷ 100) × number. That single formula handles most percent questions. For reverse problems, use (part ÷ whole) × 100, and for change over time, use ((new − old) ÷ old) × 100. Once you practice a few examples, percentage math becomes quick and intuitive.