how do i calculate a percentage of a number

The simple answer

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

(Percentage ÷ 100) × Number

So if you want to find 30% of 250:

(30 ÷ 100) × 250 = 0.30 × 250 = 75

That means 30% of 250 is 75.

Step-by-step method anyone can use

1) Turn the percent into a decimal

Move the decimal point two places left (or divide by 100).

• 50% becomes 0.50
• 8% becomes 0.08
• 125% becomes 1.25

2) Multiply by the number

Once converted, multiply that decimal by the target number.

• 15% of 80 = 0.15 × 80 = 12
• 7.5% of 64 = 0.075 × 64 = 4.8
• 2% of 950 = 0.02 × 950 = 19

Common real-life examples

Sales discount

A jacket costs $120 and is 25% off.

• Discount amount: 0.25 × 120 = $30
• Final price: 120 - 30 = $90

Tip at a restaurant

Your bill is $48 and you want to tip 18%.

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

Tax calculation

An item costs $75 and tax is 8.25%.

• Tax: 0.0825 × 75 = $6.19 (rounded)
• Total: 75 + 6.19 = $81.19

Other percentage formulas you should know

Find what percent one number is of another

Formula: (Part ÷ Whole) × 100

Example: What percent is 45 of 60?

(45 ÷ 60) × 100 = 75%

Increase a number by a percent

Formula: New value = Original × (1 + percent as decimal)

Example: Increase 200 by 10% → 200 × 1.10 = 220

Decrease a number by a percent

Formula: New value = Original × (1 - percent as decimal)

Example: Decrease 200 by 10% → 200 × 0.90 = 180

Mental math shortcuts for fast percentage calculations

• 10% = move decimal one place left. (10% of 340 = 34)
• 1% = move decimal two places left. (1% of 340 = 3.4)
• 5% = half of 10%. (5% of 340 = 17)
• 20% = double 10%. (20% of 340 = 68)
• 15% = 10% + 5%. (15% of 340 = 34 + 17 = 51)

Common mistakes to avoid

• Forgetting to divide by 100 before multiplying.
• Mixing up “percent of a number” with “what percent is this of that.”
• Applying multiple percentages in the wrong order (especially with discount + tax).
• Rounding too early—round at the final step when possible.

Quick FAQ

How do I calculate 20% of a number quickly?

Find 10% first, then double it. Example: 20% of 90 = 9 × 2 = 18.

How do I find the original number before a percentage discount?

If a final price is known after a discount, divide by the remaining percent. Example: After a 20% discount, a price is $80. Original = 80 ÷ 0.80 = $100.

Can percentages be more than 100%?

Yes. 150% means 1.5 times the original number.

Final takeaway

If you remember one formula, remember this: (percent ÷ 100) × number. From there, discounts, tax, tips, commissions, markups, and growth rates all become much easier. Use the calculator above whenever you want a fast and accurate answer.