How to Calculate Percentage of a Number

The Core Formula

A percentage is simply a way of saying "out of 100." When you want 30% of 150, you are really asking: what is 30 out of every 100 parts of 150? The arithmetic is straightforward:

Result = Number x Percentage / 100

For 30% of 150: 150 x 30 / 100 = 45. That is the whole thing. Everything else in this article is just different ways to apply or speed up that same idea.

Step-by-Step Worked Examples

Example 1: 20% of 85

Step 1: Write out the formula. 85 x 20 / 100.

Step 2: Multiply first. 85 x 20 = 1,700.

Step 3: Divide by 100. 1,700 / 100 = 17.

Answer: 17. So 20% of 85 is 17.

Example 2: 15% tip on a $60 meal

60 x 15 / 100 = 9. You would leave $9. For 20%, it is 60 x 20 / 100 = 12. The formula does not care what the percentage is for; it works the same way every time.

Example 3: 7.5% sales tax on $240

240 x 7.5 / 100 = 18. Your total becomes $258. Decimal percentages work the same way; just plug the number in.

Example 4: 45% of 360 (exam scores)

360 x 45 / 100 = 162. If 162 out of 360 marks are needed to pass, and you scored 175, you passed.

The Decimal Shortcut

Dividing by 100 at the end is equivalent to converting your percentage to a decimal first. Divide the percentage by 100 before multiplying:

• 25% becomes 0.25
• 7% becomes 0.07
• 110% becomes 1.10

Then multiply that decimal by your number. 0.25 x 80 = 20. Same as 80 x 25 / 100 = 20. On a phone calculator, many people find the decimal route faster because it is a single multiplication.

Mental Math Tricks for Common Percentages

Build up unfamiliar percentages from pieces you know. 17.5% of 200? That is 10% (20) plus 5% (10) plus 2.5% (5) = 35.

Percentages Above 100%

Nothing breaks when the percentage exceeds 100. 150% of 80 is 80 x 150 / 100 = 120. You are just getting more than the original number. This comes up in growth reports: "Sales were 150% of last year's figure" means sales grew by 50%, landing at 1.5 times the original value.

Using the Free Calculator

If you need a quick answer without mental arithmetic, the percentage calculator on the homepage handles it instantly. Type in your number and percentage, and the result appears. It also covers percentage change between two numbers and finding what percent one number is of another.

Common Mistakes to Watch For

The most frequent error: forgetting to divide by 100 and treating the percentage as a whole number. Multiplying 80 by 25 gives 2,000, not 20. Always divide by 100 or convert to a decimal first.

Second common slip: reversing the numbers. "What is 40% of 90?" is different from "What is 90% of 40?" The answers are 36 and 36, actually. That pair happens to be equal, which is a genuine mathematical fact (and a useful party trick), but it does not hold in general. "20% of 50" is 10; "50% of 20" is also 10. Coincidence holds for any pair, because multiplication is commutative. But "30% of 80" is 24, not "80% of 30" = 24. Wait, those are also equal. The reason is that A% of B = B% of A always. Worth knowing, but verify for your actual problem.

Third: confusing percentage points with percentage change. If interest rates rise from 3% to 5%, that is a 2 percentage point rise, but a 66.7% increase in rate. More on this in our article on percentage difference vs percentage change.