How to Calculate a Percentage of a Number

Quick answer: To calculate a percentage of a number, convert the percentage into a decimal and multiply by the number.

Try it: Open the free BuzBolt Percentage Calculator. No signup is required.

Three common percentage questions

People often mean one of three things when they ask for a percentage:

What is a percentage of a number? Multiply the number by the percentage expressed as a decimal.
What percentage is one number of another? Divide the part by the reference whole and multiply by 100.
How much has something changed? Use the original value as the baseline and calculate the relative difference.

Our Percentage Calculator has separate modes for these tasks so the inputs and results are not confused.

Percentage of a number

To find 18% of 250, calculate 250 × 0.18 = 45. To find 5% of 80, calculate 80 × 0.05 = 4. The answer has the same unit as the original number.

What percentage is X of Y?

To find what percentage 30 is of 150, calculate (30 ÷ 150) × 100 = 20%. The second number is the baseline or whole. If the reference number is zero, the ordinary part-over-whole calculation is undefined.

Discounts

A $90 price with a 15% discount has a reduction of 90 × 0.15 = $13.50. The final price is $90 − $13.50 = $76.50. Do not confuse the discount amount with the amount after discount.

If the final price after a 20% reduction is $80, divide by 0.80 to recover the original price of $100. Adding 20% to $80 does not undo the previous reduction.

A quick conversion table

Percentage	Decimal	Simplified fraction
10%	0.10	1/10
12.5%	0.125	1/8
20%	0.20	1/5
25%	0.25	1/4
50%	0.50	1/2
75%	0.75	3/4

Percentages communicate the share, decimals work well in multiplication, and fractions can show exact relationships.

Common mistakes

Dividing by the wrong number when calculating a share of a whole.
Using 15 instead of 0.15 as the multiplier for 15%.
Adding a discount percentage back to a discounted price instead of reversing the multiplier.
Assuming a 20% rise and 20% fall cancel each other out.

Decide what the question is really asking

“20% of 150” asks for a portion of a quantity, so the result is 30. “30 is what percentage of 150?” asks for a ratio, so the result is 20%. “150 increased to 180 by what percentage?” asks for a change, so the result is 20%. The number 20 appears in more than one of these expressions, but the arithmetic is different.

When a calculation seems surprising, rewrite the question with the words part, whole, original and new. Identifying the role of each value usually reveals the correct formula.

Verify with reverse arithmetic

If 12% of 250 is 30, divide 30 by 250 and multiply by 100 to confirm 12%. If an item is discounted from $100 to $85, subtract to find the $15 discount and divide by the original $100 to confirm 15%. These checks are simple enough to perform by hand and help prevent wrong-base errors.