How to calculate a percentage: the simple formula

Percentages are everywhere: discounts, grades, tips, tax, pay raises, survey results, nutrition labels, and progress bars. The good news is that most percentage questions use the same small idea.

A percentage means “out of 100.” So 25% means 25 out of 100, 50% means half, and 100% means the whole amount.

The basic percentage formula

Use this formula when you want to know what percent one number is of another:

percentage = part ÷ whole × 100

Example:

18 out of 60 = 18 ÷ 60 × 100 = 30%

So 18 is 30% of 60.

The most common mistake is swapping the part and the whole. The “whole” is the original full amount. The “part” is the piece you are measuring.

Example 1: test score percentage

If you answered 42 questions correctly out of 50, the part is 42 and the whole is 50:

42 ÷ 50 × 100 = 84%

Your score is 84%.

This same method works for marks, quizzes, completed tasks, attendance, and any situation where you know the number achieved and the total possible.

Example 2: discount percentage

Suppose a jacket was $80 and the discount is $20. To find the discount percentage:

20 ÷ 80 × 100 = 25%

The jacket is 25% off.

If you want the final price instead, subtract the discount from the original price:

$80 - $20 = $60

So a $20 discount on an $80 jacket is 25% off, and the final price is $60.

Example 3: percentage of a number

Sometimes the question is not “what percent is this?” but “what is 15% of 200?”

Use this version:

percentage amount = whole × percentage ÷ 100

Example:

200 × 15 ÷ 100 = 30

So 15% of 200 is 30.

A shortcut is to turn the percentage into a decimal first:

15% = 0.15
200 × 0.15 = 30

Both methods are the same.

Common percentage shortcuts

Some percentages are easy to do in your head:

Percentage	Shortcut	Example with 80
10%	divide by 10	8
5%	half of 10%	4
1%	divide by 100	0.8
25%	divide by 4	20
50%	divide by 2	40
75%	50% + 25%	60

These shortcuts are useful for quick estimates. If you need an exact answer, especially with awkward numbers like 17.5% or 23%, use the formula or a calculator.

How to avoid the three common mistakes

1. Do not forget to multiply by 100

If you calculate:

18 ÷ 60 = 0.3

that is the decimal form. To turn it into a percentage:

0.3 × 100 = 30%

2. Use the original amount as the whole

If a price goes from $80 to $60, the decrease is $20. The original price was $80, so the decrease is:

20 ÷ 80 × 100 = 25%

Do not divide by the new price unless the question specifically asks for a comparison against the new amount.

3. Keep percent and percentage points separate

If something rises from 10% to 15%, it increased by 5 percentage points. But relative to the original 10%, it increased by 50%:

5 ÷ 10 × 100 = 50%

That difference matters in finance, statistics, polls, and news articles.

When a percentage calculator is easier

For simple numbers, mental math is fine. For decimals, multiple steps, discounts, percentage change, or “what is X% of Y?” questions, a calculator saves mistakes.

You can use the percentage calculator to quickly solve common cases like:

• what is 18% of 240;
• what percentage is 42 of 50;
• what is the percentage increase from one number to another;
• what is the final price after a discount.

Quick rule to remember

If you want to know what percent a part is of a whole, use:

part ÷ whole × 100

If you want to find a percent of a number, use:

whole × percent ÷ 100

That covers most everyday percentage questions.