How to Calculate Percentages: 7 Everyday Examples

Almost every percentage question you will meet in daily life is one of three types: X% of a number (multiply by X and divide by 100), what percent one number is of another (divide and multiply by 100), or the percentage change between two numbers (difference divided by the starting value, times 100). Once you know which type you are looking at, the arithmetic is the easy part.

Below are seven ordinary situations, which formula each one needs, and the numbers worked out so you can check your own answers.

Which percentage formula do I need?

Question you are asking	Formula	Example
What is X% of Y?	Y × X ÷ 100	20% of 150 = 30
X is what % of Y?	X ÷ Y × 100	42 of 48 = 87.5%
By what % did X change to Y?	(Y − X) ÷ |X| × 100	1,200 → 1,350 = +12.5%

The trick is to spot the base, the number that represents 100%. In "what is 20% of 150", the base is 150. In "42 out of 48", the base is 48. In a change, the base is always the starting value.

7 practical ways to use percentages

1. A test score

You got 42 questions right out of 48. That is "X is what % of Y": 42 ÷ 48 × 100 = 87.5%. The base is the total number of questions, not the number you got right.

2. Sales tax or a commission

A salesperson earns 7.5% commission on a 2,400 sale. That is "X% of Y": 2,400 × 7.5 ÷ 100 = 180. The same calculation works for any flat rate applied to an amount.

3. A rent increase

Your rent goes from 1,200 to 1,350 a month. That is a percentage change: (1,350 − 1,200) ÷ 1,200 × 100 = 12.5% increase, a difference of 150.

4. A price drop

A pair of headphones falls from 64 to 48. Change again, but downward: (48 − 64) ÷ 64 × 100 = −25%, so a 25% decrease. The calculator labels the result "Decrease" and shows the difference as −16.

5. Budget share

You spend 350 of a 2,800 monthly income on groceries. "X is what % of Y": 350 ÷ 2,800 × 100 = 12.5% of your budget.

6. A mental-math tip

For 15% of 80, find 10% (8), halve it to get 5% (4), and add them: 12. Splitting a percentage into 10%, 5% and 1% chunks makes most tipping and discount math doable in your head.

7. Changes between negative numbers

Profit and loss figures can be negative. If a loss shrinks from −200 to −50, the result moved up by 150. The calculator divides by the absolute value of the starting number (200), so it reports a 75% increase. Dividing by −200 instead would give a misleading "−75%" for what is really an improvement.

How to use the Percentage Calculator

The Percentage Calculator keeps all three formulas in one place:

1. Open the Calculation menu and choose What is X% of Y?, X is what % of Y? or Percentage change from X to Y.
2. Type your numbers into the X and Y fields. Decimals and negative numbers are fine.
3. Read the result as you type. For a change, you also get the plain Difference between the two values.

The calculator runs in your browser, so there is nothing to submit and no page reload between attempts. It will stop you in two cases that have no meaningful answer: Y cannot be zero when you ask what percent X is of Y, and the starting value X cannot be zero for a percentage change (you cannot grow "by a percentage" from nothing).

Common percentage mistakes

• Using the wrong base. "Prices are 20% higher than last year" uses last year as the base. Swapping the base gives a different number.
• Adding percentages that apply in sequence. A 25% discount followed by an extra 10% is not 35% off. For stacked discounts, the Discount Calculator does the multiplication for you.
• Confusing percent with percentage points. An interest rate moving from 4% to 5% is a rise of 1 percentage point, but a 25% increase in the rate itself.
• Reversing a change with the same percentage. Going up 10% and then down 10% does not bring you back to the start. The Percentage Increase Calculator shows why.

FAQ

How do I turn a fraction into a percentage?

Divide the top number by the bottom number and multiply by 100. Three-eighths is 3 ÷ 8 = 0.375, which is 37.5%. In the calculator, that is the X is what % of Y? mode with X = 3 and Y = 8.

Can a percentage be more than 100%?

Yes. If X is larger than Y, X is more than 100% of Y: 60 is 200% of 30. A percentage change can also exceed 100%, for example going from 20 to 60 is a 200% increase.

Why do I get a different answer when I swap X and Y?

Because the base changes. 30 is 20% of 150, but 150 is 500% of 30. Always put the reference amount, the "whole", in Y.

Pick the type of question, enter two numbers, and get the answer instantly with the Percentage Calculator.