Percentage Calculator

What Is a Percentage?

The word “percent” means “per hundred.” So a percentage is simply a count out of 100. Imagine cutting something into 100 equal pieces: the percentage tells you how many of those pieces you are talking about.

• 25% is 25 pieces out of 100 (a quarter).
• 50% is 50 pieces out of 100 (half).
• 100% is all 100 pieces (the whole thing).

The grid above the calculator shows this directly. Change the percent and watch the squares fill in: the shaded squares are always “out of 100.” That picture is the whole idea behind percentages.

Finding a Percent of a Number

Once you see a percent as “so many pieces out of 100,” finding a percent of a number is two small steps:

1. Turn the percent into its size by dividing by 100. (15% becomes $\tfrac{15}{100} = 0.15$.)
2. Multiply that by the number you are taking the percent of.

$$\text{answer} = \frac{P}{100} \times N$$

So 15% of 80 is $\tfrac{15}{100} \times 80 = 12$. In the grid, 15 squares out of 100 are shaded, and 15% of 80 lands on 12.

Going Backward: A Is What Percent of B?

Sometimes you have the two numbers and want the percent. Ask “how many out of 100 would this be?” Divide the part by the whole, then multiply by 100 to scale it up to a per-hundred count.

$$\text{percentage} = \frac{A}{B} \times 100$$

For 18 out of 72: $\tfrac{18}{72} \times 100 = 25\%$. The fraction $\tfrac{18}{72}$ is the same size as 25 squares out of 100.

More Than a Whole, or Less Than Zero

Percentages are not stuck between 0 and 100. Since 100% is one whole, 150% is one and a half wholes - the idea behind growth, markups, or “the new price is 150% of the old one.” A negative percent goes the other way: -10% is a 10% decrease, taking 10 out of every 100 away. Type 150 or -10 into the calculator and the picture switches to a number line, so you can see past one whole and below zero.

Worked Examples

Problem	Steps	Answer
What is 15% of 80?	(15 / 100) x 80	12
What is 25% of 240?	(25 / 100) x 240	60
18 is what percent of 72?	(18 / 72) x 100	25%
45 is 30% of what number?	(45 / 30) x 100	150

Common Uses

• Finding discounts before buying something.
• Checking a test score or practice score.
• Comparing price, salary, or investment changes.
• Solving school percentage word problems.

FAQ

How do I calculate 10% quickly?

Move the decimal point one place left. For example, 10% of 360 is 36.

How do I calculate 25% quickly?

25% is one fourth, so divide the number by 4.

Can this be used for financial decisions?

The percentage math is exact. For important financial decisions, check the full numbers (fees, taxes, and changing rates) independently before acting.