# How to calculate percentage

Now that you know what is percentage, it's time to learn calculating them. So how do you calculate percentages? While it's certainly quick and painless just to use our percentage calculator, you don't always have an access to a computer or a smartphone. Also, it's just plain cool to be able to make calculations in your head. Maybe not as cool juggling flaming torches, but close.

The percentage tells you how number A relates to number B. A real world example could be: there are 2 girls in a group of 5 children. What's the percentage of girls? In other words, we want to know what's the ratio of girls to all children. It's 2 out of 5, or 2/5. We'll call the first number a **numerator** and the second number a **denominator**, because this is how we call these two parts of a fraction. To calculate percentage, multiply this fraction by 100 and add a percentage sign. **100 * numerator / denominator = percentage**. In our example it's 100 * 2/5 = 100 * 0.4 = 40. Forty percent of the group wears dresses and prefers dolls over toy guns. Or even real guns.

Let's go the other way around. Say we know that 70 percent of fruits in the basket are apples and there are 30 fruits altogether. How Could be worse, it could be lemons. So how many apples do we have? Let's get our old formula:

**100 * numerator / denominator = percentage**

We want to find out the numerator... let's move all the other parts of the equation to the other side. Both divide both sides by 100 (to get rid of 100 on the left) and then multiply both sides by the denominator. This is what we get:

**numerator = percentage * denominator / 100**

Let's substitute percentage and denominator with our values:

**numerator = 70 * 30 / 100**

Now it's easy:
**numerator = 1800 / 100 = 18**
We have 18 apples. Should be enough for a lunch. Or a rather violent food fight.

Now let's solve a problem with an unknown denominator. I spent 30 percent of my pocket money on bubble gum (I never said I'm a great investor). I bought 12 sticks for $1 each. So we know that $12 was 30 percent of my total budget. How much money did I have before I almost literally blew it away? Let's start with our formula:

**100 * numerator / denominator = percentage**

or **100 * 12 / denominator 30**

This time we want to find out denominator... let's multiply both sides by denominator and then divide by percentage... this way we'll get

**100 * 12 / 30 = denominator**

And the other way around...
**denominator = 100 * 12 / 30 = 1200 / 30 = 40**
We had 40 dollars. We spent 30 percent, or $12 on bubble gum. Totally worth it.

When you're done, why don't you learn how to calculate percentage increase and percentage decrease? If you still need some practice, read on how to find percentage.