Now you need to figure out something called a ratio.

A ratio is a number that says how much bigger or smaller one thing is compared to another.

Here are some examples of ratios:

There are one hundred centimeters in a meter. This means that a meter is one hundred times longer than a centimeter.

The ratio between meters and centimeters is written as 1:100. That means "one to a hundred."

There are a thousand meters in a kilometer. This means that the ratio between kilometers and meters is 1:1,000. That is the same as saying"one to a thousand."

A classroom has 25 students and one teacher. The ratio of students to teachers is twenty-five to one, or 25:1.

The order you use when you write down a ratio is very important. It has to be the same order you use when you describe the things you're comparing.

The ratio of students to teachers is 25:1.

The ratio of teachers to students is 1:25.

What is the ratio between the size of the Earth and the size of the Moon? The Earth's diameter is 12,756 km ("km" stands for kilometers.) The Moon's diameter is 3,476 km.

The ratio between the diameters of the Earth and Moon would be written as "12,756:3,476" (12,756 to 3,476). The numbers in that ratio may be accurate, but because they're fairly large they don't tell you much about these two worlds.

Simplify the ratio to make it more useful. Look at the two numbers again. You can see that the Earth (12,756 km) is bigger than the Moon (3,476 km).

Here is where a calculator comes in handy. Divide the size of the Earth by the size of the Moon.

12,756 / 3,476 = 3.7 (rounded to the nearest decimal place)

You have just figured out that Earth's diameter is 3.7 times larger than the Moon's. You have calculated that the ratio of diameters between the Earth and Moon is 3.7:1 ("3.7 to 1").

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