If you're already used to working with meters, you might be familiar with two other metric units of measure: The kilometer, which is equal to 1,000 meters, and the centimeter, which is equal to 1/100 of a meter. In both cases, the prefix gives you a valuable clue to how big, or small, the unit of measure will be. With that in mind, it's probably no surprise that a "gigameter" is really, really long. To be specific, it's equal to one billion meters.
TL;DR (Too Long; Didn't Read)
One gigameter equals one billion meters. This can be written out as 1 gm = 1,000,000,000 m, or as 1 gm = 1 × 109 m.
Get to Know the Gigameter
To put the gigameter into more familiar US terms, a single gigameter is equal to 621,371 miles. That's still so big it's hard to comprehend, so try this on for size: The earth's circumference at the equator is 24,901 miles. In other words, if you started at one spot on the equator, then traveled along the equator all the way around the Earth until you got back to the same spot, you'd go 24,901 miles. You'd have to make the trip almost 25 times to cover 621,371 miles.
So, a single gigameter is almost 25 times as long as the Earth is around. Because the gigameter measures such a big distance, it's really not useful on earth. So you're most likely to see it used when discussing astronomical measurements, such as the distance between planets.
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The gm to m Conversion
So you've been lucky enough to stumble over an elusive, Earth-side measurement given in gigameters – or perhaps you want to amuse yourself by converting astronomical measurements into the more familiar meter. Either way, to convert from gigameters to meters, you must multiply the number of gigameters by one billion.
There are two ways of doing the gm to m conversion. Either type it all out the long way, in which case you have:
Number of gigameters × 1,000,000,000 meters/gigameter = Number of meters
Or you can save space and a headache by using scientific notation, which expresses very large (or very small) numbers as a series of digits multiplied by powers of ten. In this case, your formula becomes:
Number of gigameters × 109 meters/gigameter = Number of meters
You can double-check this by multiplying out 109; the result is 1,000,000,000. You could also confirm it by counting the number of zeroes.
Usually, the method you use will be determined by how your teacher wants the answer expressed.
You can convert meters to gigameters by dividing the number of meters by 1,000,000,000 or 109, exactly the opposite of the conversion from gigameters to meters
An Example of Converting Gigameters to Miles
For now, imagine that your teacher wants the answers typed or written out longhand, and has asked you to to convert 9 gigameters to meters. You'll use the first formula, filling "9" into the space for gigameters:
9 gigameters × 1,000,000,000 meters/gigameter = 9,000,000,000 meters
Make sure you pay close attention to all those zeroes!
What if you use the second formula and scientific notation to solve the same problem? Your initial formula looks almost exactly the same:
9 gigameters × 109 meters/gigameter = ? meters
But instead of doing the long-hand multiplication, you'll leave those powers of 10 just the way they are. So, your answer will be:
9 gigameters × 109 meters/gigameter = 9 × 109 meters
Again, the result is exactly the same as typing or writing everything out the long way. 9,000,000,000 and 9 × 109 are just two different ways of expressing the same number.