If you live in the United States, the United Kingdom or their territories, you're in one of the only countries that still uses miles as the standard measure for speed and distance. Because most other countries in the world use kilometers instead, knowing how to convert from kilometers to miles – and then back again – comes in handy when you travel. It's also a vital skill if you work in a science field where the metric system predominates, or if you participate in footraces, cycling races or competitive rowing, all sports in which distances are usually given in kilometers.
TL;DR (Too Long; Didn't Read)
To convert from kilometers into miles, multiply the distance in kilometers by 0.6214.
The Km to Miles Formula
Any time you're asked to convert from one unit of length to another, all you have to do is multiply the first unit by the appropriate conversion factor. So, if you're being asked to convert km to miles, you'd do this multiplication:
The conversion factor for converting kilometers to miles is 0.62137119, but for most purposes, being accurate to four decimal places is enough – so usually you'll use 0.6214 as your conversion factor. If you're not sure how many decimal places you must be accurate to, ask your teacher.
You'll probably need to memorize conversion factors for tests and quizzes, and any conversions you do frequently in school or in real life are likely to stick in your head. But it's also very common to look up conversion factors in a reference book or your notes. Unless it's against the rules of a test, it's always better to double-check a conversion factor – and be certain you're correct – than to guess.
An Example of Converting Kilometers to Miles
Imagine that you're being asked to convert 5 kilometers into miles. You already know how many kilometers you're being asked to convert, and you know the conversion factor; so all you have to do is fill them into the conversion formula:
Once you do the multiplication, you'll have your answer:
If you've ever run or walked in a mid-distance race, this is probably a familiar number; 5k or 3.1 miles is a very popular race distance.
In this case, the miles are rounded to one decimal point because most runners don't care about the extra 0.007 miles. Context is very important when you're deciding which decimal place to round to.
Converting Miles to Kilometers
If you have reason to convert kilometers to miles, you'll probably need to do conversions in the other direction, too: going from miles to kilometers. The simplest way to do this is to perform the inverse of the operation you used to go from kilometers to miles. So if you multiplied by 0.6214 to convert from kilometers to miles, you'll divide by 0.6214 to convert from miles to kilometers.
Try it out with 3.107 miles, which from the previous problem you know is equal to 5 kilometers:
So it checks out.
Which Way Do I Go?
There's just one problem here: How do you know if you should be dividing or multiplying by the conversion factor? You can double-check by remembering which unit is larger or smaller than the other. In this case miles are longer than kilometers, so if you convert from miles to kilometers, you should end up with a bigger number. If you go the other way, from kilometers to miles, you should end up with a smaller number.
Imagine that you're asked to convert 10 kilometers into miles, but you mistakenly divide by the conversion factor instead of multiplying. Then you'd have:
But you know miles are longer than kilometers, so your result should be smaller than what you started with, not bigger. If you've double-checked your conversion factor and are sure you got it right, that means you should be multiplying instead. That gives you:
This time your result (in miles) is a smaller number than the kilometers you started with, which means you chose the right operation for your conversion.
About the Author
Lisa studied mathematics at the University of Alaska, Anchorage, and spent several years tutoring high school and university students through scary -- but fun! -- math subjects like algebra and calculus.
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