We in the United States still use the English system of measurements while most of the rest of the world uses the metric system. For example, we use pounds and ounces in food packaging. Our motoring experience is measured in miles per gallon (MPG) and miles per hour (MPH). The distances shown in road signs are in miles. Suppose you are an American driving on a French highway toward Paris and you see a road indicating a distance of 270 kilometers to Paris. How many miles are in 270 kilometers?

From my road race running experience in the past, I learned that a 5K road race is 3.1 miles long. This information provides a quick and dirty way of making the conversion from kilometers to miles and vice versa.

To convert from kilometers to miles, the key is to determine the number of multiples of five in the distance with kilometer as unit. For example, 10K would be 3.1 x 2 = 6.2 miles, 15K would be 3.1 x 3 = 9.3 miles and so on. In other words, multiple of 5K would be the same multiple of 3.1 in miles.

The distance of 270 kilometers is 54 times of 5K. Thus 270K = 54 x 3.1 = 167.4 miles.

The above is a speed limit sign from Australia. Note that 80 = 16 x 5. Thus 80 K = 16 x 3.1 = 49.6 miles. So the speed limit of 80 K/h is about 50 MPH.

To convert from miles to kilometers, we do the opposite, that is, determine the number of multiples of 3.1 (or 3) in the distance with mile as unit. For example, the driving distance from Los Angeles to New York City is about 2800 miles (903 multiples of 3.1). Thus 2800 miles is approximately 903 x 5 = 4515 kilometers.

For an even quicker estimate, let’s say the distance from LA to New York is 3000 miles. Then the distance in kilometers would be roughly 5000 kilometers.

The same conversion may be done more formally using the following conversion factors.

1 Km = 0.621371192 miles

1 mile = 1.609344 km

I would like to point out that for everyday purpose, the quick and dirty way of 5 K = 3.1 miles would work just fine. From 5 K = 3.1 miles, 1 K = 3.1/5 = 0.62 miles. Likewise, 1 mile = 5/3.1 = 1.62 K. These are quite close to the formal conversion factors indicated above. Unless you are an engineer building a highway, the approximation of 5 K = 3.1 miles would work just fine.

In the United States, sometimes miles and kilometers are shown together. In some cases, the side-by-side information can actually clue you in on the conversion factors. The following is a warning sign in Hawaii. The sign indicates that 0.5 miles = 0.8 kilometers. Thus 1 mile = 1.6 kilometers.

The following sign indicate that 100 kilometers = 62 miles. Thus, 1 kilometer = 0.62 mile.

On rare occasions, road signs in the United States are kilometer only. See the following sign from Vermont on I-89.