What Is a Kilometer?
A kilometer (symbol km) is a unit of length in the metric system equal to exactly 1,000 meters. The prefix kilo- comes from the Greek chilioi, “thousand”, and was one of the original prefixes when France introduced the metric system in 1795. Because the meter is the SI base unit of length, the kilometer inherits its definition: the BIPM defines the meter through the speed of light, so a kilometer is the distance light travels in a vacuum in 1/299,792.458 of a second.
The kilometer is the everyday unit of long distance in almost every country. Road signs, speed limits (km/h), maps, flight distances and the areas of countries (km²) all use it. The United States and the United Kingdom are the best-known exceptions that still sign their roads in miles, although both use kilometers in science, the military and most running races.
The correct symbol is lowercase “km”, with no period and no “s” in the plural: 5 km, never 5 Km or 5 kms. NIST Special Publication 811 sets out these style rules for SI units in the United States.
What Is a Mile?
The mile on every US and UK road sign is the international mile, also called the statute mile or land mile. It is exactly 5,280 feet, or 1,609.344 meters. That metric value has been exact since 1959, when six English-speaking countries signed the International Yard and Pound Agreement and defined the yard as 0.9144 m. Since the mile is 1,760 yards, the kilometer-to-mile factor became an exact number, not a measurement.
The unit itself is much older than the kilometer. Roman legions counted distance in mille passus, a thousand paces, where one pace was a full double step of about five Roman feet. That Roman mile was roughly 1,480 m. England later stretched its mile to fit the furlong, a farming length, and a 1593 Act of Parliament under Elizabeth I fixed the statute mile at 8 furlongs, the 5,280 feet still used today.
Running and Road Race Distances in Miles
Most road races are measured in kilometers, even in the United States, so runners convert km to miles constantly. The table below gives the standard race distances. The half marathon and marathon are the only common distances that are not round kilometers: they come from the 26-mile, 385-yard course of the 1908 London Olympic marathon.
| Race | Kilometers (km) | Miles (mi) |
|---|---|---|
| 5K | 5 | 3.106856 |
| 8K | 8 | 4.97097 |
| 10K | 10 | 6.213712 |
| 15K | 15 | 9.320568 |
| Half marathon | 21.0975 | 13.10938 |
| 25K | 25 | 15.53428 |
| 30K | 30 | 18.64114 |
| Marathon | 42.195 | 26.21876 |
| 50K ultramarathon | 50 | 31.06856 |
| 100K ultramarathon | 100 | 62.13712 |
Two quick facts help when you read race results. First, a pace per kilometer converts to a pace per mile by multiplying it by 1.609344: a runner holding 5:00 per km is running about 8:03 per mile. Second, a 10K is 6.21 miles, so “a 10K in 50 minutes” means the runner averaged just over 8 minutes per mile.
There is also a handy mental shortcut. Consecutive Fibonacci numbers (3, 5, 8, 13, 21) grow by a ratio of about 1.618, very close to the 1.609 km in a mile. So 5 km is about 3 miles, 8 km is about 5 miles, 13 km is about 8 miles and 21 km is about 13 miles. The exact values are 3.11, 4.97, 8.08 and 13.05 miles.
How Many Kilometers in a Mile?
There are exactly 1.609344 kilometers in one mile, so converting miles to kilometers is a multiplication by that number. A 3-mile walk is 4.83 km, and a 100-mile ultramarathon is 160.9 km.
The same factor converts speeds. A US highway limit of 65 mph is 104.6 km/h, and a European motorway limit of 130 km/h is 80.8 mph. Car speedometers sold in North America usually show both scales for this reason.
Countries that switched from miles to kilometers did it on a fixed date, because every road sign had to change at once. Australia converted its road signs to kilometers in 1974 and Canada in 1977; Ireland finished the job in 2005 by switching every speed limit to km/h. The United States passed the Metric Conversion Act in 1975, which made metric units voluntary, so American roads stayed in miles.
On foot, a kilometer is a short walk: about 12 minutes at an easy 5 km/h pace and roughly 1,300 steps for an average adult. If you count steps rather than kilometers, the steps to km calculator converts a step count by height, and the steps to miles calculator does the same in miles.