What the mile is, and why there is more than one of them
A statute mile is 5,280 feet. Since the international foot has been exactly 0.3048 meters since 1959, the mile is exactly 1609.344 meters, and every figure on this page follows from that one line. The word statute is there because the length was fixed by an English statute in 1593, which standardised a Roman unit of a thousand paces onto the existing furlong: eight furlongs of 660 feet each gives the awkward-looking 5,280.
The nautical mile is a completely separate unit that happens to share a name. It was defined as one minute of arc of latitude along a meridian, so that a navigator could step off distance directly against the latitude scale on a chart. Because the Earth is not a perfect sphere, that arc varies slightly with latitude, and in 1929 an international conference settled on a fixed value of 1,852 meters. It is 15.08 percent longer than a statute mile — you can see the figure directly: 1852 ÷ 1609.344 = 1.150779, so the excess is 15.0779 percent. Ships and aircraft use it, and speeds in nautical miles per hour are knots.
The kilometer needs no such history. It is a thousand meters, and the meter has been defined since 1983 by fixing the speed of light at exactly 299,792,458 m/s. That is why the metric side of this conversion is exact and the imperial side is exact only by borrowing it.
The formula, and the shortcuts worth remembering
Multiply miles by 1.609344 for kilometers; divide kilometers by 1.609344, or multiply by 0.621371192, for miles. The converter routes everything through meters first, so that any source unit reaches any target unit by exactly two operations and no intermediate rounding.
Two mental shortcuts are worth carrying. The first is the Fibonacci trick: consecutive Fibonacci numbers approximate the mile-to-kilometer ratio, because the ratio 1.609344 is close to the golden ratio 1.618. So 3 mi is about 5 km, 5 mi about 8 km, 8 mi about 13 km, 13 mi about 21 km. The error is 0.5 percent and it does not grow, because each step is a fresh approximation rather than an accumulation. A half marathon is 13.1 mi and 21.1 km, which is the Fibonacci pair almost exactly.
The second is the five-eighths rule: a kilometer is very nearly five eighths of a mile. 0.625 against the true 0.621371 is an overestimate of 0.58 percent, so 80 km/h reads as 50 mph when it is really 49.7. Both shortcuts are good enough for road signs and useless for anything measured.
Yards enter through the mile rather than the meter: there are 1,760 yards in a mile, which follows from 5,280 feet divided by three. That is why an 800 m race is described as a half mile even though 880 yd is 804.67 m — the two differ by about five meters, which is a full second at world-record pace.
Worked example: a marathon in miles, and 60 mph in km/h
A marathon is defined as 42.195 km. Convert it to miles.
- Into meters. 42.195 × 1000 = 42,195 m.
- Divide by the mile. 42,195 ÷ 1609.344 = 26.2187574 mi. Work it in parts if you are checking on paper: 26 × 1609.344 = 41,842.944, leaving 352.056 m, and 352.056 ÷ 1609.344 = 0.21876. So 26.21876 mi.
- Check against the historical definition. The marathon was fixed in 1921 at 26 miles 385 yards. That is 26 + 385/1760 = 26.21875 mi, and 26.21875 × 1609.344 = 42,194.988 m. The 42.195 km figure is that value rounded to the nearest meter, which is why the mile conversion comes back a hair above 26.21875.
Now convert a speed. A 60 mph speed limit in kilometers per hour:
- Convert the distance part. 60 × 1.609344 = 96.56 km/h.
- Compare with the posted limit. Countries that signed the metric road-sign conventions post 100 km/h, which is 62.14 mph — about 3.5 percent faster than a 60 mph limit, not the same limit relabelled.
Speeds convert with the same factor as distances because the time unit is unchanged on both sides. That is only true when both speeds are per hour; a knot is one nautical mile per hour, so converting knots to mph needs the 1.150779 factor instead.
How to read the result and how much precision to keep
Round road and travel distances to the nearest tenth of a unit and stop. GPS route distances are good to perhaps 1 percent because of how the road centerline is digitised, so a route reported as 412.7 km is honestly 412 or 413 km, and printing 412.7382 asserts nothing real.
Race distances are the exception: they are certified to 0.1 percent by a calibrated bicycle method, and the certified course is measured along the shortest legal running line. That is why your watch almost always reads long. A marathon watch reading of 42.5 km against a 42.195 km course is a 0.7 percent excess, which is ordinary GPS drift and imperfect tangent running rather than a mismeasured course.
For navigation, keep nautical miles and statute miles rigorously separate. A chart distance of 20 NM is 23.0 statute miles, and treating the two as interchangeable on a fuel plan understates the trip by 15 percent. Aviation compounds the trap: visibility on an American aerodrome report is in statute miles while the distances on the chart are nautical, so the same page carries both units.
Common distances in kilometers, miles and nautical miles
| Distance | Kilometers | Statute miles | Nautical miles |
|---|---|---|---|
| 1 kilometer | 1.0000 | 0.6214 | 0.5400 |
| 1 statute mile | 1.6093 | 1.0000 | 0.8690 |
| 5K road race | 5.0000 | 3.1069 | 2.6998 |
| 5 miles | 8.0467 | 5.0000 | 4.3449 |
| 10K road race | 10.0000 | 6.2137 | 5.3996 |
| 10 miles | 16.0934 | 10.0000 | 8.6898 |
| Half marathon | 21.0975 | 13.1094 | 11.3917 |
| Marathon | 42.1950 | 26.2188 | 22.7835 |
| 50 kilometers | 50.0000 | 31.0686 | 26.9978 |
| 100 kilometers | 100.0000 | 62.1371 | 53.9957 |
The nautical mile column is the kilometer column divided by 1.852; it is included because marine and aviation charts quote distances in NM even when the surrounding text uses kilometers.
Mistakes that put the wrong distance in the plan
- Using 1.6 instead of 1.609344. The shortcut is 0.58 percent short. Over a 500 mile drive that is 4.7 km, which is a fuel-stop-sized error on a marginal range.
- Treating nautical miles as statute miles. A 15.08 percent understatement of every leg. Aviation fuel planning and marine passage planning both live in NM.
- Converting a speed with a distance factor when the time unit differs. Miles per hour to kilometers per hour uses 1.609344; knots to miles per hour uses 1.150779.
- Assuming 880 yards is 800 meters. 880 yd is 804.672 m. Track records at the two distances are not interchangeable.
- Quoting a GPS distance to four decimals. Consumer GPS route distance carries roughly 1 percent of error; the decimals are arithmetic, not measurement.
- Rounding the marathon to 26.2 miles and converting back. 26.2 mi is 42.165 km, thirty meters short of the real distance.
Key terms
- Statute mile
- The ordinary land mile: 5,280 international feet, exactly 1609.344 m. Called statute because an English act of 1593 fixed it against the furlong.
- Nautical mile
- Exactly 1,852 m, originally one minute of latitude along a meridian. Used for marine and air navigation so that chart distance reads directly off the latitude scale.
- Knot
- One nautical mile per hour, 1.852 km/h or 1.150779 mph. Never write knots per hour: the time is already in the unit.
- Furlong
- One eighth of a mile, 660 feet or 201.168 m. It survives in horse racing and in the arithmetic that makes a mile 5,280 feet rather than a round number.
Where this sits among the other distance conversions
Miles and kilometers is the top of the everyday length range. Below it, structural and site dimensions run in meters and feet — use the meters to feet calculator, which also carries the US survey foot for land records. Shop and drawing dimensions drop to millimeters and inches in the millimeters to inches calculator, and a dimension that must land on a tape mark belongs in the decimal inches to fraction calculator.
Distances and heights use different formatting conventions even though both are lengths: a stature is written as feet and inches rather than decimal feet, which the feet and inches to centimeters calculator handles. Fuel planning for a long drive pairs distance with volume, so the gallons to liters calculator is the natural companion when comparing a US mpg figure with a European litres-per-100-km figure.
One conversion this page deliberately does not attempt is fuel economy, because miles per gallon and litres per 100 km are reciprocals of each other as well as unit conversions. Doubling the distance figure does not halve the consumption figure, and a single multiplication cannot carry you between them. Convert the distance and the volume separately, then form the ratio in whichever direction your reader expects.