How do you convert km to miles?
Short answer. Divide kilometers by 1.609344, or multiply by 0.621371. So 100 km ÷ 1.609344 = 62.137 miles, and 5 km = 3.107 miles. In your head, multiply by 5 and divide by 8: 100 × 5 ÷ 8 = 62.5, which runs about 0.6% high. Going the other way, multiply miles by 1.609344.
The conversion factor is not an approximation that somebody measured. It is a definition. In 1959 the international yard and pound agreement fixed the yard at exactly 0.9144 meters, and a mile is 1760 yards, so:
1 mile = 1760 × 0.9144 m = 1609.344 m = 1.609344 km 1 km = 1 ÷ 1.609344 mi = 0.6213711922373339… mi
Six decimal places going one way, an endless run of digits coming back. That asymmetry is the whole reason this conversion feels awkward, and it is why the shortcuts below exist.
The two directions
Kilometers to miles: divide by 1.609344. A 10 km race:
10 ÷ 1.609344 = 6.213711922373339… → 6.21 miles
Miles to kilometers: multiply by 1.609344. A 5 mile run:
5 × 1.609344 = 8.04672 km exact, no rounding needed
Multiplying by 0.621371 instead of dividing by 1.609344 gets you to the same place, and it is easier to type. Just remember that 0.621371 is itself rounded, so it is a shortcut and not an equal sign. More on what that costs you further down.
Doing it in your head: times five, divided by eight
Five eighths is 0.625, and the number you actually want is 0.621371. That is close enough for anything you are estimating from a car window.
80 km → 80 × 5 = 400 → 400 ÷ 8 = 50 miles Exact 80 ÷ 1.609344 = 49.709695… 100 km → 100 × 5 = 500 → 500 ÷ 8 = 62.5 miles Exact 100 ÷ 1.609344 = 62.137119…
The estimate is always high, and always high by the same fraction: 0.625 ÷ 0.6213711922373339 = 1.00584, so 0.584% over. On a 5 km run that is 29 meters. On a 1000 km drive it is 5.8 km, which is one wrong turn.
The Fibonacci trick
Consecutive Fibonacci numbers have a ratio that creeps towards 1.618, and 1.609344 is close enough to that for street use. So each pair of neighbours in the sequence is a km to miles conversion sitting there for free:
5 km ≈ 3 miles true value 3.107 8 km ≈ 5 miles true value 4.971 13 km ≈ 8 miles true value 8.078 21 km ≈ 13 miles true value 13.049 34 km ≈ 21 miles true value 21.127
It is a party trick rather than a method, but the 8 km to 5 miles pair is worth memorizing on its own. Every distance on a road sign can be pushed towards a multiple of 8.
Conversion table
| Kilometers | Miles (exact, 3 dp) | Five eighths estimate |
|---|---|---|
| 1 | 0.621 | 0.625 |
| 2 | 1.243 | 1.25 |
| 5 | 3.107 | 3.125 |
| 8 | 4.971 | 5 |
| 10 | 6.214 | 6.25 |
| 13 | 8.078 | 8.125 |
| 21 | 13.049 | 13.125 |
| 42.195 | 26.219 | 26.372 |
| 50 | 31.069 | 31.25 |
| 100 | 62.137 | 62.5 |
Speeds convert with the same number
Kilometers per hour to miles per hour is the same division, because the hours on both sides cancel and only the distance unit changes:
30 km/h ÷ 1.609344 = 18.641 mph residential 50 km/h ÷ 1.609344 = 31.069 mph town 100 km/h ÷ 1.609344 = 62.137 mph motorway 120 km/h ÷ 1.609344 = 74.565 mph motorway, elsewhere 60 mph × 1.609344 = 96.561 km/h going back
This is the one case where the five eighths trick earns its keep, because a speedometer does not need three decimal places. 100 km/h is a bit over 60 mph, and 120 km/h is a bit under 75 mph.
The marathon, which is neither number you have seen
A marathon is defined in imperial units: 26 miles and 385 yards. Convert that once and the familiar metric figure turns out to be rounded:
26 miles = 26 × 1609.344 = 41842.944 m 385 yards = 385 × 0.9144 = 352.044 m Total = 42194.988 m = 42.194988 km Printed on the entry form: 42.195 km 12 mm long Printed on the finisher shirt: 26.2 miles 30.18 m short
The half marathon inherits both problems: 21.0975 km, which is 13.109379 miles, which nobody prints either. If you have ever wondered why race distances are quoted to three decimals, this is why. The distance is exact in one system and endless in the other.
Where the rounded factor costs you
Use 0.621371 instead of the full 0.62137119223733397 and the error is invisible until the distance is large. Over 1000 km:
Rounded factor 1000 × 0.621371 = 621.371 miles Exact 1000 ÷ 1.609344 = 621.371192237… miles Difference = 0.000192 miles ≈ 31 cm
Thirty one centimeters over a thousand kilometers is not a problem. The binary arithmetic underneath is a slightly different story, because 0.621371 cannot be stored exactly in a binary float, so a machine multiplying it by 100 answers 62.137100000000004 rather than 62.1371. That trailing 4 is the same fault behind 0.1 + 0.2 = 0.30000000000000004, and it is why long chains of conversions drift. Prism works in exact rational arithmetic, so 1.609344 is held as the fraction it actually is, and 5 miles converts to 8.04672 km with nothing hiding in the fifteenth decimal place. For the digits a display cannot show at all, see what E means on a calculator.
Or let the unit converter hold the factor
Prism converts km, miles, meters, feet, yards and nautical miles in the length category, which is free, along with weight and temperature. No ads, no subscription. See the app.