KM to Miles Formula Explained
Where 0.6213711922 actually comes from, how to derive it, and how wrong the popular divide-by-1.6 shortcut really is.
Where 0.621371 comes from
The 1959 international yard and pound agreement fixed 1 yard as exactly 0.9144 metres. A statute mile is 1,760 yards, so 1 mile = 1,760 x 0.9144 = 1,609.344 metres exactly. One kilometre is 1,000 metres, therefore 1 km = 1,000 / 1,609.344 = 0.6213711922 miles.
The two formulas
| Direction | Formula | Example |
|---|---|---|
| km to mi | mi = km x 0.6213711922 | 100 km = 62.1371 mi |
| mi to km | km = mi x 1.609344 | 62.1371 mi = 100 km |
Mental math and its error
Dividing by 1.6 is the usual shortcut. It is about 0.6 percent off, not 0.02 percent - a commonly repeated mistake. Dividing by 1.609344 is exact but awkward in your head.
| km | Exact miles | km / 1.6 | Difference |
|---|---|---|---|
| 5 km | 3.1069 mi | 3.125 mi | 0.0181 mi |
| 10 km | 6.2137 mi | 6.25 mi | 0.0363 mi |
| 20 km | 12.4274 mi | 12.5 mi | 0.0726 mi |
| 42.195 km | 26.2188 mi | 26.3719 mi | 0.1531 mi |
| 100 km | 62.1371 mi | 62.5 mi | 0.3629 mi |
| 500 km | 310.6856 mi | 312.5 mi | 1.8144 mi |
Common mistakes
- Multiplying miles by 0.621371 - that converts the wrong way; use 1.609344.
- Using the nautical mile (1,852 m) for road distances; road signs use the statute mile.
- Rounding 0.621371 to 0.62 and then reporting four-decimal precision downstream.
Related pages
Frequently Asked Questions
Why is 1 km exactly 0.6213711922 miles?
Because 1 mile is exactly 1,609.344 m by the 1959 international yard agreement, and 1,000 / 1,609.344 = 0.6213711922.
Is dividing by 1.6 accurate?
It is about 0.6 percent off. The page shows the exact error at several distances; use 1.609344 for an exact division.
What is the reverse formula?
km = mi x 1.609344. Multiplying miles by 0.621371 is a common mistake.