How to Convert Km to Miles (Exact Factor, Not the Rounded 0.62)
To convert kilometers to miles, multiply by 0.621371. So 5 km is 3.10686 miles, 10 km is 6.21371 miles, and a marathon’s 42.195 km comes out to 26.2188 miles. That factor isn’t a round number chosen for convenience. It comes from the exact legal length of a mile in meters, and knowing where it comes from explains why two calculators can give you slightly different answers.
Why 0.621371, and not just 0.62
The 1959 International Yard and Pound Agreement, signed by the US, the UK, and several other countries, fixed the yard at exactly 0.9144 meters. A mile is 1,760 yards, which makes it exactly 1,609.344 meters, a legal definition rather than an estimate. Flip that ratio to get miles from kilometers and you land on 1 km = 0.6213711922… miles, a decimal that never ends. 0.621371 is that figure truncated to six places, accurate enough for almost any real use, but still a rounded stand-in for a longer exact number.
That’s why a GPS watch, a spreadsheet, and a road sign can each show a slightly different mile figure for the same distance. They’re not wrong, they’re just keeping a different number of decimal places of the same underlying ratio.
Worked examples: race distances
Runners hit this conversion constantly, since race distances are set in one system and often discussed in the other.
| Distance | Kilometers | Miles (×0.621371) |
|---|---|---|
| 5K | 5 km | 3.10686 mi |
| 10K | 10 km | 6.21371 mi |
| Half marathon | 21.0975 km | 13.1094 mi |
| Marathon | 42.195 km | 26.2188 mi |
A marathon is defined as exactly 42.195 km, not a round 42, which is why “26.2 miles” shows up on so many finisher shirts instead of a cleaner number. If you’re comparing a training plan written in kilometers against a US-based app that logs everything in miles, this is the table to keep open.
Calculate with your own numbers
Enter any distance in kilometers or miles below and get the converted value instantly, using the same 0.621371 factor used throughout this guide.
Common mistakes and edge cases
Treating a nautical mile like a regular mile. A nautical mile, used in aviation and at sea, is 1.852 km, not 1.609344 km. It’s roughly 15% longer than the everyday statute mile. A 40-nautical-mile leg on a marine chart is 40 × 1.852 = 74.08 km, which converts to 46.03 statute miles, not the 40 miles it might look like at a glance if you assume “nautical mile” and “mile” are interchangeable. Mixing the two on a flight plan or a boating route is the kind of error that changes fuel calculations, not just trivia.
Confusing an English mile with a Scandinavian “mil.” In Sweden and Norway, “mil” is still a common everyday unit, and it means 10 km, not 1.609 km. If someone tells you a hike is “2 mil,” that’s 20 km, or about 12.4 English miles, over six times longer than “2 miles” would suggest. The words look identical; the distances aren’t even close.
Rounding 0.62 instead of 0.621371 once the numbers add up. For a single run, the difference is invisible. Log a whole training year in kilometers and convert it with the shortcut and it starts to show: 2,000 km × 0.621371 = 1,242.742 miles, while 2,000 km × 0.62 gives 1,240 miles, a gap of 2.7 miles. Small per session, real once you’re summing months of mileage for a coach or an app that expects a specific unit.
A mental shortcut that actually works: Fibonacci numbers. The ratio between consecutive Fibonacci numbers (3, 5, 8, 13, 21, 34, 55…) converges toward the golden ratio, about 1.618, which happens to sit remarkably close to the real km-per-mile factor of 1.609344. That means 5 miles is close to 8 km (actual: 8.047), 8 miles is close to 13 km (actual: 12.874), and 13 miles is close to 21 km (actual: 20.921), each within about half a percent. It’s not a coincidence you can build a formula from, but it’s a genuinely useful trick for a fast estimate when you don’t have a calculator, and it’s exactly why a half marathon’s 21 km lines up so neatly with “about 13 miles.”
Frequently asked questions
How many miles is 1 km? 1 kilometer equals 0.621371 miles. The factor comes from the mile’s legal definition of exactly 1,609.344 meters, set by the 1959 International Yard and Pound Agreement.
How many miles is a 10K race? A 10K is 10 kilometers, which converts to 6.21371 miles, commonly rounded to “about 6.2 miles.”
How many miles is a marathon? A marathon is 42.195 km, which equals 26.2188 miles. The extra 0.195 km beyond a round 42 is why the distance is always quoted with a decimal in both units.
Is a nautical mile the same as a regular mile? No. A nautical mile is 1.852 km, versus 1.609344 km for the everyday statute mile used on land. That makes a nautical mile roughly 15% longer, an important distinction for aviation and marine navigation where the two units aren’t interchangeable.
Is a Scandinavian “mil” the same as an English mile? No, and the overlap in spelling makes this an easy trap. A Swedish or Norwegian “mil” equals 10 km, over six times longer than the 1.609 km English mile. Always confirm which unit someone means before converting a distance they’ve given you in “mil.”