The Formula
miles = km × 0.621371
A kilometer is about six tenths of a mile (the mile is exactly 1,609.344 m). The example above: a 10 km run is 6.21 miles.
Two Mental Tricks
Multiply by 0.6 for a quick estimate: 100 km ≈ 60 mi (exact: 62.1). Or use the elegant Fibonacci trick: consecutive Fibonacci numbers (5, 8, 13, 21, 34…) approximate the km↔mile ratio, so 8 km ≈ 5 mi, 13 km ≈ 8 mi, 21 km ≈ 13 mi — the golden ratio (1.618) happens to sit almost exactly on the km/mile factor (1.609).
Where This Conversion Shows Up
Running owns this conversion: the 5K is 3.1 miles, the 10K is 6.2, a half marathon (21.1 km) is 13.1, and the marathon (42.195 km) is the famous 26.2. Driving abroad: US/UK speed limits in mph vs km/h — 100 km/h is 62 mph, and a 65 mph interstate limit is 105 km/h. Cars: an odometer’s 100,000 miles equals 160,934 km, worth knowing when comparing used-car listings across markets.
Frequently Asked Questions
How many miles is 5 km?
3.11 miles — the standard “5K” race distance, roughly a 30-minute jog or 50-minute walk for most people.
Is 100 km/h fast in mph?
100 km/h = 62.1 mph — a typical highway speed. For quick speed conversions the same 0.6 factor applies, since the “per hour” cancels out.
Why do the US and UK use miles?
Both kept the statute mile through partial metrication — the UK converted most measures but left road distances and speeds in miles. Nearly every other country signs roads in kilometers.
What's the difference between a mile and a nautical mile?
The statute mile here is 1.609 km; the nautical mile used in aviation and shipping is 1.852 km (one minute of latitude). A ship doing 20 knots covers 37 km/h.