The Exact km to Miles Formula
To convert kilometers to miles, multiply by 0.621371. To convert miles to kilometers, multiply by 1.60934 (or divide by 0.621371). These are exact values defined by the international foot agreement of 1959, which fixed 1 yard = 0.9144 meters, making 1 mile = 1,609.344 meters = 1.609344 km exactly. For quick mental math when you don't have a calculator: 5 km ≈ 3.1 miles (actual: 3.107). The rough rule 'divide by 1.6' is close enough for most purposes — 10 km ÷ 1.6 = 6.25 miles (actual: 6.214). The Fibonacci approximation also works: consecutive Fibonacci numbers approximate the ratio (e.g., 5/8 = 0.625, 8/13 ≈ 0.615) — so 8 km ≈ 5 miles and 13 km ≈ 8 miles are both close.
km to Miles Conversion Table (Key Values)
Here are the most commonly needed conversions: 1 km = 0.621 mi | 5 km = 3.107 mi | 10 km = 6.214 mi | 15 km = 9.321 mi | 20 km = 12.427 mi | 21.1 km = 13.109 mi (half marathon) | 25 km = 15.534 mi | 42.195 km = 26.219 mi (marathon) | 50 km = 31.069 mi | 100 km = 62.137 mi. For running events specifically: a 5K is 3.107 miles, a 10K is 6.214 miles, a half marathon is 13.1 miles (21.0975 km), and a full marathon is 26.219 miles (42.195 km). These numbers are fixed by the athletics governing bodies and do not round.
When to Use km vs Miles
Kilometers are the global standard for road distances in almost every country except the United States, Liberia, and Myanmar. UK road signs use miles but the metric system is otherwise dominant. Aviation and maritime navigation use nautical miles (1 nmi = 1.852 km), a separate unit based on arc-minutes of latitude. For running and fitness tracking, most apps allow switching between km and miles. Garmin, Strava, and Apple Health all support both. If you are comparing pace, km/min and min/mile are the respective standard units — a 5:00/km pace equals approximately 8:03/mile.