Miles vs Kilometers (mi vs km)

The kilometer (km) is 1000 meters, part of the metric system used by most of the world. The mile (mi) is a US customary and imperial unit of 5280 feet.

Since the international yard and pound agreement of 1959, one foot is exactly 0.3048 m, so one mile is exactly 5280 × 0.3048 = 1609.344 meters. This is the international mile used on roads and maps.

• Miles to kilometers: multiply by 1.609344.
• Kilometers to miles: divide by 1.609344, or multiply by about 0.621371.
• Example: 26 mi × 1.609344 = 41.8429 km.
• Example: 100 km ÷ 1.609344 = 62.1371 mi.

To estimate kilometers, multiply miles by 1.6. This comes out about 0.6% low: 50 mi gives 80 km, while the exact value is 80.4672 km.

To estimate miles, multiply kilometers by 0.6. This one is less accurate, about 3.4% low: 100 km gives 60 mi, while the exact value is 62.1371 mi. Multiplying by 0.62 instead brings the error down to about 0.2%.

Most road races are named by their distance in kilometers, even in the US. The marathon is 42.195 km and the half marathon is half of that, 21.0975 km.

Speed converts with the same factor: 1 mph is exactly 1.609344 km/h. Road signs in the US and the UK show miles per hour; most other countries use kilometers per hour.

When you drive abroad, check which unit the signs and your speedometer use. Many speedometers show both scales.

The nautical mile (nmi) is used at sea and in aviation. It is exactly 1852 meters, about 1.15078 ordinary miles. A knot is one nautical mile per hour, about 1.852 km/h.

Nautical miles and knots are still standard in shipping and aviation, while land distances use statute (ordinary) miles or kilometers. Do not mix the two miles. One nautical mile is about 1.15 times as long as an ordinary mile, so the same number means a longer distance at sea.

• Dividing when you should multiply. A distance in kilometers is always a larger number than the same distance in miles.
• Using a nautical mile factor for a road distance, or the other way around.
• Rounding the factor to 1.6 in a calculation that needs precision, such as a measured course or a fuel log.
• Forgetting that speed conversions use the same factor: a speed in mph times 1.609344 gives km/h.