Miles to kilometres

The formula

kilometres = miles × 1.609344
miles = kilometres × 0.62137119

Exact by definition. A statute mile is 1,760 yards, and a yard has been exactly 0.9144 metres since 1959 - so 1,760 × 0.9144 = 1,609.344 metres.

Quick reference

Miles	km	km	Miles
1	1.609	1	0.621
2	3.219	5	3.107
5	8.047	10	6.214
10	16.093	20	12.427
20	32.187	50	31.069
26.2	42.195	100	62.137
50	80.467	200	124.27
100	160.93	500	310.69

Speed limits

mph	km/h	Typically
20	32	Residential, school zone
30	48	Urban roads (UK)
40	64	Urban arterial
55	89	US rural highway
60	97	UK single carriageway
70	113	UK motorway
75	121	US interstate

Road signs are rounded to convenient numbers rather than converted exactly. A 30 mph limit is signed as 50 km/h across the border, not 48.

Running distances

Race	Miles	km
5K	3.107	5
10K	6.214	10
Half marathon	13.109	21.0975
Marathon	26.219	42.195
50 miles	50	80.467
100K	62.137	100

The marathon’s odd distance dates from the 1908 London Olympics, where the course was extended so the race could finish in front of the royal box. It was formally standardised at 42.195 km in 1921 and has stayed there since.

The Fibonacci trick

Consecutive Fibonacci numbers (1, 2, 3, 5, 8, 13, 21, 34, 55, 89) convert miles to kilometres surprisingly well:

Miles	Fibonacci says	Actual
3	5 km	4.83
5	8 km	8.05
8	13 km	12.87
13	21 km	20.92
21	34 km	33.80
34	55 km	54.72

The ratio of consecutive Fibonacci numbers converges on the golden ratio, 1.618, which happens to sit within 0.6% of the true conversion factor of 1.609. It is a coincidence, and a genuinely useful one.

Where miles are still used

Only the United States, the United Kingdom and a small number of other countries - including Liberia and Myanmar - use miles for road distances. The UK is a notable hybrid: road signs and speedometers are in miles, while almost everything else in British life is metric.

Nautical miles and knots remain universal in aviation and maritime navigation, regardless of the country. Their advantage is geometric rather than cultural: one nautical mile is one minute of arc along a meridian, which makes chart work straightforward in a way no metric unit matches.