One international mile is exactly 1.609344 kilometres. This is not an approximation — it has been the exact legal definition since 1959, when the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa jointly agreed to standardise the international yard and pound.
For most practical purposes, 1 mile ≈ 1.6 km is accurate enough. The rounding error is just 0.6%.
Or equivalently: miles = kilometres × 0.621371
| Miles | Kilometres | Context |
|---|---|---|
| 0.25 | 0.40 | Short walk / quarter mile |
| 0.5 | 0.80 | Half mile |
| 1 | 1.609 | One mile |
| 3.1 | 5.0 | 5K run |
| 6.2 | 10.0 | 10K run |
| 13.1 | 21.1 | Half marathon |
| 26.2 | 42.2 | Full marathon |
| 60 | 96.6 | 60 mph in km/h |
| 70 | 112.7 | UK motorway speed limit in km/h |
| 100 | 160.9 | 100 miles |
| Kilometres | Miles | Context |
|---|---|---|
| 1 | 0.621 | 1 km |
| 5 | 3.107 | 5K run |
| 10 | 6.214 | 10K run |
| 21.1 | 13.1 | Half marathon |
| 42.2 | 26.2 | Full marathon |
| 100 | 62.1 | 100 km |
| 120 | 74.6 | European motorway limit in mph |
Here's a remarkable shortcut: the ratio of miles to kilometres (1 : 1.609) is very close to the golden ratio (1 : 1.618). And consecutive Fibonacci numbers (1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89...) approach the golden ratio as you go further along the sequence.
This means: consecutive Fibonacci numbers give you approximate mile/km conversions.
This works beautifully for running distances and road trip estimates — no calculator needed.
Runners often need to convert pace (time per distance unit) as well as distance. The conversion works in reverse:
Popular pace benchmarks converted:
| Min/mile | Min/km | 5K finish time |
|---|---|---|
| 7:00 | 4:21 | 21:45 |
| 8:00 | 4:58 | 24:51 |
| 9:00 | 5:35 | 27:57 |
| 10:00 | 6:13 | 31:04 |
| 12:00 | 7:27 | 37:17 |
The UK officially adopted the metric system in the 1970s, and most measurements — weight, height, volume — have transitioned to metric. Road signs, speed limits, and road distances are one of the last holdouts, remaining in miles. The government has periodically discussed conversion, but the cost (estimated at over £1 billion to replace all road signs) and public resistance have kept miles on British roads.
Meanwhile, running events, athletics tracks, cycling, and fitness apps all use kilometres — creating the situation where a British runner might train for a "10K" and drive to the start line past speed limit signs in miles.
Exactly 1.609344 kilometres by international definition since 1959. For quick calculations, 1 mile ≈ 1.6 km introduces only a 0.6% error — acceptable for most everyday purposes.
5 kilometres is 3.107 miles — typically rounded to 3.1 miles. Other common running distances: 10K = 6.21 miles, half marathon (21.1km) = 13.1 miles, full marathon (42.195km) = 26.219 miles (always quoted as 26.2).
The UK metricated most measurements in the 1970s but never changed road signs — largely due to the cost (£1bn+ estimate) and public attachment to miles. Running events, however, are internationally standardised in metric distances (5K, 10K, etc.) and have used kilometres globally since long before the UK metrication debate.
Multiply by 1.609344. So 60 mph = 96.56 km/h, and the UK's 70 mph motorway limit = 112.65 km/h. Quick estimate: multiply by 1.6. To convert back, divide by 1.609 or multiply by 0.621.