The exact factor, and a shortcut you can do in your head
A mile is defined as exactly 1.609344 km, so one kilometre is 0.621371 miles. For a rough figure, multiply the kilometres by 0.6; for a closer one, divide by 1.6. 100 km ÷ 1.6 = 62.5 mi, against the exact 62.1.
The Fibonacci trick actually works
Consecutive Fibonacci numbers (5, 8, 13, 21, 34, 55, 89…) sit almost exactly one conversion factor apart, because the golden ratio 1.618 is within 0.5% of 1.609. So the next number up converts miles to kilometres, and the previous one goes back.
- Miles to km, step up. 5 mi → 8 km, 8 mi → 13 km, 13 mi → 21 km, 55 mi → 89 km.
- Km to miles, step down. 8 km → 5 mi, 21 km → 13 mi, 89 km → 55 mi.
- For any number, split it. 100 mi = 89 + 8 + 3, so 144 + 13 + 5 = 162 km (exact: 160.9).
Quick reference — common distances
| Kilometres | Miles | What it is |
|---|---|---|
| 1 km | 0.621 mi | — |
| 5 km | 3.107 mi | 5K run |
| 10 km | 6.214 mi | 10K run |
| 21.097 km | 13.109 mi | half marathon |
| 42.195 km | 26.219 mi | marathon |
| 100 km | 62.137 mi | ultramarathon |
Speed limits convert with the same factor: 100 km/h is 62 mph, 120 km/h is 75 mph — useful before you drive a hire car in another country.
Common questions
How many miles are in a kilometre?
One kilometre is 0.621371 miles, or a little over three fifths of a mile. The factor is exact: a mile is defined as 1.609344 km.
How do I convert km to miles in my head?
Multiply by 0.6 for a quick low estimate, or divide by 1.6 for a closer one. 80 km divided by 1.6 is 50 miles (the exact answer is 49.7).
What is a marathon in miles?
A marathon is 42.195 km, which is 26.219 miles. A half marathon is 21.097 km, or 13.109 miles.
