Miles to Kilometers in Your Head: The 1.6 Rule and the Fibonacci Trick
October 21, 2026
One mile is exactly 1.609344 kilometers, so multiply miles by 1.609 for an accurate result: 10 miles is 16.09 km and a 26.2-mile marathon is 42.16 km. For quick estimates, many people multiply by 1.6, or add half and then a tenth: 10 + 5 + 1 = 16. A neat trick uses the Fibonacci sequence, because the ratio of consecutive terms approaches 1.618, very close to 1.609.
Skip the math and get your answer instantly:
Open the Length Converter →The Fibonacci Shortcut
The Fibonacci numbers are 1, 1, 2, 3, 5, 8, 13, 21, 34, 55. Each term divided by the one before it is about 1.618. So if a distance in miles is a Fibonacci number, the next Fibonacci number is approximately the same distance in kilometers: 5 miles ≈ 8 km, 8 miles ≈ 13 km, 13 miles ≈ 21 km, 21 miles ≈ 34 km.
Check: 5 × 1.609 = 8.05, 8 × 1.609 = 12.87, 13 × 1.609 = 20.92. Each is off by less than 1%. For other distances, split them into Fibonacci pieces: 10 miles = 8 + 2 gives 13 + 3 = 16 km.
Common Distances
Running and driving distances are frequently quoted in miles in the US and UK and in kilometers elsewhere.
| Distance | Miles | Kilometers |
|---|---|---|
| 5K race | 3.11 | 5.00 |
| 10K race | 6.21 | 10.00 |
| Half marathon | 13.11 | 21.10 |
| Marathon | 26.22 | 42.20 |
| Highway limit 65 mph | 65 | 104.6 (km/h) |
| Speed limit 50 mph | 50 | 80.5 (km/h) |
Going From Kilometers to Miles
Multiply by 0.621, or divide by 1.609. A rough mental rule is to take 60% of the kilometers and add a little: 100 km is about 62 miles. The same Fibonacci pattern works in reverse, so 21 km is about 13 miles.
Remember that speed conversions use the same factor: 100 km/h is about 62 mph, and 60 mph is about 97 km/h.
Frequently Asked Questions
How many kilometers are in a mile?
1.609344 exactly, or about 1.61.
Why does the Fibonacci trick work?
The ratio of consecutive Fibonacci numbers approaches the golden ratio, 1.618, which is close to the 1.609 km-per-mile factor.
How far is a marathon in kilometers?
42.195 km, or 26.219 miles.
What is 100 km/h in mph?
About 62 mph.