The conversion factor
One kilometer equals exactly 0.621371 miles. For 5 kilometers: 5 × 0.621371 = 3.106856 miles. For a single kilometer, the factor itself is the answer: 0.621371 miles.
A famous real-world example, the marathon distance
A marathon is officially 42.195 kilometers. Converting exactly: 42.195 × 0.621371 ≈ 26.218757 miles — not the "26.2 miles" figure commonly quoted, which is itself a rounded approximation of the exact conversion, not the marathon's actual definition.
Converting the other direction, miles to kilometers
Going from miles to kilometers uses the reciprocal factor, 1.609344. Converting the popular rounded "26.2 miles" back gives 26.2 × 1.609344 ≈ 42.164813 kilometers — noticeably short of the marathon's true 42.195 km, which shows why relying on a pre-rounded figure and converting it again compounds the rounding error.
Why the marathon distance reveals the rounding gap
The marathon is defined in kilometers (42.195 km) by the sport's official rules, and its mile equivalent is simply whatever that converts to — 26.218757 miles, not a round number. "26.2 miles" is a convenient, memorable approximation for everyday use, but converting distances precisely means always working from the original, unrounded figure rather than an already-rounded one.