A round trip that returns exactly where it started
Converting 1,609.344 meters to feet gives exactly 5,280 feet — not 5,279.9 or 5,280.1, but precisely 5,280. Converting 5,280 feet back to meters gives exactly 1,609.344 meters again. This isn't a coincidence of rounding; it's because 1,609.344 meters is the international mile's exact legal definition, and 5,280 feet is the same distance in a different unit.
Where the exactness comes from
In 1959, the International Yard and Pound Agreement fixed the foot at exactly 0.3048 meters — not an approximation to be refined later, but a permanent definition. Since a mile has always been exactly 5,280 feet, that makes 1 mile exactly 5,280 × 0.3048 = 1,609.344 meters, with no rounding at any step of that chain.
Contrast: a real-world distance that isn't a round number
The marathon's distance, 42.195 kilometers, is not an exact conversion of some round imperial number — it converts to 26.218757 miles, an unremarkable decimal, because 42.195 km was chosen historically for reasons unrelated to the mile. The commonly quoted "26.2 miles" is a convenient rounding of that result, not a second, equally exact definition — which is the opposite situation from meters-to-feet or miles-to-kilometers, where the conversion factor itself is the definition.
The practical takeaway
When a conversion factor comes from an internationally fixed standard (like the foot or the mile), converting back and forth always returns the exact original number, with no accumulating error. When a figure has already been rounded before you convert it — like a marathon's distance quoted as "26.2 miles" instead of the exact 42.195 km — expect the result to drift slightly from the true original value, as the companion article on kilometers to miles demonstrates directly.