**How large is the ancient Greek length measure
“stadium” ( plural: “stadii”) in today’s metres?
**

Know: Circumference of earth is 240 000 stadii as measured by
Eratosthenes.

Definition: Metre is one ten-millionth the distance from the Pole to
the equator through Paris. Therefore the circumference of the earth is
40 000 000 m or 4 × 10^{7} m.

(240 000 stadii) / (4×10^{7 }m) = 1

240 000 stadii = 4×10^{7}m

1 stadium = (4×10^{7}m) / (2.4 × 10^{5} )^{
}= 1.67 × 10^{2} m

= 167 m

= 0.167 km

that's about 0.1 mile

Trivia:

The distance from Alexandria to Syene is about 500 miles and it was
measured by walking the distance and counting the steps (pacing).

The value of the earth’s circumference determined by Eratosthenes (and
his indefatigable assistant) was within 5 % of the correct value!

There is still some debate about the exact value of the ancient stadium.