Under the Wiki page it states,

We can use Eratosthenes' shadow experiment to determine the diameter of the flat earth.

Syene and Alexandria are two North-South points with a distance of 500 nautical miles. Eratosthenes discovered through the shadow experiment that while the sun was exactly overhead of one city, it was 7°12' south of zenith at the other city.

7°12' makes a sweep of 1/25th of the FE's total longitude from 90°N to 90°S (radius).

Therefore we can take the distance of 500 nautical miles, multiply by 25, and find that the radius of the flat earth is about 12,250 nautical miles. Doubling that figure for the diameter we get a figure of 25,000 miles.

These figures do on the face of it work out correct. But so too do these figures if you hypothesize (as Eratosthenes did) that the Earth is a sphere. Or a least round but then he would have had to have been on the edge of the Earth to detect any curvature of a flat Earth. He based his calculation on the fact that 7°12' (=7.2°) is 1/50 of a complete circle (7.2° x 50 = 360°) and using the known distance from Alexandria to Syene he worked out that 50x500miles meant that the Earth must have a 25,000 mile circumference. Based on the modern figure according to RET that is very close to the right answer. A very good result for Eratosthenes time.

The FE Wiki version of Eratosthenes experiment however gives 25,000 miles as the diameter of the Earth. Using Eratosthenes figure for the circumference, based on his assumption of a round Earth and dividing that by pi gives a diameter of 7957 miles. That is the accepted value according to RET.

Searching through the Internet at length and several books I have on historical astronomy, all accounts of Eratosthenes experiment appear to agree that from his observations and measurements he deduced that the Earth was spherical with a 25,000 mile circumference. There is no other mention (that I can find at least) of a 12,250 mile radius or 25,000 mile diameter as the FE Wiki version seems to suggest. I'm sure that Eratosthenes was aware of Aristotles assumptions but he drew his own conclusions from his own observations and measurements and they led him to reach a figure of 25,000 miles for the circumference of the Earth.