Well, man, you're asking a pretty interesting question. The circumference of the earth, if you really dive into it, it's fascinating. The ancient Greeks, dudes like Aristarchus, figured this out without all the tech we have today. So at a certain time of day, a certain time of the year, the sun is directly overhead at noon. If you look down a deep well, you'll see the reflection of the sun directly in it. But if you have a buddy measure the angle of the sun relative to a well like 100 miles away, the angle won't be the same. This difference in angles reveals that the earth is indeed round and allows you to calculate its circumference.

Now, if you want specifics, the earth is about 7,920 miles in diameter. If you imagine drawing a line straight through the earth from the Tropic of Cancer to the Tropic of Capricorn, it would be about this length. To get the circumference, you gotta multiply this diameter by pi (which is about 3.14). So the earth's circumference ends up being somewhere in the ballpark of 24,901 miles.

Of course, these are all average numbers. The earth isn't a perfect sphere, it's more like an oblate spheroid. It's a bit wider at the equator and narrower at the poles because of its spin. But regardless of the specifics, the fact remains that our planet is certainly not flat.