The surface Area of the earth is 510,072,000 km. Surface area of a sphere is the area of that same sphere flattened out.
A= pi(r^2)
510,072,000/pi = 162,360,960.3
sqrt(162,360,960.3) = 12,742.09403
r= 12,742.09403 x 2 = 25484.18806
Pretty close, right?
and circumference is just that times pie. Just looking at the diameter now, it's definitely around 78k.