What is the diameter of the earth's moon?

The Moon isn't a sphere, so it doesn't have a single diameter.

The moon is an oblate spheroid — a sort of "squashed sphere" in which the equatorial radius is greater than the polar radius.

The equatorial diameter of the Moon is 3 476.28 km. And the polar diameter of the Moon is 3 471.94 km.

So, the Moon’s diameter from side to side is 4.34 km more than its diameter from pole to pole.