What is the Moon's mean radius in kilometers?

The Moon’s mean radius is 1,737.4 kilometers.

“Mean radius” is a volume-equivalent measurement: it is the radius of a perfect sphere having the same volume as the Moon. The Moon is not perfectly spherical, so astronomers also quote separate equatorial and polar radii. NASA lists its volumetric mean radius as 1,737.4 km and its equatorial radius as about 1,738.1 km.

This distinction explains why nearby numbers can differ slightly. Earth’s mean radius is about 6,371 km, while Mars’s mean radius is about 3,389.5 km; both are plausible-looking distractors but describe different worlds. The Moon’s mean diameter is therefore about 3,474.8 km.

The measurement is an average over a body with mountains, craters, basins, and a slight equatorial bulge. It is also much smaller than Earth: the Moon’s mean radius is about 27.27% of Earth’s mean radius, a useful comparison when estimating lunar scale.