What is the Moon's diameter in kilometers?

The Moon’s diameter is approximately 3,474 kilometers.

That measurement refers to the Moon’s average width from one side to the other through its center. The Moon is not a perfect sphere: its equatorial diameter is slightly larger than its polar diameter, but 3,474 km is the standard rounded figure used in general astronomy and trivia.

The Moon is about one-quarter of Earth’s diameter. That makes it unusually large compared with the planet it orbits, which is one reason the Earth–Moon system is often described as an important planetary pair rather than a typical planet-and-tiny-satellite arrangement.

The Moon’s diameter also helps explain its appearance in the sky. Although it is much smaller than the Sun, it is also much closer to Earth. Their apparent sizes are therefore similar, allowing the Moon to cover the Sun during a total solar eclipse. By comparison, Earth’s diameter is about 12,742 km.