How big is the moon compared to the earth The Moon is much smaller than Earth. Earth's diameter is approximately 12,742 kilometers (7,918 miles), while the Moon's diameter is about 3,474 kilometers (2,159 miles). This means the Moon is roughly one-quarter the size of Earth in terms of diameter and about 1/6th of Earth's mass. This significant difference in size is why the Moon has lower gravity and lacks a substantial atmosphere compared to Earth.
