What is the approximate diameter of Mars in kilometers?

Mars has an approximate mean diameter of 6,779 kilometres. That makes the Red Planet about half the diameter of Earth, whose mean diameter is roughly 12,742 kilometres.

Mars is smaller because it formed from less material in the early Solar System than Earth. Its smaller size also contributes to its weaker gravity—about 38% of Earth’s surface gravity—and its inability to retain a thick atmosphere over geological time. Today, the Martian atmosphere is extremely thin and composed mostly of carbon dioxide.

The number 6,779 kilometres is a mean diameter, meaning it represents an average across the planet. Mars is not a perfect sphere: its rotation causes a slight difference between its equatorial and polar dimensions. Its equatorial diameter is a little larger than its polar diameter.

Mars’s size helps explain several familiar comparisons. Its surface area is approximately comparable to the total dry land area of Earth, despite Mars having much less volume and mass. Its two moons, Phobos and Deimos, are tiny by comparison—only tens of kilometres across.