May 2, 2026
How Big Is Mars Compared to Earth?
Mars is roughly half the diameter of Earth: 6,792 km across at the equator, versus Earth's 12,756 km. By mass, the gap is even larger — Mars has only about 10.7% of Earth's mass. But Mars is also surprisingly large in the places that count for human exploration: its surface area is about the same as Earth's dry land, and its low gravity makes Olympus Mons the tallest volcano in the solar system. Below: every dimension worth knowing, in NASA-cited numbers.
Photo: Unsplash
Mars vs. Earth: The Numbers
| Dimension | Mars | Earth | Mars / Earth |
|---|---|---|---|
| Equatorial diameter | 6,792 km | 12,756 km | 53% |
| Equatorial radius | 3,396 km | 6,378 km | 53% |
| Mass | 6.417 × 10²³ kg | 5.972 × 10²⁴ kg | 10.7% |
| Volume | 1.6318 × 10¹¹ km³ | 1.0832 × 10¹² km³ | 15.1% |
| Surface area | 144.8 M km² | 510.1 M km² | 28% |
| Surface gravity | 3.72 m/s² | 9.81 m/s² | 38% |
| Day length | 24 h 37 min | 23 h 56 min | ~103% |
| Year length | 687 Earth days | 365.25 days | ~188% |
| Mean distance from Sun | 227.9 M km | 149.6 M km | ~152% |
| Moons | 2 (Phobos, Deimos) | 1 | 2× |
Source: NASA Planetary Fact Sheet. Mars is consistently smaller than Earth — but a Martian day (the "sol") is just 41 minutes longer than ours, which is one of the reasons it remains a plausible target for human exploration.
Mars Is About Half the Diameter of Earth
Mars's equatorial diameter is 6,792 km. Earth's is 12,756 km. So a one-dimensional "how wide" comparison makes Mars about 53% of Earth.
But the volume comparison is much steeper. Volume scales with the cube of radius, so a planet with half the radius has only one-eighth the volume. Mars is closer to 15% of Earth's volume — meaning you could fit about 6.5 Marses inside Earth. The planet is not just shorter; it is dramatically less material.
Mars Is Bigger Than the Moon (Yes, Really)
One of the most common misconceptions is that Mars is similar in size to Earth's Moon. The bright reddish dot Mars makes in the night sky encourages it. The reality:
Mars diameter: 6,792 km. Moon diameter: 3,474 km. Mars is roughly twice the Moon's diameter. By volume, Mars is about 7 times larger than the Moon. By mass, Mars is about 9 times the Moon's mass.
That gap matters for exploration. Mars has enough mass to retain a thin atmosphere, drive seasonal weather patterns, and host both polar ice caps and recurring slope features. The Moon, with one-ninth the mass, holds essentially no atmosphere at all.
Mars vs. the Other Planets
Among the 8 planets, Mars is the second-smallest. Only Mercury (4,879 km diameter) is smaller. Here is the full size ranking, smallest to largest:
Mercury (4,879 km) → Mars (6,792 km) → Venus (12,104 km) → Earth (12,756 km) → Neptune (49,528 km) → Uranus (51,118 km) → Saturn (120,536 km) → Jupiter (142,984 km).
Mars is in a different league from the giants. Jupiter alone has more than 5,700 times Mars's volume. For more on planetary scale, see our guide to comparing planets at scale and how big the solar system actually is.
Mars's Surface Area: Equal to Earth's Dry Land
This is the comparison that most surprises students. Mars's total surface area is 144.8 million km². Earth's total surface area is 510.1 million km² — but 71% of that is ocean. Earth's dry land covers about 148.9 million km².
So Mars and Earth's land have roughly the same area. A future explorer landing on Mars would have access to as much continental ground as exists across all of Earth's continents combined — without an ocean breaking up the map. That fact does much to explain why Mars looms so large in the imagination of long-term human spaceflight planning.
Gravity: 38% of Earth's
Surface gravity on Mars is about 3.72 m/s², compared to Earth's 9.81 m/s². That is roughly 38% of Earth gravity. Practical consequences:
A 100 kg astronaut weighs about 38 kg-force on Mars. A jump that gets you 0.5 m off the ground on Earth would clear about 1.3 m on Mars. A meter-tall fall hits the ground at about half the velocity. Walking, running, and lifting objects all become roughly 2.6 times easier in raw effort terms — though the lower gravity also creates problems (bone density loss, fluid redistribution) that remain unsolved for long-duration missions.
Why Mars Is So Small: The Grand Tack
By the standards of solar system formation, Mars looks too small. Models predict that the rocky material in its orbital region should have produced a planet roughly the size of Earth. Instead, Mars is barely larger than Mercury.
The leading explanation is the Grand Tack hypothesis, proposed in 2011. In the early solar system, Jupiter migrated inward toward the Sun until it reached about 1.5 AU (roughly Mars's current distance), then was pulled back outward by gravitational interaction with Saturn. Jupiter's inward swing scattered most of the rocky material that would have built up Mars to a larger size, leaving Mars as a "stunted" planet. Earth and Venus formed in regions where less material had been scattered, so they grew to full planetary size.
For more on how planetary formation produces such different worlds, read our explainer on why planets are round and our overview of all 8 planets in order.
The Olympus Mons Paradox
Mars is small. Yet it hosts the tallest known volcano in the solar system. Olympus Mons rises about 21.9 km above the surrounding plains — roughly 2.5 times the height of Mt. Everest above sea level — and spans approximately 600 km at its base. You could not stand at the edge of Olympus Mons and see the summit; the curvature of Mars itself would put it below the horizon.
How does a small planet host such a giant feature? Two reasons. First, low surface gravity (38% of Earth's) means a volcanic structure can grow taller before collapsing under its own weight. Second, Mars has no plate tectonics — the crust does not move over its mantle hot spot the way Earth's does. Earth's Hawaiian Islands are a chain of volcanoes precisely because the Pacific Plate moves over a fixed hot spot, building one volcano at a time and then sliding it away. On Mars, the same hot spot just keeps building the same volcano for billions of years.
Explore Mars and the Solar System
Want more on Mars specifically? Read Mars Facts: Everything We Know About the Red Planet for the full fact file (atmosphere, water, missions, Olympus Mons, Valles Marineris). Or jump to the Mars Fast Facts page for a structured data view.
Test your knowledge of all 8 planets in the interactive Solar System game, or play the daily GeoProwl game where every clue is built from real geographic and scientific data.