Surface Gravity Calculator

Surface gravity is the gravitational acceleration felt by an object resting at the surface of a planet, moon or star. It determines how much an object weighs there and how fast it accelerates when dropped. It is quoted in metres per second squared, the same units as any acceleration.

It is g = G x M / r squared, where G is the gravitational constant (6.674e-11 N m^2 / kg^2), M is the body's mass in kilograms and r is its radius in metres. Gravity scales directly with mass and falls with the square of the radius, so doubling the radius cuts gravity to a quarter.

Using Earth's mass and mean radius of 6.371e6 m, and ignoring rotation, the formula returns about 9.8195 m/s^2. The textbook standard of 9.80665 m/s^2 is slightly lower because it includes the centrifugal reduction from Earth's spin and an average over latitude and terrain.

It is the surface gravity divided by 9.80665 m/s^2, Earth's standard gravity, so it expresses the result in g units. A value of 1 g feels like Earth, the Moon comes out near 0.17 g, and Jupiter is about 2.64 g, meaning a weight there would read more than two and a half times its Earth value.

Yes. It treats the body as a uniform sphere and uses a single mean radius, which is why it works from the centre outward. Real planets bulge at the equator and vary in density, so actual measured gravity differs slightly by location and is lower at the equator than at the poles.

No. The acceleration is identical for a feather and a boulder, so both fall at the same rate in a vacuum. Only the mass and radius of the planet, moon or star set the value; the falling object's own mass cancels out.

With the Moon's mass and radius the tool gives about 1.623 m/s^2, roughly one sixth of Earth's gravity, or 0.17 g. That low pull is why Apollo astronauts could bound across the lunar surface in their heavy suits.

No. The engine expects SI units, mass in kilograms and radius in metres, because the gravitational constant is defined in those units. Convert first: 1 kilometre is 1000 metres, and note that many sources list planet radii in kilometres, so multiply by 1000 before entering them.

No. The calculation is a single formula evaluated locally in your browser, so the mass and radius you type never leave your device. The tool also keeps working offline once the page has loaded.