Free worldbuilding planetary gravity calculator. Set planet mass and radius (relative to Earth or absolute) and a character's Earth weight; outputs local surface gravity (g), adjusted weight, vertical-jump height, and narrative fall-damage notes.
Built and maintained by Paul Clark, Redmoon Software · Last verified · Sources
Use when building a sci-fi setting with multiple inhabited worlds. Gravity affects daily life, athletics, combat, and engineering — get the number first.
Real astrophysics calculators want SI units and rotational data. This is a writer's shortcut using Earth-relative inputs.
Fall damage scaled by 1.00× of Earth.
In Earth-relative units, g = M ÷ R². A planet with twice Earth's mass but the same radius has 2g surface gravity.
Local weight = Earth weight × g. Vertical jump height = Earth jump ÷ g.
Above 1.5g, characters tire faster and fall damage increases proportionally.
No. It only computes surface gravity from mass and radius. Atmospheric effects need a separate model.
g = G·M/R². In Earth-relative units this simplifies to mass / radius².
Surface gravity follows g = G·M/R^2, so it scales linearly with mass but with the inverse square of radius. A planet twice Earth's mass but also twice its radius actually has half Earth's surface gravity.
Mass is the amount of matter in your body and never changes with location, while weight is mass times local gravity. On a low-g world you weigh less because g is smaller, even though you contain exactly the same matter.
Input
Mass 2× Earth, radius 1× Earth, 160 lb character.
Output
Surface gravity: 2.00 g. Local weight: 320 lb. Jump: 0.25 m.
g = mass/radius² = 2/1² = 2g. Weight scales linearly; jump height scales inversely.
Surface gravity scales with mass and falls with the square of radius, so the two inputs pull in opposite directions and radius pulls harder. A planet with twice Earth's mass and twice its radius has half Earth's surface gravity, which is the result people find least intuitive.
Reference figures come from published planetary data, so the solar-system comparisons are real numbers rather than approximations.
Weight changes with gravity; mass does not. A person on Mars weighs about 38% of their Earth weight and has exactly the same mass, which is why manoeuvring something heavy in low gravity is easier to lift and just as hard to stop once moving. That distinction matters for any scene involving low-gravity physical action.
The other consequence worth knowing for worldbuilding is that surface gravity constrains a lot besides walking: it sets whether an atmosphere is retained over geological time, how tall structures and organisms can grow, and how much energy it takes to reach orbit. A low-gravity world with a thick atmosphere needs an explanation.
The method behind this tool, so you can check it rather than take it on trust.
Surface gravity, mass and radius figures for the solar-system bodies used as reference points.
Once you have picked a planet's surface gravity, how do you actually play it? Concrete rules for jump distance, fall damage, carrying capacity, movement, and long-term strain — all keyed to the one number that drives them.
How surface gravity really works, what high- and low-g worlds do to bodies, jumps, and buildings, and how to pick numbers that keep a fictional planet physically honest.
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