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.
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.
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.
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