Running low- and high-gravity worlds: turning surface g into table rules

Deciding that your moon has 0.38 g or that your prison planet sits at 2.4 g is the easy part. The hard part comes at the table, when a player asks "so how far can I jump?" or "does the fall kill me?" and the whole illusion depends on you answering consistently. A believable gravity setting isn't a number in a worldbuilding document — it's a set of small, repeatable rulings that all trace back to that number. This piece is about turning a single surface-gravity figure into the handful of concrete effects you'll actually adjudicate during play.

Everything scales from one ratio

Surface gravity relative to Earth — call it g — is the master dial. On Earth, g = 1. A world at g = 0.38 pulls a little over a third as hard; a world at g = 2 pulls twice as hard. Almost every physical consequence a character experiences scales directly, inversely, or predictably from that ratio, which is why you only need to track one figure. Get g first (mass and radius determine it), then derive the rest.

The useful mental model: weight and fall damage scale with g, while jump height and the distance a thrown object travels scale inversely with g. Double the gravity and a character weighs twice as much and hits the ground twice as hard, but can jump only half as high. Halve the gravity and the reverse holds. That single pair of relationships covers most of what comes up in play.

Weight and carrying capacity

A 160 lb character on a 0.38 g world effectively weighs about 61 lb; on a 2 g world, 320 lb. This matters less for the character's own body — muscles adapt or strain in proportion — and more for anything they carry. Encumbrance rules that assume Earth weight get looser in low g and brutally tighter in high g. A pack that's a comfortable load on Earth becomes a back-breaking anchor at 2.5 g and a trivial one on a small moon. If your system tracks carrying limits, scale the effective weight of gear by g before checking against the limit.

Jump height and reach

A standing vertical jump of roughly half a metre on Earth becomes about 1.3 m at 0.38 g and only about 0.25 m at 2 g. Divide the Earth jump by g and you have the local jump. This is the effect players notice first and love most, so make it concrete: on a low-g world, describe characters clearing obstacles that would be impossible at home, reaching high ledges, and covering horizontal distance in long, floating bounds. On a heavy world, describe the opposite — a jump that barely clears a curb, every leap costing real effort. The planet gravity calculator reports the local jump directly, so you can quote a believable number instead of eyeballing it.

Fall damage

Fall damage scales with g: a drop that does a certain amount of harm on Earth does roughly g times as much on your world. At 0.38 g, characters can survive falls that would break bones at home — useful for a setting where daring leaps are part of the culture. At 2.5 g, an ordinary stumble down a staircase becomes genuinely dangerous, and a fall from any height is potentially lethal. If your rules assign fall damage by distance, multiply the result by g. A practical consequence worth foreshadowing: architecture and safety norms on a heavy world would be paranoid about height, while a low-g culture might build tall, thin, and vertiginous.

Movement, combat, and stamina

Below about 0.5 g, characters bound rather than run, thrown weapons and projectiles carry farther and flatter, and anything not tied down drifts on impact — great for cinematic, floaty action but a nightmare for careful footing. Above about 1.5 g, movement turns sluggish, characters tire faster, and falls hurt more, so encounters should run shorter and lean on positioning over acrobatics. Past roughly 3 g, an unadapted human is in medical trouble: breathing is labored, blood pools, and sustained exertion risks blacking out. If your players are visiting a high-g world without conditioning or tech support, that's not flavor text — it's a mechanical penalty on physical actions and a hard cap on how long they can push.

Picking numbers for the story you want

Work backward from the experience. Want heroic, mobility-driven action where characters leap between rooftops? Aim low — somewhere around 0.3–0.5 g gives dramatic jumps without turning every step into a floating hazard. Want a grinding, oppressive world where simply surviving is an achievement? Push into the 1.8–2.5 g range, where weight, fatigue, and fall risk all bite at once. Want strange-but-livable? Stay within about 0.7–1.3 g, close enough that humans function normally while thrown objects and jumps feel subtly off. The point is to choose g for its downstream effects, not for how impressive the raw figure sounds.

Consistency is what sells a fictional world, and consistency is easy once every ruling traces to the same number. Set your planet's mass and radius, read off the surface gravity, jump height, and adjusted weight from the planet gravity calculator, and keep those figures next to your notes. When a player asks how far they jump or whether the fall hurts, you'll have an answer that's the same this session as it was last session — which is the whole trick to a setting that feels real.