How this instrument works
Weight and mass get used as synonyms in everyday speech, but the formula this instrument runs, W = m·g, is really just Newton's second law, F = ma, applied to one specific acceleration: the local pull of gravity. Mass is a fixed property of the object — the amount of matter in you, measured in kilograms, unchanged whether you are standing in your kitchen or orbiting Jupiter. Weight is the force that gravity exerts on that mass, and because it depends on g, it is different on every world with a different surface gravity.
The g in the formula is not a universal constant — it is Newton's law of universal gravitation, g = GM/r², evaluated for one specific body's mass M and radius r. That is why size alone does not predict it: the Moon is smaller and lighter than Mars, so its 1.62 m/s² is the gentlest gravity in the Planet menu, while Jupiter's enormous mass gives it 24.79 m/s² despite being made mostly of gas rather than rock. A 70 kg mass registers about 113 N on the Moon and about 1,735 N on Jupiter — the identical matter, a fifteenfold spread in the force it produces.
Every value in the Planet menu is a nominal, mean figure, and that is a real simplification worth knowing about. Jupiter, Saturn, Uranus and Neptune have no solid ground at all; their listed gravity is conventionally measured at the 1-bar level, the atmospheric depth where pressure matches Earth's sea level, because there is no other natural boundary to measure from. On rotating, non-spherical worlds like Earth, surface gravity also drifts a little with latitude — roughly 9.78 m/s² at the equator versus 9.83 m/s² near the poles — so the single number shown here is a representative value, not a promise that a scale reads identically everywhere on that planet.
- Type your mass into the Mass field in kilograms, or switch its unit to pounds if that is what you have on hand.
- Open the Planet menu and choose a world — each option loads that body's own measured surface gravity, from the Moon's 1.62 m/s² to Jupiter's 24.79 m/s².
- Read the result in the Weight field, reported in newtons by default; switch its unit to lbf for a pounds-force reading.
- Leave Mass untouched and cycle through the Planet menu to watch weight alone change — the quickest way to see the mass/weight split in action.
Worked example — 70 kg on Mars
Set Mass to 70 kg, about an average adult, and choose Mars from the Planet menu, which loads a surface gravity of 3.72076 m/s². The formula multiplies straight through: W = 70 × 3.72076 = 260.4532 N, the exact figure the Weight field returns, no rounding involved.
Switch the Planet menu to Earth without touching Mass, and the same 70 kg reports 70 × 9.80665 = 686.4655 N instead. Mars's gravity is only about 38% of Earth's, so the reading fell by well over half even though nothing about the person changed — a demonstration, not just a definition, of why mass and weight are different quantities.
Questions
Why does the same 70 kg mass weigh differently on different planets?
Because weight is a force, mass times the local gravitational acceleration g, and g is different on every world. Mass measures how much matter you have and stays 70 kg everywhere; weight is what a scale or a support structure actually feels, and it scales directly with whichever g the Planet menu loads — 260.4532 N on Mars, 686.4655 N on Earth, from the identical 70 kg.
What's the real difference between the Mass and Weight fields?
Mass, in kilograms, is an intrinsic property — the quantity of matter — and does not change when you travel. Weight, in newtons, is a force equal to m·g, and depends on where you are, specifically on the gravitational acceleration of whichever body is selected in the Planet menu. Mixing the two up is the most common mistake in this part of physics, which is exactly why they are kept as separate fields here.
Where do the gravity values in the Planet menu come from?
Each figure is that body's measured surface gravitational acceleration, derived from Newton's law of universal gravitation, g = GM/r², using the body's measured mass and radius. Earth's entry is the internationally adopted standard, 9.80665 m/s²; the Moon's gentle 1.62 m/s² and Jupiter's crushing 24.79 m/s² come from the same kind of measurement, not rounded guesses.
Why is there no solid-ground reading for gas giants like Jupiter?
Because there is no solid surface to stand on. For Jupiter, Saturn, Uranus and Neptune, 'surface gravity' is a defined convention: it is evaluated at the 1-bar pressure level, the atmospheric depth where pressure matches Earth's sea level, the closest thing those planets have to a boundary. The 24.79 m/s² listed for Jupiter is measured there, not at any physical ground.
Does the calculator account for latitude, altitude, or a planet's spin?
No — each Planet option is a single nominal value, typically an equatorial or globally averaged figure. Real surface gravity varies slightly with latitude, because a spinning planet bulges at its equator, and with elevation; Earth's own g ranges from about 9.78 m/s² at the equator to 9.83 m/s² near the poles. For comparing worlds against each other, the nominal figure is the right number to use.
Is this accurate enough for real spacecraft or lander engineering?
It gives the correct static weight as a first-pass figure — the same m·g relationship engineers use when sizing landing legs, actuators, or structural mounts for hardware headed to another world. Full mission design layers on dynamic landing loads, local terrain, and safety margins, but the number in the Weight field is the physically correct starting point for that work.