Mass and weight are not the same thing
Mass is how much matter is in an object: the number of atoms and their kind, packed together. It does not care where the object is. Weight is different. Weight is the force gravity exerts on that mass, and it changes the moment the pull of gravity changes. Take a 70 kg person from Earth to the Moon and their mass is still 70 kg, every atom accounted for, but their weight drops to a fraction of what it was, because the Moon pulls far more gently than Earth does. This mass and weight calculator is built around that single distinction: mass in kilograms stays fixed, weight in newtons moves with gravity.
Everyday speech blurs the two constantly. "How much do you weigh?" gets answered in kilograms or pounds, both properly units of mass, and nobody thinks twice about it, because on the surface of one planet the mix-up never causes a problem. It only matters once gravity itself becomes a variable, whether that's a physics problem, a spacecraft, or simple curiosity about the Moon landings. If the question instead is how much mass a lump of some material has, given its density and volume, that's a job for the density calculator; this page starts from a mass you already know and asks what it weighs.
The formula: W = m × g
Weight is mass times the local gravitational acceleration:
A newton is the SI unit of force: the push needed to accelerate a 1 kg mass by 1 m/s² every second. Earth's gravitational acceleration at the surface averages close to 9.8 m/s², though it varies slightly with latitude and altitude, from about 9.78 m/s² near the equator to about 9.83 m/s² near the poles. Because engineers and scientists needed one fixed figure to calculate with, standard gravity was defined by international agreement in 1901 as exactly 9.80665 m/s². This calculator defaults to the commonly rounded 9.81, close enough for any everyday sum, but switch to the exact 9.80665 if a result needs to match a textbook or a specification to the last decimal place.
Rearranged, the same short formula answers the other two questions this tool covers:
Solving for mass needs the weight and the gravity already known; solving for gravity needs the weight and the mass. Switching the seg above swaps which field disappears, and the answer for whichever quantity you picked gets its own conversions worked out underneath rather than a unit switch to fiddle with, since the point here is keeping mass and weight visibly separate.
Why a bathroom scale still shows kilograms
A bathroom scale does not measure mass directly. It measures the force your body presses down on it with, exactly the weight this page calculates, then does the arithmetic in reverse: it divides that force by an assumed value of g (standard Earth gravity) and displays the result as if it were a mass, in kilograms. That shortcut works because everyone using a bathroom scale stands on the same planet, at close enough to the same gravity, that the assumed g is always correct in practice. Carry the same scale to the Moon and it would still divide by Earth's gravity, so it would report roughly a sixth of your true mass, having no way to know the ground underneath had changed. The scale was never wrong about the force it measured; it was only ever right about the mass on the assumption that gravity stayed put.
Kilogram-force and pound-force, weight measured the old way
Before the newton was adopted as the standard unit of force, weight was often stated directly in kilograms-force (kgf) or pounds-force (lbf), units built specifically around Earth's standard gravity. A kilogram-force is defined as the weight of a 1 kg mass under standard gravity, so 1 kgf = 9.80665 N exactly. A pound-force is the weight of one avoirdupois pound (0.45359237 kg, also an exact figure) under the same standard gravity, giving 1 lbf = 4.44822 N. Both units are still legal and still turn up, in older engineering drawings, in some cooking and tyre-pressure gauges, and in aerospace where "pounds of thrust" is really pounds-force. They're convenient exactly because they let you read off a weight in numbers that look like an everyday mass, at the cost of baking in an assumption about gravity that a newton figure never makes.
Why astronauts are weightless, not massless
Orbiting astronauts look weightless, floating free inside the cabin, yet nothing about their mass has changed and gravity up there has barely weakened either. The International Space Station orbits roughly 400 km up, where Earth's gravitational pull is still around 90% of its surface strength, nowhere near zero. What removes the sensation of weight is that the station and everyone in it are in continuous freefall, all falling toward Earth together at the same rate as they hurtle sideways fast enough to keep missing it, which is what an orbit actually is. With nothing pushing back against that fall, there's no force left to feel as weight, even though gravity is still doing almost all of its usual work. Mass, meanwhile, hasn't moved an inch: the astronaut still has exactly as much matter in them in orbit as they did on the launch pad, which is why they still need the same force to push off a wall, and why "weightless" is never quite the same claim as "massless".
Two worked examples
A 70 kg person under Earth's standard gravity (9.81 m/s²): W = 70 × 9.81 = 686.7 N. Converted the older way, that's 686.7 ÷ 9.80665 = about 70.0 kgf, and 686.7 ÷ 4.44822 = about 154.4 lbf, both landing close to the mass figure itself only because Earth's gravity is exactly what those units were built around.
Now the reverse: a parcel presses down with 980 N of force on Earth. Rearranging for mass: m = 980 ÷ 9.81 = about 99.9 kg. Take the same parcel to the Moon (1.62 m/s²) and it would only press down with 99.9 × 1.62 = about 161.8 N, roughly a sixth of its Earth weight, while its mass sits at exactly the same 99.9 kg it always was.
Reading the other-worlds chart and table
The chart and table above take your own mass and multiply it by each world's own surface gravity, so you can see directly what you'd weigh if you somehow stood on it. Earth's bar is marked out from the rest for a quick reference point. The Moon and Mars sit low because their gravity is weak; Jupiter's sits high because a planet fourteen hundred times Earth's volume pulls hard even though it has no solid surface to stand on (the figure quoted for it, and for Saturn, Uranus and Neptune, is measured at the visible cloud tops rather than a surface that doesn't exist). The Sun is left out of the bars because at roughly 28 times Earth's gravity it would flatten every other bar into an unreadable sliver, but its figure is in the table, for anyone curious just how much a person would (hypothetically) weigh there. None of this changes the mass entered at the top of the page; only the weight column moves, row by row, which is the entire point of separating the two.
Questions people ask
What's the actual difference between mass and weight?
Mass is the amount of matter in an object and stays the same wherever it goes. Weight is the force gravity exerts on that mass, so it changes with location: the same mass weighs less on the Moon and more on Jupiter, purely because their gravity differs from Earth's.
If weight is a force, why does my bathroom scale show kilograms?
The scale measures the force you press down with, then quietly divides by Earth's standard gravity and displays the result as a mass. That only works because it assumes you're standing on Earth; the same scale on the Moon would still divide by Earth's gravity and understate your true mass.
What are kgf and lbf, and are they still valid units?
Kilogram-force and pound-force are older units of weight, defined as the force a 1 kg mass or a 1 lb mass exerts under standard Earth gravity (1 kgf = 9.80665 N, 1 lbf = 4.44822 N). They're still used in some engineering and everyday contexts, but they bake in an assumption of Earth gravity that a plain newton figure doesn't.
Why are astronauts in orbit weightless if gravity up there is still strong?
Gravity at typical orbital altitudes is still around 90% of its surface strength. Weightlessness comes from continuous freefall, the station and everyone in it falling toward Earth together while moving sideways fast enough to keep missing it, not from an absence of gravity. Their mass is completely unchanged throughout.
Would I weigh the same amount on the Moon as I do on Earth?
No. Your mass would be identical, but your weight would drop to roughly a sixth of its Earth value, since the Moon's gravity (about 1.62 m/s²) is only about a sixth of Earth's (about 9.81 m/s²). Try it above: enter your mass and pick the Moon from the gravity chips.
Why does the calculator default to 9.81 instead of the exact 9.80665?
9.80665 m/s² is the exact figure fixed by international agreement in 1901 as standard gravity. 9.81 is simply that value rounded to three figures, close enough for any everyday calculation and the more commonly seen default; switch the gravity field to 9.80665 if a result needs to match that exact standard.
What happens if I set gravity to zero?
Weight comes out at exactly zero, whatever the mass, since there's nothing left pulling on it. That's the honest meaning of "weightless": the object still has all of its mass, there simply isn't a gravitational force acting on it to produce a weight.
Surface gravity figures are cross-checked against NASA/JPL planetary data and standard astronomical references; gas giant figures are measured at the visible cloud tops rather than a solid surface. This calculator is for general and educational use, not for engineering or safety-critical calculations.