The formula
W = m · g
What it means
Mass is the amount of matter and travelling does not change it: seventy kilograms are seventy kilograms anywhere. Weight is the force pulling that body down, and it depends on the local gravity, so it does change. A bathroom scale actually measures a force and converts it to kilograms assuming Earth's gravity, which is why on the Moon it would read under twelve: you have not lost weight, the scale is simply lying with a straight face.
How to work it out by hand
- Note your mass in kilograms, which is the part that does not change
- Look up the gravity of the body, in metres per second squared
- Multiply mass by gravity: that is the weight, in newtons
- Divide that weight by 9.81 to see what an Earth scale would read
What is worth knowing
Gas giants have no surface to stand on, so the gravity quoted for Jupiter, Saturn, Uranus and Neptune is the value at the level where the pressure matches Earth's atmospheric pressure: it is the figure everyone uses, but it is worth knowing what it means. Curiously Uranus has almost the same gravity as Venus despite being fourteen times more massive, because it is also far larger and its surface sits much further from the centre. And the Sun is an exercise: you could not stand there.