Mass vs Weight — What's the Difference?
Mass
Mass is the amount of matter in an object. It measures inertia — how strongly the object resists changes to its motion — and it does not depend on location.
Unit: kilogram (kg)
Example: A 70 kg astronaut has a mass of 70 kg on Earth, on the Moon, and floating in deep space.
Weight
Weight is the gravitational force exerted on an object's mass. It depends on the local strength of gravity, so it changes from place to place.
Unit: newton (N)
Example: That 70 kg astronaut weighs about 686 N on Earth but only about 114 N on the Moon, where gravity is roughly one-sixth as strong.
Key differences
| Aspect | Mass | Weight |
|---|---|---|
| What it measures | Amount of matter / inertia | Force of gravity on that matter |
| Unit | Kilogram (kg) | Newton (N) |
| Changes with location? | No — constant everywhere | Yes — weaker on the Moon, zero in deep space |
| Type | Scalar | Vector (points toward the gravitating body) |
| Measured with | A balance (compares masses) | A spring scale or bathroom scale (measures force) |
Bathroom scales are marked in kilograms, which makes people say 'I weigh 70 kg.' Strictly, the scale measures the force you exert on it (your weight, in newtons) and converts it to the mass that would produce that force on Earth. In orbit, astronauts are weightless but not massless — shoving a massive space station module is still hard because its inertia is unchanged.