Mass is the amount of matter in an object and stays constant everywhere, while weight is the force of gravity acting on that mass, so it changes depending on where you are.
Mass is measured in kilograms and describes how much matter something contains — it doesn't change whether you're on Earth, the Moon, or in deep space. Weight is a force, measured in newtons, calculated as mass multiplied by the local gravitational acceleration (W = mg).
An astronaut has the same mass on the Moon as on Earth, but weighs about one-sixth as much, because the Moon's gravity is weaker. In everyday language people use "weight" loosely to mean mass (like on a bathroom scale calibrated for Earth's gravity), but in physics the two are distinct quantities with different units.
Last reviewed: September 2026