Mass vs. Weight: What's the Difference?
They're often used interchangeably, but they're not the same. Mass is the amount of matter in an object — it never changes, whether you're on Earth, the Moon, or in space. Weight is the force of gravity pulling on that mass, so it changes with location. You'd have the same mass on the Moon but weigh about a sixth as much.
See the difference, explained visually.
Watch a 2-minute animated lesson comparing mass and weight.
At a glance
| Mass | Weight | |
|---|---|---|
| What it measures | Amount of matter | Force of gravity on the matter |
| Changes with location? | No — constant everywhere | Yes — depends on gravity |
| Unit | Kilogram (kg) | Newton (N) |
| Measured with | A balance | A spring scale |
| On the Moon | Same as on Earth | About 1/6 of Earth weight |
Which should you use?
Mass
Use mass when you mean how much stuff an object is made of — a fixed property that doesn't change no matter where the object is.
Weight
Use weight when you mean how hard gravity pulls on that object — a force that gets weaker on the Moon and stronger on a larger planet.
Frequently asked questions
- Why do you weigh less on the Moon but have the same mass?
- Mass is the matter in you, which never changes. Weight is gravity's pull, and the Moon's gravity is about a sixth of Earth's — so you weigh less while your mass stays identical.
- Aren't kilograms a unit of weight?
- Strictly, kilograms measure mass. Everyday scales display weight in kilograms by assuming Earth's gravity, but scientifically weight is a force measured in newtons.
- Can something be weightless but still have mass?
- Yes. In orbit or free fall, astronauts feel weightless because there's no support force, but their mass — and thus their inertia — is unchanged.