Mass vs Weight
Side-by-side comparison, when-to-use-each guide, and instant conversion. Reviewed for 2026.
Science, physics, chemistry, nutrition. Anything where the answer must be true off Earth.
Everyday language. When people say 'I weigh 70 kg' they mean their mass is 70 kg (technically weight is in newtons).
| Aspect | Mass | Weight |
|---|---|---|
| SI unit | Kilogram (kg) | Newton (N) |
| Constant? | Yes — same on Moon, Mars, space | No — changes with gravity |
| Measured by | Balance scale | Spring scale / weighing scale |
| Formula | — | W = mg |
| Common confusion | 'Weight' in everyday speech means mass | 'Force of gravity' is the strict meaning of weight |
Frequently asked
If I 'lose weight' in space, do I lose mass?
No. You become weightless (near-zero gravitational force) but your mass is unchanged — you still have all the same atoms. To lose mass, you'd need to actually shed matter (eat less, exhale CO₂, etc.).
How does a bathroom scale work?
Most bathroom scales measure the compression force on a load cell (in newtons or pounds-force) then divide by 9.81 to display in kilograms. They're calibrated for Earth's gravity, so taking one to the Moon would under-read your mass by ~83%.
Frequently asked questions
If I 'lose weight' in space, do I lose mass?
No. You become weightless (near-zero gravitational force) but your mass is unchanged — you still have all the same atoms. To lose mass, you'd need to actually shed matter (eat less, exhale CO₂, etc.).
How does a bathroom scale work?
Most bathroom scales measure the compression force on a load cell (in newtons or pounds-force) then divide by 9.81 to display in kilograms. They're calibrated for Earth's gravity, so taking one to the Moon would under-read your mass by ~83%.