Density vs Mass vs Weight
Side-by-side comparison, when-to-use-each guide, and instant conversion. Reviewed for 2026.
Buoyancy calculations, material science, comparing materials by volume (why boats float).
Everyday weighing, medicine, sports — whenever you care about how much stuff there is, not its compactness.
| Aspect | Density | Mass vs Weight |
|---|---|---|
| Definition | Mass per unit volume (kg/m³) | Mass: amount of matter / Weight: gravitational force |
| Unit | kg/m³ or g/cm³ | Mass: kg / Weight: Newton |
| Water | 1,000 kg/m³ | Varies with gravity |
| Gold | 19,300 kg/m³ | — |
| Changes with gravity? | No | Weight yes, mass no |
Frequently asked
Why do objects float?
An object floats when its average density is less than the fluid. Ice (917 kg/m³) is less dense than water (1,000 kg/m³). Ships float because the steel hull encloses air — the average density of the whole vessel is less than water.
What is specific gravity?
Specific gravity is density relative to water (density / 1,000 kg/m³). A specific gravity of 1 = same density as water. Gold specific gravity = 19.3. Cork = ~0.24 (floats because well below 1). Used in brewing, geology, and precious metal testing.