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.
Frequently asked questions
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.