Give it any two of density, mass and volume and it returns the third, in both metric and imperial units.
Density is mass divided by volume — ρ = m / V — and it tells you how much matter is packed into a given space. It is the property that decides whether something floats, how much a full tank weighs, and whether two liquids will mix or sit in layers.
Water is the reference point worth memorising: 1 g/cm³, which is the same as 1000 kg/m³ or about 62.4 lb/ft³. Anything below that floats in fresh water and anything above sinks, which is the whole of Archimedes’ principle reduced to a comparison.
A block measuring 10 × 10 × 10 cm has a volume of 1000 cm³. If it weighs 1000 g, its density is exactly 1 g/cm³ — it is, near enough, water. If the same block weighs 2700 g, the density is 2.7 g/cm³, which is aluminium. At 7870 g it is iron, and at 19,300 g it is gold.
That last figure is why gold is such a reliable thing to test: at 19.3 g/cm³ it is nearly two and a half times as dense as steel, and no common counterfeit metal comes close. Tungsten, at 19.25, is the one exception, and it is the reason serious assay involves more than a density check.
| Material | g/cm³ | kg/m³ | lb/ft³ |
|---|---|---|---|
| Air (sea level, 20 °C) | 0.0012 | 1.2 | 0.075 |
| Cork | 0.24 | 240 | 15 |
| Petrol / gasoline | 0.75 | 750 | 47 |
| Ice | 0.92 | 920 | 57 |
| Water (4 °C) | 1.00 | 1000 | 62.4 |
| Seawater | 1.03 | 1030 | 64 |
| Concrete | 2.4 | 2400 | 150 |
| Aluminium | 2.7 | 2700 | 169 |
| Steel | 7.85 | 7850 | 490 |
| Lead | 11.34 | 11340 | 708 |
| Gold | 19.3 | 19300 | 1204 |
Almost every substance is denser as a solid than as a liquid, because cooling packs the molecules closer. Water is the conspicuous exception. Its molecules form a hydrogen-bonded lattice on freezing that holds them slightly further apart than in the liquid, so ice is about 8% less dense than water and floats.
It is a small quirk of chemistry with outsized consequences. Lakes freeze from the top down, insulating the water beneath and letting things live through winter. Were ice denser than water, they would freeze from the bottom up instead.
Density is not a fixed property of a material — it depends on conditions. Liquids and solids expand slightly when heated, so their density falls. Gases are far more sensitive: heat air and it expands enough to rise, which is what drives everything from hot air balloons to weather. Published densities are quoted at a stated temperature for exactly this reason, usually 20 °C or 4 °C for water, where it happens to reach maximum density.
Specific gravity, or relative density, is a material’s density divided by water’s. Because it is a ratio it has no units, and it is numerically identical to density in g/cm³ — steel at 7.85 g/cm³ has a specific gravity of 7.85. It shows up in brewing, in battery testing, and anywhere a hydrometer is used, precisely because a unitless ratio sidesteps arguments about which measurement system to use.
It floats if it is less dense than the fluid it sits in. An object displaces its own volume of fluid, and if the weight of that displaced fluid exceeds the object’s own weight, the upward buoyant force wins. This is why a steel ship floats despite steel being nearly eight times denser than water — the hull encloses a large volume of air, so the ship’s average density, steel and air together, comes out below 1 g/cm³.
Specific gravity is density divided by the density of water, which makes it a pure ratio with no units. Numerically it equals density expressed in g/cm³, so a material at 2.7 g/cm³ has a specific gravity of 2.7. The unitless form is convenient because it is the same number regardless of whether you work in metric or imperial, which is why hydrometers in brewing and battery testing are calibrated in it.
Yes. Most materials expand when heated, so density falls as temperature rises. Gases change dramatically — warm air rises precisely because it has become less dense than the air around it. Liquids and solids change much less but measurably. Water is unusual in reaching its maximum density at about 4 °C rather than at freezing, which is why published values always state a temperature.
Displacement. Fill a container with water, note the level, submerge the object fully, and note the new level — the difference is the object’s volume. Weigh the object separately and divide. This is essentially the method attributed to Archimedes, and it remains the practical answer for anything you cannot measure with a ruler. The object must be fully submerged and must not absorb water for the reading to be valid.
Because they do not mix and oil is less dense, at roughly 0.9 g/cm³ against water’s 1.0. Immiscible liquids settle into layers ordered by density, lightest at the top. The same principle explains why an oil spill spreads across a sea surface rather than dispersing downward, and why a density column of syrup, water and oil stays in stable, visible bands.
Related: Molecular Weight Calculator · Molarity Calculator · Volume Calculator · Unit Converter For bulk materials by the load rather than by the sample, the Gravel Calculator converts volume to tonnage for gravel, sand and topsoil. Density is exactly why kitchen conversions need the ingredient named: the Cooking Measurement Converter applies it per ingredient.