Density, and what it lets you work out
Density is how much mass sits in a given volume: ρ = m ÷ V. Steel is 7.85 g/cm³, water is 1.00, air is 0.0012 — a range of nearly four orders of magnitude between the materials you handle every day. Once you know any two of mass, volume and density, the third follows, which is why this calculator asks which one you are missing rather than making you rearrange the formula yourself.
The units, and how they convert
- 1 g/cm³ = 1 g/mL = 1,000 kg/m³ = 62.43 lb/ft³. That single chain covers almost every textbook and workshop number.
- 1 kg/m³ = 0.001 g/cm³ — so 2,700 kg/m³ (aluminium) is 2.70 g/cm³.
- Water is the anchor: 1,000 kg/m³, which is why 1 litre of water weighs 1 kg and 1 cubic metre weighs 1 tonne.
Some densities worth remembering
| Material | g/cm³ | kg/m³ | lb/ft³ | Specific gravity |
|---|---|---|---|---|
| Air (20 °C, sea level) | 0.001205 | 1.205 | 0.08 | 0.001205 |
| Cork | 0.24 | 240 | 14.98 | 0.24 |
| Pine (dry) | 0.5 | 500 | 31.21 | 0.5 |
| Petrol / gasoline | 0.74 | 740 | 46.2 | 0.74 |
| Oak (dry) | 0.75 | 750 | 46.82 | 0.75 |
| Ethanol | 0.789 | 789 | 49.26 | 0.789 |
| Ice | 0.917 | 917 | 57.25 | 0.917 |
| Water (4 °C) | 1 | 1000 | 62.43 | 1 |
| Seawater | 1.025 | 1025 | 63.99 | 1.025 |
| Milk | 1.03 | 1030 | 64.3 | 1.03 |
| ABS plastic | 1.04 | 1040 | 64.93 | 1.04 |
| PVC | 1.4 | 1400 | 87.4 | 1.4 |
| Honey | 1.42 | 1420 | 88.65 | 1.42 |
| Concrete | 2.4 | 2400 | 149.83 | 2.4 |
| Glass (window) | 2.5 | 2500 | 156.07 | 2.5 |
| Aluminium | 2.7 | 2700 | 168.56 | 2.7 |
| Steel (mild) | 7.85 | 7850 | 490.06 | 7.85 |
| Stainless steel | 8 | 8000 | 499.42 | 8 |
| Copper | 8.96 | 8960 | 559.35 | 8.96 |
| Lead | 11.34 | 11340 | 707.93 | 11.34 |
| Mercury | 13.55 | 13550 | 845.9 | 13.55 |
| Gold | 19.3 | 19300 | 1204.86 | 19.3 |
Specific gravity
Specific gravity (SG) is density divided by the density of water — a dimensionless number, which makes it convenient when you do not want to carry units around. Water is 1.00 by definition, aluminium is 2.70, mild steel is 7.85, ethanol is 0.789. In industries that measure liquids with hydrometers (brewing, winemaking, battery acid, urine testing), SG is the working unit. To convert back, multiply by 1,000 kg/m³.
Floating and sinking
An object floats when its average density is lower than the fluid's, and it floats with exactly enough of itself above the surface that the displaced water's weight equals its own. Ice (0.917) floats with about 8% showing, which is why icebergs are mostly hidden. A human body at roughly 0.985 g/cm³ floats, but exhale fully and the average density rises above 1 and you start to sink — the basis of every "float on your back" safety instruction.
Why your calculation may disagree with a table
Three reasons. Temperature: most materials expand when heated, so density falls; a table without a stated temperature is only approximate. Purity and alloying: "steel" covers 7.75–8.05 g/cm³ depending on the alloy, and seawater is denser than fresh water by about 2.5%. Porosity and voids: the density of a "dry" material often means bulk density including air gaps, which can be half the true solid density — pine at 0.50 g/cm³ is a bulk figure, while the cell wall material itself is about 1.5.
Where the numbers get used
Shipping and freight charge by volumetric weight, which is a density calculation in disguise; concrete mixes are designed by mass per cubic metre; buoyancy and ballast calculations in boatbuilding are pure density arithmetic; and in the lab, weighing a known volume is the fastest way to identify an unknown liquid.
Frequently asked questions
What is the density formula?
Density = mass ÷ volume (ρ = m ÷ V). Rearranged, mass = density × volume and volume = mass ÷ density. Units must match: grams with cubic centimetres, or kilograms with cubic metres, or the answer will be out by a factor of a thousand.
What is the density of water?
1.000 g/cm³ (1,000 kg/m³) at 4 °C, which is its densest point; at 20 °C it is 0.998 g/cm³. That is why 1 litre of water weighs 1 kg and 1 cubic metre weighs 1 tonne, and why water is the reference for specific gravity.
How do I convert g/cm³ to kg/m³?
Multiply by 1,000: 2.70 g/cm³ is 2,700 kg/m³. For lb/ft³, multiply g/cm³ by 62.43, so aluminium is about 168 lb/ft³. Specific gravity is the same number as g/cm³, just without units.
What is specific gravity?
Density expressed relative to water: SG = ρ ÷ 1,000 kg/m³. It is dimensionless, so it works in any unit system — steel is 7.85, ethanol 0.789, mercury 13.55. Brewers and battery technicians measure it directly with a hydrometer.
Will an object float or sink?
It floats if its average density is lower than the fluid it is in. Ice at 0.917 g/cm³ floats on water with about 8% above the surface; aluminium at 2.70 sinks immediately; a sealed hollow aluminium can may still float because its average density includes the air inside.
Why does my measured density differ from the table?
Temperature, alloy or purity, and porosity. Most materials expand when warm so their density falls; "steel" varies from 7.75 to 8.05 g/cm³ by alloy; and bulk densities of wood, sand or soil include air gaps, which can halve the figure compared with the solid material.