GlossaryKind of term: Manufacturing termDensity
What is density?
Density is the mass per unit volume of a material, usually given in kg/dm³ for metals and plastics; it turns the volume of stock into a weight to price.
The value in kg/dm³ is numerically identical to the value in g/cm³. In the shop it turns the volume of the stock into a mass, and the mass into a material cost through a price per kilo. Specific weight, in N/m³, and relative density, compared with water and without a unit, are related but different quantities.
What does density depend on?
Density depends mainly on composition, so it varies between alloys of one family: EN AW-6082 is about 2.70 kg/dm³, EN AW-7075 about 2.8, and austenitic stainless is denser than plain carbon steel. Structural design to the Eurocodes uses fixed values, such as 7.85 kg/dm³ for steel and 2.70 for aluminium; EN 10088-1 gives values per stainless grade, such as 7.9 for 1.4301 and 8.0 for 1.4404. In plastics, fillers and fibres change the value; in castings and sintered parts, porosity does too.
How does density work in practice?
The volume of a block is length × width × height; of a round bar, π/4 × d² × length. Work in decimetres and volume × density gives kilograms directly. For example, an aluminium block of 100 × 100 × 50 mm is 0.5 dm³ and weighs 1.35 kg at 2.70 kg/dm³; a steel bar ⌀50 × 200 mm is about 0.39 dm³ and weighs about 3.1 kg at 7.85 kg/dm³. Such a weight is calculated on nominal sizes, not weighed. Price on the stock you buy, including allowance, saw cut and clamping margin, not on the volume of the finished part.
What is density often confused with?
Density is not specific gravity: specific gravity compares a material with water, has no unit and for solids is numerically close to density in kg/dm³. Nor is it the weight per metre in a stockholder's table, which already combines density and cross-section into kg per metre of bar or kg per square metre of plate.
Frequently asked questions
How do you calculate the weight of a block or bar?
Work out the volume in dm³: length × width × height for a block, π/4 × d² × length for a round bar. Multiply by the density in kg/dm³. Use the size you buy, including allowance and saw cut.
Where do I find the density of a material?
On your supplier's data sheet for the exact grade. Design standards such as the Eurocodes, and EN 10088-1 for stainless, give reference values. Alloys within one family differ, so use the value for the grade you buy.
Is density the same as specific gravity?
Not quite. Density is mass per volume, in kg/dm³. Specific gravity is the ratio to the density of water and has no unit; for solids the two numbers are close, which is why they are often mixed up.