Density of steel, aluminium and copper, and how to work out weight

2 November 2026 · KERF

The density of mild steel is 7.85 g/cm³, or 7,850 kg/m³. Aluminium is about 2.7 g/cm³, so it weighs about a third as much as steel. Stainless is a little heavier than mild steel, and brass and copper are heavier again.

Density is the number you need to work out what a plate, a bar or a cut part weighs. This post gives the density of each metal we cut, the formula, and a few worked examples. If you just want the answer, the metal weight calculator does the maths as you type.

The density of each metal

These are the densities our quotes use to weigh every part.

• Mild steel (Grade 250): 7.85 g/cm³, 7,850 kg/m³.
• Stainless steel 304 and 316: 8.0 g/cm³, 8,000 kg/m³.
• Aluminium 5005 and 6061: 2.7 g/cm³, 2,700 kg/m³.
• Aluminium 5083: 2.66 g/cm³, 2,660 kg/m³.
• Brass: 8.5 g/cm³, 8,500 kg/m³.
• Copper: 8.96 g/cm³, 8,960 kg/m³.

To turn g/cm³ into kg/m³, multiply by 1,000. The two numbers say the same thing. A cubic metre of mild steel weighs 7.85 tonnes.

Why the number moves with the grade

Every alloy is a slightly different mix, so every grade has a slightly different density. Marine grade 5083 aluminium carries magnesium, which is lighter than aluminium, so it comes in a touch under 2.7. Brass is copper and zinc, and its density runs from about 8.4 to 8.7 depending on how much zinc is in it. Pure copper sits at the top of the list.

The differences inside a family are small. For a plate or a bracket, 2.7 for aluminium and 7.85 for steel will be within a percent or two. When weight really matters, use the figure for the exact grade.

How to work out the weight of a plate

Weight is volume times density. With every size in millimetres, the formula for a plate is:

Weight in kg = length × width × thickness × density ÷ 1,000,000

Use the density in g/cm³. The million turns cubic millimetres into the right units, so you never have to convert anything else.

Take a 250 × 250 mm base plate in 10 mm mild steel. 250 × 250 × 10 × 7.85 ÷ 1,000,000 = 4.91 kg.

The shortcut: kilograms per square metre

For plate, there's a quicker way. One square metre of plate weighs its thickness in millimetres times the density, in kilograms.

• 6 mm stainless: 6 × 8.0 = 48 kg per square metre.
• 12 mm aluminium 5083: 12 × 2.66 = 31.9 kg per square metre.
• 3 mm brass: 3 × 8.5 = 25.5 kg per square metre.

Multiply that by the area in square metres and you have the weight. A full 2400 × 1200 mm sheet is 2.88 square metres. So a sheet of 3 mm stainless weighs 3 × 8.0 × 2.88 = 69.1 kg. That's why full sheets go by freight, not in the post.

Discs and rings

A disc works the same way. The area is π × radius², then multiply by the thickness and the density.

A 150 mm disc of 4 mm brass: 3.1416 × 75 × 75 × 4 × 8.5 ÷ 1,000,000 = 0.60 kg.

For a ring, work out the outside disc, then take away the hole.

The same plate in five metals

Here's one plate, 500 × 500 × 6 mm, in each metal. The size doesn't change. The weight does, by more than three times.

• Aluminium 5083: 3.99 kg.
• Aluminium 5005: 4.05 kg.
• Mild steel: 11.78 kg.
• Stainless 304: 12.00 kg.
• Brass: 12.75 kg.
• Copper: 13.44 kg.

We stock copper up to 4 mm, so that last one is for comparison. At 4 mm, the same copper plate weighs 8.96 kg.

Bar and tube

The calculator also weighs bar, tube and angle, worked out per metre. If you'd rather do it by hand, two formulas cover most of it.

• Round bar: kg per metre = diameter² × density × 0.000785. A 20 mm mild steel bar is about 2.47 kg per metre.
• Round tube: kg per metre = (outside diameter − wall) × wall × density × 0.00314. A 48.3 mm tube with a 3.2 mm wall, in mild steel, is about 3.56 kg per metre.

Square tube and angle have rounded corners in real life, so a sharp-cornered sum comes out a little heavy. Check the supplier's catalogue weight when it matters.

A cut part weighs less than its plate

Holes and cut-outs weigh nothing. Our 250 × 250 mm base plate weighs 4.91 kg as a blank. Cut four 18 mm bolt holes in it and about 80 g comes out, so the finished part weighs 4.83 kg.

For a simple part, take the holes off by hand. For a shaped one, upload the drawing instead. The quote shows each part's weight, worked out from its actual outline.

Why weight matters

• Delivery. Delivery is priced at checkout by weight and address, with Australia Post. Orders over 22 kg, or parts over 1000 mm long, are too big for Australia Post, and checkout offers to quote freight for those.
• Choosing a metal. If a part has to be light, aluminium at a third of the weight of steel is often the first swap. Thicker aluminium can match the stiffness of thinner steel and still come out lighter.

Skip the maths

The metal weight calculator does all of this for you. Pick a shape: plate, disc, ring, round bar, flat bar, square bar, round tube, square tube or angle. Pick the metal, type your sizes in millimetres, and the weight updates as you type. It shows the weight each, the total for your quantity, and the weight per square metre or per metre.

For any metal not on our list, choose Other metal and type its density. For a plate in a metal and thickness we stock, one button opens the quote with that plate ready to price.

It's free, there's nothing to sign up for, and it uses the same densities as our quotes.