Steel sheet weight is found with the formula thickness x width x length x density. For carbon steel the density is taken as 7.85 g/cm3, which means one square metre of 1 mm sheet weighs about 7.85 kg.
In practice the calculation splits in two: the weight of a single plate, and the weight per square metre. The second is used when comparing quotations and stock; the first when planning loading, stacking and transport.
The formula and keeping units straight
The whole calculation is a single line. Confusion almost always comes from units: giving thickness in millimetres and length in metres produces a wrong answer.
The safe approach is to convert every measurement to metres and use density in kg/m3. For carbon steel that is 7,850 kg/m3.
| Formula | Weight (kg) = thickness (m) x width (m) x length (m) x 7,850 |
|---|---|
| Short cut | kg/m2 = thickness (mm) x 7.85 |
| 1 mm sheet | 7.85 kg/m2 |
| 2 mm sheet | 15.70 kg/m2 |
| 3 mm sheet | 23.55 kg/m2 |
| Worked example | 2 mm x 1.50 m x 3.00 m -> 15.70 x 4.5 = 70.65 kg |
Calculating from a coil
If the length of a coil is unknown, it can be derived from the total weight: length = total weight / (thickness x 7.85 x width).
Example: a 5 tonne coil, 1.50 m wide and 2 mm thick -> 5,000 / (15.70 x 1.50) = about 212 metres. This is a theoretical figure; edge trimming and thickness tolerance mean the actual length will differ slightly.
Density changes with the material
For materials other than carbon steel the multiplier changes. Stainless steel is slightly denser; aluminium is close to one third. An aluminium plate of the same size weighs roughly a third of the steel one.
| Carbon steel | 7.85 g/cm3 |
|---|---|
| Stainless steel (304/316) | 7.9 - 8.0 g/cm3 |
| Aluminium | 2.70 g/cm3 |
| Copper | 8.96 g/cm3 |
Does galvanising change the weight
Yes, but only slightly. A zinc coating adds weight measured in grams per square metre, and that figure rises with the coating class. On thin sheet the proportional effect is noticeable; on thick sheet it is negligible.
When comparing quotations, check whether the coating class is stated: it affects both the weight and the corrosion performance.
Three common mistakes
- Mixing millimetres and metres without converting
- Treating theoretical weight as actual weight - thickness tolerance changes the result
- Using the flat plate formula for trapezoidal or corrugated sheet, where covering width and sheet width are not the same
Why it matters
Weight is not only a commercial figure: vehicle load limits, rack capacity and crane selection all depend on it. The weight of goods entering a bonded warehouse is used in planning the stacking layout and the floor loading.