Steel Calculation

Imagine filling your steel section with water. The volume of water it holds — after you subtract any hollow cavity — is exactly the volume of steel in the piece. Multiply that volume by how dense steel is (7,850 kg for every cubic meter), and you have the weight. That is the entire calculation.

The only complexity lies in computing the cross-section area correctly for each shape. A solid round bar uses the circle area formula. A hollow section subtracts the inner void area from the outer area before multiplying by length. A flat bar and a plate are rectangles — width times thickness. The tool handles all six common profiles so you do not need to recall each formula under pressure.

Weight per metre is the most useful intermediate result for a fabricator. Once you know that a given section weighs, say, 12.3 kg per metre, you can price any quantity or length without recalculating from scratch. Most steel suppliers publish theoretical weight per metre in their catalogues, and this tool lets you verify those figures or calculate them for non-standard sizes.

Use this tool when sizing a steel order, estimating structural dead load for a frame, checking whether a vehicle or lifting equipment can handle the load, or preparing a bill of materials for quoting. It works best as a sanity check against supplier quotes and as a rough dead-load input before detailed structural analysis.

It is also useful when a fabrication shop needs to estimate offcut weights for scrap value, or when a purchasing team wants to cross-check the mass stated on a delivery note against nominal dimensions. A discrepancy of more than 5 percent warrants a query to the supplier.

Do not rely on this tool as the sole basis for structural design load calculations. It gives you mass accurately, but a structural member's suitability depends on section modulus, moment of inertia, buckling resistance, and connection capacity — none of which this tool addresses. Feed the weight output into your structural analysis, but do not skip that analysis.

Entering diameter when the section is hollow, then skipping wall thickness. A 100 mm hollow tube and a 100 mm solid bar differ by roughly a factor of five in weight per metre. The tool requires wall thickness for hollow sections to prevent this silent error — but double-check whether your section is solid or hollow before you accept a result.

Confusing outer diameter with inner diameter. Some suppliers spec tube by bore (inner diameter). The tool expects outer diameter. If you enter bore dimensions for a tube, the calculated weight will be too low. Check your material certificate or catalogue page to confirm which dimension you are using.

Using nominal dimensions without accounting for quantity tolerance. For bulk orders, steel is typically sold and invoiced by actual weighed tonnage, not calculated nominal weight. The 2 to 5 percent difference between theoretical and actual weight on a 10-tonne order is 200 to 500 kg — material in cost terms. Use this calculator for estimation and procurement budgeting, not final invoice verification.

Cross-section area drives everything. For a solid round bar: A = pi times (D/2)^2, where D is diameter in metres. For a round hollow section: A = pi times ((D_outer/2)^2 minus (D_inner/2)^2), where D_inner = D_outer minus 2 times wall thickness. For square sections, substitute side lengths. For flat bar and plate: A = width times thickness.

Volume per piece = A times length. Mass per piece = volume times 7,850. Total mass = mass per piece times quantity. Cost = total mass times price per unit mass. All arithmetic uses SI base units internally (metres, cubic metres, kilograms), then converts for display into the selected unit system.

The weight per metre figure (kg/m) equals A times 7,850 and is independent of length. That is why suppliers can list it as a catalogue property — it depends only on the section profile, not how long you cut it.

The formula assumes constant cross-section along the full length. Tapered sections, rolled sections with varying flange or web thickness (standard I-beams and channels have slightly tapered flanges), and cold-formed sections with corner radii all deviate from the idealised geometry. Published section tables from steel producers incorporate these geometrical corrections and are more accurate than a pure rectangular or circular model for catalogue profiles.

For hollow sections, corner radii on cold-formed SHS and RHS reduce the effective material volume slightly — typically 1 to 3 percent below the idealised square-corner calculation. If you are specifying large quantities of SHS and the budget is tight, use the manufacturer's published mass per metre rather than this tool's rectangular-corner model.