How to calculate steel surface area
The area you need to paint or galvanise is the external perimeter of the section run along its length:
Painting area per metre (m²/m) = Perimeter (mm) ÷ 1,000
Painters and galvanisers usually price per tonne, so the key figure is the area per tonne:
Area per tonne (m²/t) = Area per metre ÷ Weight per metre (in tonnes)
Thin, spread-out sections (light angles, thin plate) have a high m²/t and cost more to coat per tonne; heavy compact sections have a low m²/t. The calculator computes both directly from the section geometry, and estimates paint litres from your coats and coverage.
Typical painting area per tonne
| Section | Approx. area (m²/tonne) |
|---|---|
| 10 mm plate (both faces) | ≈ 25 |
| Heavy universal column | ≈ 9–13 |
| Universal beam | ≈ 12–20 |
| Light angle / channel | ≈ 20–35 |
| Small hollow section | ≈ 25–40 |
For rolled beams and columns, use the "Other section — by area & perimeter" option and enter the cross-section area and surface perimeter from a steel section table.