Steelwork Paint Calculator
Tonnage to surface area to paint quantity for protective coating.
Quick answer
With the default inputs: Steel tonnage 10 t · Area per tonne 28 m²/t · Number of coats 2 courses etc.Paint to order 136.8 kg
Edit the inputs below to recalculate.Inputs
Result
Paint to order
136.8kg
| Painting area | 280.0m² |
| Total dry film | 120μm |
| Theoretical quantity | 85.53kg |
| With loss | 136.84kg |
| Per square metre | 0.489kg/m² |
| Per tonne of steel | 13.68kg/t |
| Drums at 20 kg | 7cans |
Area per tonne is the crux here: thinner sections carry more surface per tonne of steel. Rolled sections sit around 25-35 m²/t, heavy plate sections only 15-25 m²/t, and light purlins or space frames reach 40-55 m²/t. Theoretical quantity is kg = area × DFT(μm) × density ÷ (1000 × volume solids). Use volume solids, not weight solids — the two differ a lot. The loss factor covers overspray, drum residue and touch-up.
Formula
| Painting area | A = tonnage × area per tonne |
| Theoretical quantity | G = A × total DFT × density ÷ (1000 × volume solids) |
| Actual quantity | G = theoretical × loss factor |
| Per tonne | g = actual ÷ tonnage (kg/t) |
10 t of sections at 28 m²/t, two 60 μm coats, 55% volume solids, density 1.4 and a 1.6 loss factor: 280 m², 85.53 kg theoretical, 136.8 kg to order, 13.7 kg per tonne.
FAQ
What is the right area-per-tonne figure?
Best is to total the member surface areas from the take-off, or sum section perimeters times lengths. For estimates, use bands by member type: H-section columns and beams 25-30 m²/t, angle and channel trusses 30-40, cold-formed purlins 45-55, heavy plated members (box columns, deep beams) 15-22. Picking the wrong band doubles or halves the paint.
Volume solids or weight solids?
Volume solids is how much solid volume remains after drying and it alone sets the film thickness — you buy paint by volume and deliver by thickness, so volume solids is what the calculation needs. Weight solids depends on pigment density, is used for VOC, and cannot give film build. The two typically differ by ten points or more; using the wrong one leaves you short.
How is dry film thickness controlled?
On site, use a wet film gauge in real time: wet = dry ÷ volume solids, so a 60 μm dry film at 55% solids needs about 110 μm wet. After curing, test with a magnetic gauge at several points per member, taking mean and minimum — the minimum must not fall below 80% of spec and at least 85% of readings must meet it. Too thin kills durability, too thick cracks.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.