Coatings
Paint or galvanising surface area per metre and per tonne
- where
- surface area per metre (m²/m)
- painted perimeter of the section (mm)
- surface area per tonne (m²/t)
- mass per metre (kg/m)
- total surface area (m²)
- total length (m)
- total steel weight (t)
Explain this equation
Works out the area of steel to paint, blast or galvanise, per metre, per tonne and in total. Use it to order paint or price coating.
A metre length of section has a surface area equal to its perimeter; dividing millimetres by 1000 gives square metres per metre. Galvanisers and painters often price by area per tonne, which is the area per metre divided by the mass per metre, times 1000 kg. The totals follow from the length.
Surface area of a prism is perimeter times length.
Sense check: a 203 × 133 × 25 UB has about 0.915 m² per metre and about 36 m² per tonne. Heavy sections have far less area per tonne than light ones.
Watch out: use the perimeter of the faces that are actually coated, for example leave out the top flange under a slab.
Area to paint or galvanise for a length of steel section, and the area per tonne used to price coatings and estimate zinc and paint.
Enter your values to see the result.
Good to know
The surface area per metre equals the perimeter of the section. Rough perimeters: flats and boxes 2(B + H); tubes πD; I-sections about 2h + 4b − 2tw (the Blue Book also corrects for the root radii, which reduces it slightly). The Blue Book lists surface area per metre and per tonne for every rolled section. Light sections have much more area per tonne than heavy ones, which is why galvanisers price thin, light work higher per tonne.
Common mistake
Forgetting the inside of hollow sections when galvanising (the zinc coats inside and out, so double the area for zinc estimates), or pricing paint per tonne using a figure for a heavier section.
Limits
Outside surface only. Add ends, cleats and plates separately. Paint quantities also need volume solids, dry film thickness and loss factors from the paint data sheet.
Sources
- SCI / Steel for Life Blue Book: Steel building design – design data (section property formulas and tables for UKB, UKC, hot-finished and cold-formed hollow sections)
- Galvanizers Association: hot dip galvanizing (zinc pick-up and pricing are related to steel weight and section thickness; surface area per tonne)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the welding procedure specification (WPS), BS EN ISO 15614/9606, BS EN 1090, BS EN 1993 and manufacturers’ data. Structural welds need qualified welders and engineered designs.