Zinc Consumption Calculator

How zinc consumption is calculated

Galvanizing coats surface, but steel is bought and sold by weight. So zinc consumption per tonne is the coating mass per square metre multiplied by how much surface area a tonne of that steel has — then a margin for dross and ash:

Coating mass is about 7.14 g/m² per micron of zinc (zinc density 7.14 g/cm³). Surface area per tonne is the variable that dominates: a tonne of thin, complex fabrication has far more area than a tonne of heavy plate, so it consumes much more zinc. The thickness bands above set surface area from S ≈ 255 ÷ thickness in mm, the flat-plate figure counting both faces; complex fabrication runs higher, so override it with your own measured area when you have one.

The coating that matters is the one you deliver, not the one you are asked for. Zinc on the product is driven by your average gauge reading, not by the specified minimum. A plant averaging 140 µm where Grade 100 is specified is fully compliant and using 40% more zinc than the contract requires, and on most plants that gap is worth several times the entire dross and ash prize. Enter what your readings actually average; if you do not know it, that is the first number to go and measure.

Dross and ash are counted per tonne of steel, not as a percentage of zinc. A percentage of zinc moves when the coating changes even though the by-products have not, which is why it makes a poor control figure. The defaults of 5.5 and 3.5 kg/t are the rates a well-managed batch plant achieves; both are converted to contained zinc at 95% and 85% before being added.

Worked example

A 3–6 mm structural job specified to ASTM A123 Grade 75 but delivering an average of 115 µm, with dross at 5.5 and ash at 3.5 kg per tonne of steel:

1. Coating mass = 7.14 × 115 = 821 g/m²
2. Surface area ≈ 57 m²/t (S ≈ 255 ÷ 4.5 mm)
3. Zinc on product = 821 × 57 ÷ 1000 ≈ 47 kg/t
4. Zinc into dross = 5.5 × 0.95 ≈ 5 kg/t
5. Zinc into ash = 3.5 × 0.85 ≈ 3 kg/t
6. Total zinc consumption ≈ 55 kg/t (≈ 5.5% of steel mass)

Of that 47 kg on the product, 16 kg/t is above what Grade 75 requires — the specification would be met at 31. That single line is worth more than the dross and ash together, and it is the reason average coating thickness, not by-product recovery, is where plant zinc cost is won or lost.

Set the delivered coating to the specified grade and you get the theoretical floor — what the contract would cost if every piece came out exactly at minimum. Set it to what your gauge actually reads and you get your real consumption. The example above lands at 55 kg/t, which is where our Galvanizing Cost Calculator puts a 3–6 mm programme from whole-plant figures. Two routes, same answer, which is the check worth doing on your own numbers. Note also that the surface areas here are flat-plate; complex fabrication carries more, so override the figure when you have measured your own. The gap between specification and delivery is the subject of Zinc Consumption KPIs.

Tracking this against actual zinc purchased is one of the most important plant KPIs — see Zinc Consumption KPIs, and cost it with the Galvanizing Cost Calculator.