While hot dip galvanising is usually priced in dollars per tonne, it is desirable to also convert this to dollars per square metre to allow comparison with alternative coatings.

In addition, the conversion to square metres allows accurate estimation of weight increase through the addition of the hot dip galvanised coating. The surface area per tonne can also be calculated using the following formula:

Surface Area per tonne = 255/Section Area in mm

Mass per square metre of steel can be calculated using the following formula:

Mass per Square Metre (g/m²) = Section Thickness (mm) x .85

Section Thickness (mm)	Surface Area/Tonne (m2/tonne)	Mass/m2 kg/m2	Min Coating Thickness (µm) per AS 1650*	Mass Increase (%)**
1mm	255	7.85	45µm (320g/m2)	4.10
2mm	127	15.70	55µm (390g/m2)	2.85
3mm	85	23.55	55µm (390g/m2)	1.65
4mm	64	31.40	70µm (500g/m2)	1.59
5mm	51	39.25	70µm (500g/m2)	1.25
6mm	42	47.10	70µm (500g/m2)	1.05
8mm	32	62.80	85µm (600g/m2)	0.95
10mm	25	78.50	85µm (600g/m2)	0.75
12mm	21	94.20	85µm (600g/m2)	0.65
15mm	17	117.75	85µm (600g/m2)	0.50
20mm	13	157.00	85µm (600g/m2)	0.40
25mm	10	196.25	85µm (600g/m2)	0.30

* Hot rolled steel sections and heavier steel sections will generate galvanised coatings considerably thicker than required by the AS 1650 Standard. To convert coating thickness in microns to equivalent coating mass in grams per square metre (g/m 2 ), use the following formula:

Coating mass (g/m²) = Coating Thickness (µm) x 7.05

Coating Thickness (µm) = Coating Mass (g/m²) x 0.14

** Actual zinc pickup after galvanising will depend on average coating thickness and section design.

Poor drainage, zinc entrapment and large horizontal surfaces will result in higher zinc pickup.

Note: Actual zinc usage in hot dip galvanising is significantly higher than physical zinc pickup. Zinc usage in hot dip galvanising is typically 5-7% of the mass of steel dipped because of zinc consumed on jigs and in generating zinc ash and zinc dross in the galvanising process.