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Steel thickness versus area

m2 vs thickness,mm;
R/tonne to Rands/m2;
zinc thickness,um to mass, g/m2

Hot dip galvanizing is usually priced in Rands per tonne of galvanized steel. However, organic coatings are frequently costed in Rands per square metre of surface to be coated. This often leads to confusion and it is desirable to convert Rands/tonne to Rands/square metre and vice versa in order to arrive at a cost comparison. In addition, the conversion facilitates accurate estimation of weight increase through the addition of the hot dip galvanized coating. The surface area can be calculated using the following formula:

Surface area per tonne = 255/Section thickness in mm

The mass per square metre can be calculated from the following:

Mass per square metre (kg/m2) = Section thickness in mm x 7.85

The figure below shows, graphically, the relationship between thickness in mm and surface area in m2/tonne.

The figure below allows conversion from Rands per tonne to Rands per m2:

Key to use:

The Figures at the end of each line represent section thickness in mm

To convert R/t to R/m2:

Say quoted price = R1500/tonne
Say average thickness = 10mm
Follow vertical R1500 line to 10mm line
At intersection read off R/m2 = R58-00

Hot rolled steel and heavier steel sections will generate coatings thicker than those required in SANS 121 (ISO 1461). To convert coating thickness in microns (um) to equivalent coating mass in grams per square metre (g/m2), use the following formula:

Coating mass (g/m2) = Coating thickness in microns x 7.05
Coating thickness (um) = Coating mass (g/m2) x 0.14

Actual zinc pickup after galvanizing will depend upon the average coating thickness and section design. Poor drainage, zinc entrapment and large horizontal surfaces will result in higher zinc pickup.

The table below shows the mass increase resulting from the galvanizing process:

General Applications Total mass increase % Heavy duty Applications* Total mass increase %

Section thickness mm Surface area/tonne m2/tonne Mass/m2 of surface kg/m2 thickness,
um g/m2 per side thickness,
um g/m2 per side

1 254 3.9 45 321 8.18 55 393 9.99

1.5 17 5.9 55 393 6.66 70 500 8.48

2 127 7.9 55 393 5.00 70 500 6.36

3 85 11.8 70 500 4.24 88 628 5.33

4 64 15.7 70 500 3.18 88 628 4.00

5 51 19.7 70 500 2.54 88 628 3.20

6 42 23.6 85 607 2.57 106 757 3.21

8 32 31.4 85 607 1.93 106 757 2.41

10 25 39.3 85 607 1.54 106 757 1.93

12 21 47.2 85 607 1.29 106 757 1.60

15 17 59.0 85 607 1.03 106 757 1.28

20 13 78.6 85 607 0.77 106 757 0.96

25 10 98.3 85 607 0.62 106 757 0.77

* Heavy duty may be specified in terms of the SABS permit conditions

( Note - zinc consumption is normally taken as 30% over the theoretical mass increase to allow for thickness coverage greater than the minimum and ash and dross formation)

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