Hot-Dip Galvanized Coating Weight and Thickness

What a galvanized coating weighs, from the thickness it is specified in. The two are the same statement in different units and the conversion is a MATERIAL CONSTANT rather than a process figure: zinc has a fixed density, so one mil over one square foot weighs 0.5940 oz regardless of how it got there. Specifications use thickness because that is what corrosion life depends on and what a magnetic gauge measures; galvanizers think in weight because that is what they buy. THE ECONOMICS OF THE TRADE SIT ENTIRELY IN THE SURFACE-AREA-TO-WEIGHT RATIO, and it is why galvanizing prices look strange to a fabricator. Zinc consumption, kettle time, racking and handling all track AREA, while the purchase order is written in TONS. Heavy structural shapes run roughly 100 to 150 sq ft per ton and light angle, grating and fabricated assemblies run 300 to 500, so two loads of identical weight can differ by a factor of three or four in the zinc they take. That is what minimum charges and light-section pricing exist to recover, and it is not a markup. THE PICKUP FIGURE IS THE ONE THAT MAKES IT VISIBLE: zinc as a percentage of steel weight is a SURFACE-AREA quantity reported against a WEIGHT basis, which is exactly why it moves so much for a fixed coating grade. A comparison section is computed alongside so the ratio is shown rather than asserted. AND THE COATING IS AN AVERAGE, NOT A UNIFORM VALUE. ASTM A123 specifies a mean thickness with minimums, and the coating forms by reaction between molten zinc and the steel, so its thickness depends on the steel's silicon and phosphorus content, its section thickness, the bath temperature and the immersion time. A silicon-reactive steel grows a thick dull grey coating that meets the specification and looks nothing like the bright pieces beside it on the same rack. That is metallurgy rather than a defect, and it is the single most common galvanizing complaint. This is a quantity and conversion calculation on entered figures. Surface area per ton is the dominant input and must come from the fabricator's own takeoff or the galvanizer's estimate for the actual pieces, since the range across real work spans a factor of four and no table substitutes for the drawings. It does not specify the coating -- ASTM A123 assigns a grade from the material category and the steel thickness, with fasteners under A153 and sheet under other standards. It does not predict the thickness achieved, which is measured on the finished work. It does not address the galvanizer's total zinc consumption, which includes kettle losses to dross, ash and skimmings well beyond what leaves on the steel, or the venting, draining, distortion and embrittlement requirements that govern whether a fabrication can be galvanized at all. ASTM A123 and the applicable coating standard, the fabricator's surface area takeoff, and the galvanizer govern.

thickness in mils = microns / 25.4; coating weight in oz per sq ft = mils x 0.5940 (the fixed zinc density); total area = tonnage x surface area per ton; zinc = area x coating weight / 16; pickup = zinc / (tons x 2,000), reported as a percent of steel weight.

ASTM A123 assigns the coating grade in microns from the material category and the steel thickness; the grade is ENTERED here rather than selected. Fasteners fall under A153 and sheet under other standards.

A material density conversion and two unit conversions.

Estimate. AHJ and licensed professional govern.

Field names used by the API: coating_grade_um, steel_tons, area_per_ton_ft2, alt_area_per_ton_ft2, thickness_mils, coating_oz_ft2, total_area_ft2, zinc_lb, pickup_pct, zinc_lb_per_ton, alt_zinc_lb_per_ton, alt_pickup_pct, zinc_ratio

• Zinc density conversion 1 mil over 1 sq ft = 0.5940 ozthe density of zinc
• Surface area per ton is entered from the fabricator's takeoff, not a tablethe drawings for the actual pieces
• Kettle losses are not counted dross, ash and skimmings are beyond what leaves on the steelthe galvanizer's own consumption records