Finding the weld cross section area
How to work out the cross section area of fillet and groove welds in mm2, and how deposition efficiency turns deposited weight into electrode purchased.
Why area drives everything
Deposited weld metal is just a long prism: a constant cross section pulled along the joint. That is why volume is simply area times length, and why the area is the number worth getting right. A small error in the cross section scales straight through to volume, weight and cost across the whole run. Measure the finished weld profile, not the drawing symbol, because reinforcement and convexity add real metal that the nominal size ignores.
Fillet weld area
A fillet weld approximates a right triangle sitting in the corner of the joint. For equal legs of size L millimetres, the triangular area is L squared divided by 2. A 6 mm fillet therefore has a nominal area of 18 mm2 before any convex cap. Real fillets are rarely flat faced, so add roughly 10 to 20 percent for the convex reinforcement most welders lay down. Enter that adjusted figure in the Cross section area field to reflect the metal you will actually deposit.
Groove weld area
Groove welds are built from simpler shapes stacked together. A single V joint is a triangle set by the included angle and the plate thickness, plus a rectangle for any root opening, plus a thin cap for the reinforcement. Compute each piece in mm2 and sum them. For a 60 degree bevel on 12 mm plate with a 2 mm root gap, the bevel triangle plus root rectangle plus a modest cap often lands near 90 to 110 mm2. Summing shapes is more reliable than a single rule of thumb because joint geometry varies so widely.
From deposited weight to electrode purchased
This tool reports deposited metal, the weight that ends up fused in the joint. Buying electrode requires more, because some is lost as spatter, slag, stub ends and fume. Deposition efficiency captures that loss: stick electrodes run near 55 to 70 percent, flux cored around 80 to 90 percent, and solid MIG wire above 90 percent. Divide the deposited weight from the calculator by the efficiency as a decimal to size the purchase. For 785 g of deposited steel at 65 percent efficiency you would buy about 1208 g of electrode.