SIZE OF WELDSFull Penetration Butt Welds.The general rules are: design throat thickness = thickness of the thinner part joined.cap width = prep width + 10% either side (for open prep, not specified in BS 499)Partial Penetration Butt Welds. The term partial penetration strictly implies butt welds that are designed to have less than full penetration. Failure to achieve full penetration when it is wanted should be listed as the defect incomplete penetration.The design throat thickness of a partial penetration weld is t1 and the actual throat thickness is t2. With a partial penetration weld made from both sides, the design throat thickness is t1 + t1 and the actual throat thickness is t2 + t2. Note that the degree of penetration must be known.t1 t2t2 t1t1 t2 t1t2 t1t2 t1 t2Fillet Welds.Fillet weld sizes are calculated by reference to allowable shear stress on the throat area, i.e.throat area = design throat thickness x length of weld.The size required is specified on drawings in terms of leg length (l).For fillet welds with equal leg lengths l = 1.4 t1. This does not apply to concave fillet welds. l l t1 = t2t1 t1 t2 If an asymmetrical weld is required, both leg lengths are specified and t1 is taken as the minimum throat dimension. l1 l2 t1Deep penetration fillet weld.With high current density processes, e.g. submerged arc and MIG (spray), penetration along the joint line can be produced. This gives an increase in throat thickness with no change in leg length.l