Fillet weld size, throat, and how much metal it takes
A fillet weld is the triangular bead that joins two surfaces meeting at an angle, like a tee or a lap joint. Its strength rides on the throat — the shortest path from the root to the face — while the amount of filler it swallows rides on the cross-sectional area. This calculator gives you both, plus the weight of weld metal so you can plan wire and cost.
The formulas
For an equal-leg fillet of leg size L, the effective throat is a = 0.707 x L. For unequal legs the throat is (L1 x L2) / sqrt(L1 squared + L2 squared). The theoretical cross-section area of the weld triangle is (L1 x L2) / 2, and weld metal volume is that area times the weld length.
Why the throat matters
Design codes rate a fillet weld on its throat, not its leg. Doubling the leg roughly doubles both throat and deposited metal, but the deposited area grows with the square of leg size — so a 5/16 in fillet uses about 56 percent more wire than a 1/4 in fillet for the same length. Oversizing fillets is one of the most common ways shops burn through filler and labor.
Weld metal weight
Weight equals weld volume times material density. The calculator defaults to carbon steel at 7.85 g/cm3; switch the preset for stainless or aluminum. Real beads carry some convexity and reinforcement, so add roughly 10 to 20 percent to the theoretical weight when ordering filler.
Worked example
A 1/4 in equal-leg fillet, 12 in long, in steel: throat = 0.707 x 0.25 = 0.177 in, area = 0.25 x 0.25 / 2 = 0.031 in squared, volume = 0.375 in cubed, and weight is about 0.11 lb of weld metal.
FAQ
Leg size or throat — which do I specify?
Drawings call out the leg size, and inspectors gauge the leg. The throat is derived from it and is what the strength calculation uses.
Does a convex fillet add strength?
Not in design terms — the rated throat is still measured to the theoretical flat face. Extra convexity just adds weight and can raise stress at the toe, so flat or slightly convex is the goal.