This calculator works out the strength of a fillet weld from its size, finding the throat thickness, the effective area and the shear capacity. Fillet welds, the triangular welds that join two surfaces meeting at an angle, are the most common type in steel fabrication, and their strength depends on a key dimension called the throat, the shortest distance through the weld where failure occurs. The throat is smaller than the visible leg size of the weld, related by a fixed geometric factor, and it is the throat, not the leg, that carries the load. Sizing fillet welds correctly is fundamental to structural and mechanical design: too small and the joint fails, too large and you waste weld metal, time and money. This calculator does the calculation. You enter the leg size of the weld, the total length of weld, and the allowable shear stress of the weld metal, and the calculator returns the shear capacity of the weld in kilonewtons, the throat thickness, the effective weld area, and the leg size for reference. The results update as you type. Use it for structural and fabrication design, for checking a welded joint, or for engineering study. The throat thickness of a standard fillet weld is the leg size multiplied by 0.707, since the throat runs across the right-angled triangle of the weld at forty-five degrees. The effective area is the throat times the length of weld, and the capacity is that area times the allowable shear stress of the weld metal. Because the throat is what resists the load, increasing the leg size or the length both raise the capacity, as does using a higher-strength weld metal. Comparing the calculated capacity against the load the joint must carry, with the appropriate code safety factors, tells you whether the weld is adequate. This is the core sizing calculation for fillet-welded connections, where getting the weld size right is essential to a safe, economical structure.
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40.3 kN
weld shear capacity
Throat thickness4.24 mm
Weld area424 mm²
Leg size6 mm
Throat = 0.707 x leg size. Area = throat x length. Capacity = area x allowable shear stress. Compare against the design load with code safety factors. Dimensions in mm.
How it works
The throat thickness of a fillet weld is 0.707 times the leg size, the distance across the weld's right-angled triangle at forty-five degrees, which is where the weld fails. The effective area is that throat thickness times the length of the weld. The shear capacity is the effective area multiplied by the allowable shear stress of the weld metal.
Worked example
A 6 millimetre fillet weld 100 millimetres long has a throat of 0.707 times 6, about 4.24 millimetres. Its effective area is 4.24 times 100, about 424 square millimetres. With an allowable shear stress of 95 megapascals, the capacity is 424 times 95, about 40,300 newtons, or 40.3 kilonewtons.
