Fillet Weld Calculator
The throat carries the load, not the leg.
Work out Fillet Weld. The throat carries the load, not the leg. Answers in the units the job actually uses.
Weld capacity
61.1 kN
4.24 mm throat from a 6 mm leg · adequate for the load
The throat carries the load, not the leg, and for an equal-legged fillet the throat is 0.707 of the leg. Sizing on the leg length overstates the capacity by 41%, which is the most common error in fillet weld calculations. Weld metal volume grows with the square of the leg length. Going from a 6 mm to a 10 mm fillet gives 67% more capacity and needs nearly three times the weld metal, time and heat input — which is why oversizing welds is expensive as well as distorting. A fillet larger than about 8 mm cannot be laid in a single pass and needs multiple runs, with the interpass temperature controlled. It is usually cheaper and stronger to run a longer small fillet than a short large one.
How the Fillet Weld Calculator works
Fillet weld capacity from leg length, weld length and electrode strength, with the length needed for a given load. The throat is 0.707 of the leg, and sizing on the leg overstates capacity by 41%.
Also known as: how strong is a 6mm fillet weld · weld throat versus leg · how long does the weld need to be · fillet weld capacity kn
Frequently asked questions
How strong is a fillet weld?
Its throat area times the allowable shear stress, conventionally 0.3 of the electrode tensile strength. A 6 mm fillet 100 mm long in E70 carries roughly 61 kN.
What is the throat thickness?
The shortest distance through the weld, 0.707 of the leg for an equal-legged fillet. It carries the load, and sizing on the leg length overstates capacity by 41%.
What size fillet should I use?
Rarely more than the thinner plate's thickness, and often less. Weld metal volume grows with the square of leg length, so oversizing costs time, heat and distortion for capacity you do not need.
Is a longer small weld better than a short large one?
Usually — cheaper, less distorting, and the stress is spread further. A fillet above about 8 mm cannot be laid in one pass and needs interpass temperature control.
Can I rely on this for structural work?
No. This gives an indicative capacity for a simple fillet in shear. Real design accounts for load direction, fatigue, eccentricity, root access and the applicable code.
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