Weld Strength Calculator
Calculate the load capacity of a fillet weld from its leg size and length, using the standard AISC formula for E70 electrode welds.
How this calculator works
A fillet weld doesn’t fail along the flat leg you measure with a gauge — it fails through the diagonal throat running between the two legs. This calculator converts your leg size into that effective throat thickness, then multiplies by the allowable shear stress for an E70 electrode and the total weld length to find the load the weld can carry.
Formula: Fillet weld capacity = 0.707 × leg size × 0.3 × electrode strength (Fexx) × weld length. The 0.707 factor converts the leg size to the weld’s effective throat thickness; 0.3 × Fexx is the allowable shear stress on that throat (AISC ASD, E70 electrode, Fexx = 70,000 psi).
Worked example
A 1/4-inch (0.25 in) leg fillet weld, 10 inches long, E70 electrode:
- Capacity = 0.707 × 0.25 × 0.3 × 70,000 × 10
- Capacity ≈ 37,118 lb
Notes
E70 (Fexx = 70,000 psi) is the most common electrode strength for mild steel welding, covering E70XX stick electrodes and ER70S wire. It’s the right default for most general structural and fabrication work unless your drawings specify otherwise.
A larger leg size increases capacity linearly for a fixed weld length — doubling the leg size roughly doubles the weld’s strength for the same length, since leg size appears as a simple multiplier in the formula.
Real connections often use multiple welds, or a continuous weld running around several sides of a joint. In those cases, calculate each segment’s capacity separately using its own length, then sum them for the total connection capacity.
How to use
Enter your fillet weld’s leg size and total weld length, and select the electrode strength (E70 is the default for most mild steel work). The calculator returns the allowable load capacity for that weld in pounds, based on the standard AISC ASD formula.
This is a planning estimate using standard AISC/AWS engineering formulas, not a substitute for engineering design. Confirm actual weld and connection sizing with a licensed structural engineer.
Frequently asked questions
Why is there a 0.707 factor in the weld strength formula?
A fillet weld's leg size is measured along its two flat faces, but the weld actually fails through its diagonal throat, not the leg. For an equal-leg fillet weld, that throat thickness works out to 0.707 times the leg size, so the formula converts leg size to throat thickness before applying the allowable stress.
What does E70 electrode mean?
E70 refers to an electrode with a minimum tensile strength of 70,000 psi (Fexx = 70,000 psi), the most common strength class for welding mild steel. It covers common stick electrodes like E70XX and wire electrodes like ER70S. Higher-strength electrodes, like E80 or E90, allow a smaller weld for the same load.
How do I find the total capacity of a weld with multiple segments?
Calculate the capacity of each weld segment separately using its own length, then add the results together. This applies to welds on multiple sides of a joint, intermittent welds, or any connection where the weld isn't one continuous run.
Is this the same as the AWS D1.1 allowable stress?
Yes — 0.3 times Fexx is the AISC ASD allowable shear stress on a fillet weld throat, which lines up with AWS D1.1 provisions for E70 electrodes. Load and resistance factor design (LRFD) methods use a different factor and design philosophy, so don't mix the two approaches in the same check.
Estimates only. Verify quantities with your supplier before purchasing.