Weld Strength Calculator
Calculate weld strength for fillet and groove welds per AISC. Determine effective area, allowable load, and shear capacity with directional effects.
About this calculator
This calculator sizes fillet and groove welds under AISC allowable-stress design and reports how much load the joint can carry before hitting that limit. For a fillet weld, it doesn't use the leg size directly as the load-bearing dimension — it first computes the effective throat, 0.707 times the leg size, which is the shortest distance through the weld's triangular cross-section and the plane along which shear actually governs failure. That throat area is then multiplied by an allowable shear stress of 0.30 times the electrode's rated tensile strength (FEXX) — so a common E70XX electrode gives an allowable stress of 21,000 psi. A groove (full-penetration butt) weld is treated differently: its effective area is the weld size itself times length, and because it's typically loaded in tension rather than shear, the allowable stress is set at 0.60×FEXX.
The calculator also applies AISC's directional strength enhancement for fillet welds only: loads applied transverse to the weld axis (90°) can be up to 50% stronger than loads along the axis (0°), captured here as 1.0 + 0.50·sin¹·⁵(θ). Shear capacity is reported separately from allowable load so you can see the un-enhanced baseline. The built-in factor of safety of 2.0 isn't calculated from your inputs — it's simply the safety margin already baked into the allowable-stress method by definition, not a measure of how close your actual applied load is to failure. Remember to subtract twice the weld size from your measured length for fillet weld end returns, per AISC convention.
Inputs
Results
Allowable Load
22,271 lb
≈ 7 small cars
Shear Capacity
22,271 lb
≈ 7 small cars
How to Use This Calculator
- Enter the Weld Length in inches — for fillet weld returns subtract 2× the weld size per AISC.
- Enter the Weld Size (Leg) in inches for fillet welds, or throat thickness for groove welds. Common fillet sizes: 3/16", 1/4", 5/16".
- Enter the Electrode Strength (FEXX) in psi: E60XX = 60,000 psi, E70XX = 70,000 psi, E80XX = 80,000 psi.
- Select the Weld Type — Fillet for lap and T-joints, Groove (Butt) for full-penetration connections.
- Enter the Load Angle in degrees: 0° = longitudinal shear, 90° = transverse loading (strongest for fillets, ~50% stronger).
- Review Allowable Load in lb and Shear Capacity — if the applied load exceeds these, increase weld size or length.
How the result changes with Weld Length
| Weld Length | Allowable Load | Shear Capacity |
|---|---|---|
| 3 | 11,135 lb | 11,135 lb |
| 4.5 | 16,703 lb | 16,703 lb |
| 9 | 33,406 lb | 33,406 lb |
| 15 | 55,676 lb | 55,676 lb |
What each input means
- Weld Length
- Total effective length of the weld. Subtract 2× weld size for fillet weld returns (AISC).
- Weld Size (Leg)
- Fillet weld leg size or groove weld throat. Common fillet sizes: 3/16", 1/4", 5/16", 3/8".
- Electrode Strength (FEXX)
- Electrode classification tensile strength. E60XX = 60,000; E70XX = 70,000; E80XX = 80,000 psi.
- Weld Type
- Fillet welds are triangular and join perpendicular surfaces. Groove welds fill a prepared joint for full penetration.
- Load Angle
- Angle between the load direction and the weld axis. 0° = longitudinal shear; 90° = transverse (strongest for fillets).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersWeld Length = 6, Weld Size (Leg) = 0.25, Electrode Strength (FEXX) = 70000, Weld Type = 1 = 5 input(s) provided
- Calculate Allowable LoadAllowable Load22271 = 22271
- Calculate Shear CapacityShear Capacity22271 = 22271
- Calculate Effective Throat AreaEffective Throat Area1.0605 = 1.0605
- Calculate Allowable Weld StressAllowable Weld Stress21000 = 21000
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator use 0.707 times the leg size instead of the leg size itself as the throat?
For a fillet weld, the leg size is the length of each side of the triangular cross-section, but the weld actually fails along its shortest through-thickness dimension — the throat — which runs at 45° to the legs for an equal-leg fillet. That throat dimension works out to 0.707 (cos 45°) times the leg size, so the calculator uses it to compute effective area since that's the plane where shear stress governs failure.
Why is the allowable stress different for fillet welds (0.30×FEXX) versus groove welds (0.60×FEXX)?
Fillet welds are assumed to fail in shear along the throat, and AISC sets the allowable shear stress at 0.30 times the electrode's rated tensile strength. Full-penetration groove welds are typically loaded in tension across their cross-section like the base metal itself, so they're given a higher allowable stress of 0.60×FEXX, reflecting the different failure mode.
What does the load angle input actually change, and why only for fillet welds?
Fillet welds loaded transverse to the weld axis (90°) carry meaningfully more load than the same weld loaded parallel to the axis (0°), a directional strength effect captured here as 1.0 + 0.50·sin¹·⁵(θ), giving up to 50% more capacity at 90°. Groove welds don't get this bonus in the calculator because a full-penetration weld is treated as base-metal-equivalent regardless of load direction.
Why does the Factor of Safety always show 2.0 no matter what I enter?
That figure isn't computed from your specific applied load — it's the safety margin already built into the allowable-stress design method itself, since values like 0.30×FEXX are derived by dividing the weld's actual ultimate strength by roughly 2. If you want a safety factor specific to your real applied load, compare that load against the Allowable Load output directly.
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