Weld Strength Calculator (Fillet Weld)
Fillet welds are the most common weld type in structural steel construction. Their shear capacity is computed on the effective throat area: throat = 0.707 x weld leg size for a 45-degree fillet weld. Per AISC 360 Section J2.4, the design strength is phi * 0.60 * FEXX * Aw, where phi = 0.75, FEXX is the electrode classification number, and Aw is the effective throat area (throat x total weld length). This calculator computes the total LRFD design shear capacity for a specified weld configuration. It applies to welds loaded parallel or perpendicular to the weld axis, using the simplified directional strength increase where appropriate.
Fillet weld strength formula (AISC 360 LRFD)
Throat = 0.707 × w
Aw = Throat × Lw
phi Rn = 0.75 × 0.60 × FEXX × Aw
Where w is the weld leg size and Lw is the total length of weld. The effective throat for a standard 45-degree fillet weld is 0.707w.
Weld design rules of thumb
- A 1/4-in E70 fillet weld per linear inch of weld provides approximately 2.78 kips capacity (AISC Weld Coefficient C1 = 1.0 for E70).
- Weld return at corners (minimum 2x leg size) helps prevent stress concentrations at weld ends.
- Minimum weld length must be at least 4 times the weld size to be counted as effective per AISC 360 Section J2.2b.
- Prequalified joint designs in AWS D1.1 simplify qualification; non-prequalified joints require procedure qualification tests.
Frequently asked questions
How is fillet weld strength calculated?
The design strength of a fillet weld per AISC 360 is phi*Rn = 0.75 * 0.60 * FEXX * Aw, where FEXX is the electrode classification strength, Aw = 0.707 * w * L is the effective throat area (w = weld leg size, L = weld length), and 0.60 represents shear on the effective throat.
What does 0.707 represent in the weld throat?
The effective throat of a 45-degree fillet weld is 0.707 times the leg size (w). This comes from the geometry: throat = w * sin(45 degrees) = 0.707 w. The throat is the minimum cross-sectional dimension and is where weld shear failure is assumed to occur.
What electrode strength FEXX should I use?
Common electrodes: E60XX (FEXX = 60 ksi), E70XX (FEXX = 70 ksi, most common for structural steel), E80XX (80 ksi), E90XX (90 ksi). Select an electrode that matches or exceeds the base metal strength per AWS D1.1.
What is the minimum fillet weld size?
AISC 360 Table J2.4 specifies minimum fillet weld sizes based on the thicker part joined: for material up to 0.25 in thick, minimum size is 0.125 in; for 0.25-0.5 in, 0.1875 in; for 0.5-0.75 in, 0.25 in; for over 0.75 in, 0.3125 in.
What is the maximum fillet weld size?
For material 0.25 in or more thick, the maximum fillet weld size along edges is the material thickness minus 1/16 in, to avoid burning through the base metal. Along edges of material less than 0.25 in, the maximum weld size equals the material thickness.
Official sources
- American Institute of Steel Construction: AISC 360 Specification, Chapter J2 Welds.
- American Welding Society: AWS D1.1 Structural Welding Code (Steel).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.