Weld Strength Calculator (Fillet Weld)

Find the design strength of a fillet weld from its leg size, length, and electrode classification strength, using the AISC LRFD shear strength equation.

🔗 Weld Strength Calculator (Fillet Weld)
mm
mm
MPa
Design strength (φRn)
—
Nominal strength (Rn)
—
Effective throat (te)
—
Step-by-step working

🔗 What is Fillet Weld Strength?

Fillet weld strength is the design shear capacity of a triangular fillet weld joining two structural steel elements. Under the AISC LRFD approach, it is computed as phi Rn = phi(0.6 FEXX)(0.707w)Ln, where FEXX is the electrode's classification strength, w is the weld's leg size, L is its length, n is the number of weld lines sharing the load, and phi is the resistance factor.

Structural engineers use this calculation whenever a connection relies on welding, connecting a bracket to a column, a stiffener to a beam web, or two plates in a lap joint, converting the weld's geometry and electrode strength into a design capacity that gets compared against the applied factored load.

A common point of confusion is the 0.707 factor in the effective throat thickness te = 0.707w. This comes directly from the 45-degree geometry of a standard equal-leg fillet weld's triangular cross-section, the throat (the shortest failure plane through the weld) is always 0.707 times the leg size for that geometry, regardless of the weld's overall size.

This calculator computes the effective throat, nominal strength, and design strength for a longitudinally loaded fillet weld, supporting multiple weld lines so a two-sided lap joint's combined capacity can be found in a single step.

📐 Formula

φRn = φ(0.6FEXX)(0.707w)Ln
φRn = design (factored) weld strength
φ = resistance factor (commonly 0.75 for LRFD)
FEXX = electrode classification strength (MPa)
w = fillet weld leg size (mm), te = 0.707w (effective throat)
L = weld length per line (mm), n = number of weld lines
Example: w = 8 mm, L = 200 mm, FEXX = 482 MPa, φ = 0.75, n = 2 → φRn ≈ 490.71 kN.

📖 How to Use This Calculator

Steps

1
Enter the fillet weld leg size. Type w, the fillet weld's leg size, in millimeters.
2
Enter the weld length. Type L, the length of a single weld line, in millimeters.
3
Enter the electrode strength and resistance factor. Type FEXX, the electrode classification strength, in megapascals, and phi, the resistance factor (commonly 0.75).
4
Enter the number of weld lines. Type n, the number of weld lines carrying the load together (use 1 for a single weld).

💡 Example Calculations

Example 1 — Double-Fillet Lap Joint With E70XX Electrodes

w = 8 mm, L = 200 mm, FEXX = 482 MPa (E70XX), phi = 0.75, n = 2

1
te = 0.707 × 8 = 5.6560 mm
2
Rn = 0.6 × 482 × 5.6560 × 200 × 2 = 654,286 N = 654.29 kN
3
φRn = 0.75 × 654.29 = 490.71 kN
φRn = 490.71 kN
Try this example →

Example 2 — Single Fillet Weld With E60XX Electrodes

w = 6 mm, L = 150 mm, FEXX = 414 MPa (E60XX), phi = 0.75, n = 1

1
te = 0.707 × 6 = 4.2420 mm
2
Rn = 0.6 × 414 × 4.2420 × 150 × 1 = 158,057 N = 158.06 kN
3
φRn = 0.75 × 158.06 = 118.54 kN
φRn = 118.54 kN
Try this example →

Example 3 — Heavy Bracket Connection

w = 10 mm, L = 300 mm, FEXX = 482 MPa (E70XX), phi = 0.75, n = 2

1
te = 0.707 × 10 = 7.0700 mm
2
Rn = 0.6 × 482 × 7.0700 × 300 × 2 = 1,226,794 N = 1,226.79 kN
3
φRn = 0.75 × 1,226.79 = 920.09 kN
φRn = 920.09 kN
Try this example →

❓ Frequently Asked Questions

What is a fillet weld?+
A fillet weld is a triangular-shaped weld used to join two surfaces at roughly a right angle, such as a plate to a flange, without full penetration into the base metal. It is the most common structural weld type due to its simplicity and tolerance for minor fit-up variation compared to groove welds.
What is the formula for fillet weld strength?+
phi Rn = phi x (0.6 FEXX) x (0.707w) x L x n, where FEXX is the electrode classification strength, w is the fillet weld leg size, L is the weld length, n is the number of weld lines, and phi is the resistance factor (commonly 0.75 for LRFD).
What is the effective throat thickness of a fillet weld?+
The effective throat te is the shortest distance from the root of the weld to its face, assumed to fail in shear. For a standard equal-leg fillet weld, te = 0.707w, where w is the leg size, the 0.707 factor comes from the 45-degree geometry of the weld's triangular cross-section.
What does the electrode classification FEXX mean?+
FEXX is the minimum specified tensile strength of the weld filler metal, given in the electrode's AWS classification (for example E70XX means FEXX = 70 ksi, about 482 MPa). It represents the weld metal's own strength, separate from the base metal being joined.
Why is the weld strength formula based on 0.6 times FEXX?+
A fillet weld loaded parallel to its axis is assumed to fail in shear along its 45-degree throat plane. The 0.6 factor converts the electrode's tensile strength FEXX to an assumed shear strength, following the standard von Mises-based approximation that shear strength is about 0.6 times tensile strength for ductile weld metal.
Does weld strength depend on the load angle?+
Yes, in the full AISC procedure. A fillet weld loaded transversely (perpendicular to its axis) can carry more load than one loaded longitudinally, captured by the factor 1.0 + 0.5sin(1.5*theta). This calculator computes only the baseline longitudinal (parallel-loaded) case, which is the more conservative value.
What resistance factor phi should I use for weld design?+
AISC LRFD specifies phi = 0.75 for the shear rupture strength of fillet weld metal, this is the standard value for weld metal design under the AISC Specification, always confirm against your governing design code.
Why does the calculator ask for a number of weld lines n?+
Many connections use more than one weld line to share the same applied load, such as fillet welds on both sides of a lap joint or plate. Setting n to the number of weld lines lets the calculator sum their combined strength directly rather than requiring you to run the formula multiple times and add results by hand.
Does this calculator check the base metal strength?+
No, this calculator checks only the weld metal's shear strength. A complete weld connection design also verifies the base metal strength along the fusion faces, using a similar shear-based check with the base metal's own yield or tensile strength, and this can sometimes govern instead of the weld metal.
What units does this calculator use?+
Leg size and weld length are entered in millimeters, electrode strength in megapascals, and the resistance factor and number of weld lines are dimensionless. Design and nominal strength results are shown in kilonewtons.
Tags: Weld Strength Calculator Fillet Weld Calculator Aisc Weld Design Calculator Weld Throat Thickness Calculator Fillet Weld Shear Strength Calculator Formula Free Online

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What is a fillet weld?

A fillet weld is a triangular-shaped weld used to join two surfaces at roughly a right angle, such as a plate to a flange, without full penetration into the base metal. It is the most common structural weld type due to its simplicity and tolerance for minor fit-up variation compared to groove welds.

What is the formula for fillet weld strength?

phi Rn = phi x (0.6 FEXX) x (0.707w) x L x n, where FEXX is the electrode classification strength, w is the fillet weld leg size, L is the weld length, n is the number of weld lines, and phi is the resistance factor (commonly 0.75 for LRFD).

What is the effective throat thickness of a fillet weld?

The effective throat te is the shortest distance from the root of the weld to its face, assumed to fail in shear. For a standard equal-leg fillet weld, te = 0.707w, where w is the leg size, the 0.707 factor comes from the 45-degree geometry of the weld's triangular cross-section.

What does the electrode classification FEXX mean?

FEXX is the minimum specified tensile strength of the weld filler metal, given in the electrode's AWS classification (for example E70XX means FEXX = 70 ksi, about 482 MPa). It represents the weld metal's own strength, separate from the base metal being joined.

Why is the weld strength formula based on 0.6 times FEXX?

A fillet weld loaded parallel to its axis is assumed to fail in shear along its 45-degree throat plane. The 0.6 factor converts the electrode's tensile strength FEXX to an assumed shear strength, following the standard von Mises-based approximation that shear strength is about 0.6 times tensile strength for ductile weld metal.

Does weld strength depend on the load angle?

Yes, in the full AISC procedure. A fillet weld loaded transversely (perpendicular to its axis) can carry more load than one loaded longitudinally, captured by the factor 1.0 + 0.5sin(1.5*theta). This calculator computes only the baseline longitudinal (parallel-loaded) case, which is the more conservative value.

What resistance factor phi should I use for weld design?

AISC LRFD specifies phi = 0.75 for the shear rupture strength of fillet weld metal, this is the standard value for weld metal design under the AISC Specification, always confirm against your governing design code.

Why does the calculator ask for a number of weld lines n?

Many connections use more than one weld line to share the same applied load, such as fillet welds on both sides of a lap joint or plate. Setting n to the number of weld lines lets the calculator sum their combined strength directly rather than requiring you to run the formula multiple times and add results by hand.

Does this calculator check the base metal strength?

No, this calculator checks only the weld metal's shear strength. A complete weld connection design also verifies the base metal strength along the fusion faces, using a similar shear-based check with the base metal's own yield or tensile strength, and this can sometimes govern instead of the weld metal.

What units does this calculator use?

Leg size and weld length are entered in millimeters, electrode strength in megapascals, and the resistance factor and number of weld lines are dimensionless. Design and nominal strength results are shown in kilonewtons.

📌 Quick Tips

💡This calculator computes strength for a weld loaded parallel to its axis (longitudinal shear). AISC allows an increase in strength for welds loaded transversely or at an angle, via the 1.0 + 0.5sin(1.5*theta) factor, which is not included here.
💡The effective throat thickness te = 0.707w applies to standard equal-leg fillet welds, the 0.707 factor comes from the 45-degree throat geometry of a right-triangle fillet weld cross-section.
💡Common electrode classifications include E70XX (FEXX = 70 ksi, about 482 MPa) and E60XX (FEXX = 60 ksi, about 414 MPa), match your electrode strength to the base metal per AWS/AISC prequalified electrode requirements.
💡Use n to model multiple weld lines carrying the same load simultaneously, such as both sides of a double-fillet lap joint, rather than running the calculation twice and adding results by hand.
💡This calculator checks the weld metal shear strength limit state only, a complete weld design also checks that the base metal itself has adequate strength along the fusion faces, which typically uses a similar equation with the base metal's own strength.