Fillet Weld Size Chart — Minimum, Maximum & Shear Capacity (AISC 360, AS 4100, EN 1993)

Fillet weld size is governed by two code limits: a minimum to ensure adequate heat input and fusion, and a maximum to prevent excessive weld metal and distortion. Between these limits, the weld size is determined by the connection's strength requirement. This reference covers both limits, effective throat geometry, shear capacity per unit length, and multi-code requirements from AISC 360, AS 4100, and EN 1993.

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Minimum Fillet Weld Size — AISC 360 Table J2.4

The minimum fillet weld leg size is governed by the thickness of the thicker part being joined. The purpose is metallurgical: the weld must have enough heat input to fuse the base metal without creating an undersized weld on a thick heat sink.

Use the thickness of the thicker part at the weld location. Do not base the minimum on the thinner part unless the governing code explicitly does so.

Important: Where the minimum fillet weld size specified in the table exceeds the thickness of the thinner part, the minimum size is limited to the thickness of the thinner part (AISC 360-22 Section J2.2b).

Maximum Fillet Weld Size — AISC 360 Section J2.2b

The maximum fillet weld size is limited to prevent notching and stress concentration at the edge of the connected part:

Example: For a 1/2 in plate edge, the maximum fillet weld leg size is 1/2 - 1/16 = 7/16 in. For a 3/16 in angle leg, the maximum is 3/16 in.

This limit only applies to welds along the edge of a part. Welds on the flat face of a plate (not at the edge) are not subject to this maximum.

Effective Throat of Fillet Welds

The effective throat (E) is the shortest distance from the root to the face of the weld diagram. For an equal-leg fillet weld with leg size (a):

E = a * cos(45 deg) = a / sqrt(2) = 0.707 * a

For unequal-leg fillet welds, the effective throat is the perpendicular distance from the root to the hypotenuse of the weld cross-section, which must be computed from the actual geometry.

Fillet Weld Shear Capacity — AISC 360 Section J2.4

The nominal shear strength per unit length of fillet weld is:

Rn = F_w * E

Where Fw = 0.60 * FEXX (electrode classification tensile strength). For E70XX electrodes: F_w = 0.60 * 70 = 42 ksi.

Shear Capacity per 1/16 inch of Weld Size (E70XX, phi = 0.75)

Calculated as phiRn = 0.75 _ 0.60 _ 70 _ E = 31.5 _ E kips/in. Per foot: multiply by 12.

Capacity by Electrode Strength

Per 1/16 in per ft = 0.75 _ 0.60 _ F*EXX * (1/16)/sqrt(2) _ 12.

E70XX is the most common electrode for structural steel (A36, A992). Matching or overmatching is not required for fillet welds per AISC; E70XX is routinely used on all grades.

Minimum Fillet Weld Size — AS 4100:2020

AS 4100 Clause 9.7.2 specifies minimum fillet weld sizes based on the thinner part thickness:

Note: AS 4100 bases the minimum on the thinner part, while AISC bases it on the thicker part. This difference can affect connection design when joining dissimilar thicknesses.

Minimum Fillet Weld Size — EN 1993-1-8

Eurocode 3 does not prescribe a single minimum weld size table but provides a formula (Clause 4.5.2):

a_min = sqrt(max(t ; 3 mm) / 0.5)

But a_min shall not be less than 3 mm. In practice:

EN 1993 also specifies a maximum leg size: the fillet weld leg shall not exceed the thickness of the thinner part minus 2 mm (for t >= 4 mm) or the full thickness (for t < 4 mm).

Multi-Code Comparison — Minimum Fillet Weld Size

Key difference: AISC uses the thicker part; AS 4100 and EN 1993 use the thinner part. When joining a 3/8 in plate to a 1 in plate, AISC requires 1/4 in (based on the 1 in part), while AS 4100 would allow 6 mm (based on the 3/8 in part).

Worked Example — Fillet Weld Size Selection

Problem: A 1/2 in A992 gusset plate is fillet-welded to a 3/4 in A992 column flange. The connection must transfer 45 kips in shear. E70XX electrodes. Find the minimum weld length for a 5/16 in fillet weld.

Step 1: Check minimum weld size

Thicker part = 3/4 in. AISC Table J2.4 minimum = 1/4 in for 1/2 < t <= 3/4. Our 5/16 in > 1/4 in. OK.

Step 2: Check maximum weld size

Edge weld along 1/2 in plate: max = 1/2 - 1/16 = 7/16 in. Our 5/16 in < 7/16 in. OK.

Step 3: Calculate capacity per unit length

Effective throat E = 5/16 * 0.707 = 0.221 in.

phiRn = 0.75 _ 0.60 _ 70 * 0.221 = 6.96 kips/in.

Step 4: Calculate required weld length

For two fillet welds (one each side of the gusset):

L = 45 / (2 * 6.96) = 3.23 in per side.

Use 4 in per side minimum (round up). Check that L >= 4*a = 4 * 5/16 = 1.25 in. 4 > 1.25. OK.

Result: Two 5/16 in fillet welds, 4 in long each side. phiRn = 2 _ 6.96 _ 4 = 55.7 kips > 45 kips. OK.

Key Points for Weld Size Selection

• Minimum size is a code floor, not the final weld size. The actual size is determined by the strength demand on the connection.
• Electrode strength matters for capacity. E70XX is the default for structural steel, but higher-strength electrodes (E80XX, E110XX) can reduce the required weld length.
• Effective throat is always less than leg size. A 1/4 in fillet weld has an effective throat of only 0.177 in. The design capacity uses the effective throat, not the leg size.
• Longitudinal vs transverse welds. AISC 360-22 allows a 1.5x strength increase for transversely loaded fillet welds (load perpendicular to weld axis). This can significantly reduce the required weld length for certain configurations.
• Base metal strength can govern. The weld shear capacity on the base metal side (phi _ 0.60 _ Fu * t) must also be checked. For thin material, base metal shear may govern over weld metal shear.

Frequently Asked Questions

What is the minimum fillet weld size for 1/2 inch plate? AISC 360 Table J2.4: the thicker part governs. If both parts are 1/2 in (t between 1/4 and 1/2), the minimum is 3/16 in. If one part is thicker, use that thickness to determine the minimum.

What is the maximum fillet weld size along an edge? For material 1/4 in or thicker: material thickness minus 1/16 in. For material thinner than 1/4 in: equal to the material thickness. This prevents notching at the edge of the connected part.

How do I calculate fillet weld shear capacity? For E70XX electrodes: phiRn = 0.75 _ 0.60 _ 70 _ E = 31.5 _ E kips per inch, where E is the effective throat. For a 1/4 in fillet weld: phiRn = 31.5 * 0.177 = 5.57 kips/in per weld.

Why does AISC use the thicker part for minimum weld size? The thicker part acts as a heat sink. A small weld on a thick plate may not achieve adequate fusion because the thick plate draws heat away from the weld pool too quickly. The minimum size ensures sufficient heat input.

Do I need to use matching electrodes for higher-strength steel? No. AISC 360 Section J2.4 permits undermatched electrodes (e.g., E70XX on A992 with Fu = 65 ksi). The weld strength is based on the electrode classification, not the base metal. For fillet welds loaded in shear, the 0.60 * F_EXX capacity is conservative for most combinations.

What is the difference between fillet weld leg size and effective throat? Leg size (a) is the actual weld dimension measured along the plate surface. Effective throat (E) is the perpendicular distance from the root to the weld face, which is E = 0.707 * a for equal-leg 45-degree fillets. Design capacity uses the effective throat.

AISC Table J2.4 — complete minimum weld size reference

AISC 360-22 Table J2.4 specifies the minimum fillet weld size based on the thickness of the thinner connected part. These minimums ensure adequate heat input for proper fusion and prevent rapid cooling defects.

Minimum fillet weld size by material thickness (AISC Table J2.4)

Notes per AISC J2.4:

• The minimum size applies to single-pass fillet welds.
• For multi-pass welds, the first pass must meet the minimum size.
• For material thicker than 3/4 in., the minimum weld size is 5/16 in. regardless of thickness.
• The minimum applies to both longitudinal and transverse fillet welds.
• Preheat requirements per AWS D1.1 Table 3.2 may govern for thick material.

Maximum effective weld length (AISC J2.2b)

For fillet welds loaded longitudinally, the effective length is the actual length. However, for very long welds, the end-effect reduces average stress. AISC limits the maximum effective length for fillet welds in lap joints to 70 times the weld size (70w). For end-loaded welds longer than 70w, the effective length is reduced by the ratio beta = 1.0 - 0.01*(L/w - 70), but not below 0.60.

Fillet weld capacity table by leg size (E70XX, FEXX = 70 ksi, phi = 0.75)

The nominal capacity of a fillet weld per unit length is phi*Fnw = 0.75 x 0.60 x FEXX x (0.707 x a), where a is the leg size.

For E80XX (FEXX = 80 ksi), multiply all capacity values by 80/70 = 1.143.

Prequalified joint details per AWS D1.1

AWS D1.1 Clause 3 (Figures 3.3 through 3.13) defines prequalified joint details that do not require procedure qualification testing. The most common prequalified fillet weld joints:

Prequalified fillet weld requirements (AWS D1.1 Clause 5.23):

• Minimum effective throat per AISC Table J2.4
• Maximum single-pass weld size: 1/4 in. for flat, 5/16 in. for horizontal, 1/2 in. for vertical down
• Interpass temperature minimum per Table 3.2 (typically 50-150 deg F for A992)
• Maximum concavity: 1/16 in. below theoretical throat
• Minimum convexity: per Table 6.1 (tolerance on profile)

Run This Calculation

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