path: /blog/fillet-weld-size-guide/ canonical: https://steelcalculator.app/blog/fillet-weld-size-guide/ meta_title: 'Fillet Weld Size Selection Guide -- Minimum Size, Strength & Design (2026)' meta_description: 'Complete fillet weld size selection guide. Minimum weld sizes per AISC 360 Table J2.4, AS 4100 Table 9.7.3.10, and EN 1993-1-8 Table 4.2. Throat thickness, leg length, weld stress calculation, and common sizing mistakes.' robots: 'index,follow' lastmod: '2026-05-20' schema_file: 'schema/blog_fillet-weld-size-guide.json' FAQPage: '@type': 'FAQPage' mainEntity: - '@type': 'Question' 'name': 'What is the minimum fillet weld size?' 'acceptedAnswer': '@type': 'Answer' 'text': 'The minimum fillet weld size is determined by the thicker part being joined. Per AISC 360 Table J2.4: for material up to 1/4 in (6 mm), minimum weld size is 1/8 in (3 mm); 1/4 to 1/2 in (6-13 mm): 3/16 in (5 mm); 1/2 to 3/4 in (13-19 mm): 1/4 in (6 mm); over 3/4 in (19 mm): 5/16 in (8 mm). AS 4100 Table 9.7.3.10 and EN 1993-1-8 Table 4.2 have similar but not identical requirements tied to the thinner part.' - '@type': 'Question' 'name': 'How is fillet weld strength calculated?' 'acceptedAnswer': '@type': 'Answer' 'text': 'Fillet weld strength is calculated using the effective throat thickness (0.707 x leg length for a 90-degree fillet) multiplied by the effective weld length. The design strength per unit length is phi x F_w x A_we where F_w is the weld electrode strength and A_we is the effective throat area per unit length. For AISC 360, F_w = 0.60 x F_EXX. For EN 1993-1-8, the directional or simplified method uses stress components on the throat plane.' - '@type': 'Question' 'name': 'What is the difference between leg length and throat thickness in fillet welds?' 'acceptedAnswer': '@type': 'Answer' 'text': 'The leg length is the distance from the weld root to the toe measured along the base plate surface. The throat thickness (theoretical throat) is the shortest distance from the weld root to the weld face, equal to leg length x cos(45) = 0.707 x leg length for a 90-degree fillet. Weld strength calculations use the effective throat, not the leg length. Design specifications always reference leg length for minimum size requirements and throat thickness for strength calculations.' - '@type': 'Question' 'name': 'What is the maximum fillet weld size?' 'acceptedAnswer': '@type': 'Answer' 'text': 'Per AISC 360 Section J2.2(b), the maximum fillet weld size along edges is: (1) for material less than 1/4 in (6 mm) thick, the weld size shall not exceed the material thickness, (2) for material 1/4 in (6 mm) or more thick, the weld size shall not exceed the material thickness minus 1/16 in (2 mm). This accounts for the rounded edge profile of rolled sections and prevents weld metal from flowing beyond the edge.' - '@type': 'Question' 'name': 'How does weld length affect fillet weld design?' 'acceptedAnswer': '@type': 'Answer' 'text': 'Weld length directly affects the total strength of the connection (strength per unit length x total length). However, AISC 360 requires that the effective weld length be at least 4 times the weld leg size for minimum weld length -- any shorter weld is considered ineffective due to stress concentration at start/stop points. For longitudinally loaded welds longer than 100a (where a = throat thickness), strength reduction is required per AISC 360 Section J2.2(c).'

Fillet Weld Size Selection Guide -- Minimum Size, Strength, and Design per AISC 360, AS 4100, EN 1993

Fillet welds account for roughly 80 percent of all welds in steel construction, yet their apparent simplicity hides important code requirements that can trip up experienced designers. Getting the fillet weld size right matters because undersized welds crack from rapid cooling, oversized welds distort the base metal, and an incorrect throat thickness produces a connection that either fails or costs far more than it should.

This guide covers fillet weld sizing across four major design codes -- AISC 360 (US), AS 4100 (Australia), EN 1993-1-8 (Europe/UK), and CSA S16 (Canada) -- with worked examples and practical guidance on common sizing mistakes.

PRELIMINARY -- NOT FOR CONSTRUCTION. This guide is an educational reference only. Every weld design must be independently verified by a licensed Professional Engineer before use in any project.

Fillet Weld Geometry Fundamentals

Before discussing code requirements, we need to be precise about fillet weld geometry. The three critical dimensions are the leg size, the throat thickness, and the effective length.

Leg Size

The leg size (denoted w or s) is the distance from the weld root to the weld toe measured along the surface of the base plate. For a typical equal-leg fillet weld, both legs are the same length. The leg size is what designers specify on drawings: "6 mm fillet weld" means each leg is 6 mm long.

Throat Thickness

The theoretical throat thickness (a or t_t) is the shortest distance from the weld root to the weld face. For a 90-degree fillet weld with equal legs and a flat face, the relationship is:

a = w * cos(45 deg) = 0.707 * w

A 6 mm leg-size fillet weld has a throat thickness of 6 * 0.707 = 4.2 mm. This is the dimension that actually resists stress, and all strength calculations use the throat area, not the leg area. A convex weld face reduces the effective throat below the theoretical value, but design codes assume a flat face profile unless otherwise documented.

Effective Length

The effective length L_eff is the total length of weld that contributes to strength. For straight fillet welds, this is normally the overall weld length minus start/stop craters. In practice, designers use the full specified length for welds longer than the minimum length requirement.

The effective throat area for strength calculations is simply:

A_we = a * L_eff

Minimum Fillet Weld Size by Code

Minimum fillet weld sizes prevent rapid cooling cracking. When weld metal deposits onto a relatively cold base plate, it shrinks as it solidifies. If the weld is too small relative to the plate thickness, the cooling rate is too fast, and the weld or its heat-affected zone can develop hydrogen-induced cracking. Larger plates act as bigger heat sinks, so minimum weld size increases with plate thickness.

AISC 360 Table J2.4

AISC 360-22 Table J2.4 specifies minimum fillet weld leg sizes based on the thickness of the thicker part joined:

The weld leg size need not exceed the thickness of the thinner part joined. This prevents the absurd situation where welding a thin plate to a thick plate would require a weld larger than the thin plate itself.

EN 1993-1-8 Table 4.2

EN 1993-1-8 Clause 4.5 specifies minimum throat thickness based on the thinner part thickness:

Note that EN 1993 specifies throat thickness (a), not leg size. Converting to leg size: w = a / 0.707. A 5 mm minimum throat corresponds to roughly a 7 mm leg size. The Eurocode approach differs conceptually from AISC in two ways: it references the thinner part rather than the thicker part, and it specifies throat thickness rather than leg size. This makes EN 1993 requirements generally more conservative for connections between plates of similar thickness.

AS 4100 Table 9.7.3.10

AS 4100-2020 Clause 9.7.3.10 specifies minimum leg lengths based on the thickness of the thicker plate joined:

AS 4100 also requires that the leg size not exceed the thickness of the thinner plate for edge distances along the direction of load.

CSA S16 Clause 13.13

CSA S16-24 Clause 13.13 follows AISC 360 Table J2.4 closely:

The Canadian code also permits the weld size to be limited by the thickness of the thinner part, consistent with AISC practice.

Cross-Code Summary

All four codes agree thicker plates need larger minimum welds. AISC 360 and CSA S16: sized by the thicker part, specified as leg length. EN 1993: sized by the thinner part, specified as throat thickness. AS 4100: sized by the thicker part, specified as leg length. For typical structural connections (plates in the 10-20 mm range), all four codes give similar minimum sizes.

Maximum Fillet Weld Size

Minimum sizes prevent cracking. Maximum sizes prevent weld metal from flowing onto the plate surface.

AISC 360 Section J2.2(b)

Along edges of material:

• For material less than 1/4 in (6 mm) thick: weld size shall not exceed the material thickness
• For material 1/4 in (6 mm) or more thick: weld size shall not exceed the material thickness minus 1/16 in (2 mm)

The 1/16 in (2 mm) reduction accounts for the rounded edge profile of rolled sections. Hot-rolled shapes have small corner radii rather than perfectly sharp 90-degree edges. Without this reduction, a weld specified at the full plate thickness would extend beyond the edge.

EN 1993-1-8 and AS 4100 do not specify explicit maximum fillet weld sizes in the same way. Instead, they tie practical maximums to corrosion protection requirements, accessibility, and through-thickness loading limits (lamellar tearing).

Fillet Weld Strength Calculation

The strength of a fillet weld depends on the effective throat area, the electrode strength, and the applicable resistance factor.

Per AISC 360-22 Section J2.4

phi * R_n = phi * 0.60 * F_EXX * A_we

Where phi = 0.75, F_EXX = electrode classification strength (e.g., 70 ksi = 482 MPa for E70XX), and A_we = effective throat area = a * L.

For a 6 mm fillet weld using E70XX electrodes (a = 0.707 * 6 = 4.24 mm):

phi * R_n / mm = 0.75 * 0.60 * 482 * 4.24 / 1000 = 0.92 kN/mm

Per EN 1993-1-8 Clause 4.5.3

EN 1993-1-8 provides two methods: the simplified method and the directional method.

Simplified method:

F_w,Rd = f_vw_d * a

Where fvw_d = f_u / (sqrt(3) * betaw * gamma_M2), f_u = ultimate tensile strength of the weaker part, beta_w = correlation factor (0.80 for S235, 0.85 for S275, 0.90 for S355, 1.00 for S420/S460), and gamma_M2 = 1.25.

For S355 steel (f_u = 510 MPa) with a 6 mm weld:

f_vw_d = 510 / (sqrt(3) * 0.90 * 1.25) = 262 MPa
F_w,Rd = 262 * 4.24 / 1000 = 1.11 kN/mm

Directional method:

Resolves throat stress into three components (sigma_perp, tau_perp, tau_para) with the acceptance criterion:

sigma_perp^2 + 3(tau_perp^2 + tau_para^2) <= f_u / (beta_w * gamma_M2)

The directional method gives higher capacity for transversely loaded welds where compressive stress through the throat is significant.

Per AS 4100-2020 Clause 9.7.3.10

phi * V_w = phi * 0.6 * f_u_w * t_t * L_w

Where phi = 0.80, f_u_w = nominal tensile strength of weld metal (typically 490 MPa for E49XX).

For E49XX electrode with a 6 mm leg (t_t = 4.24 mm):

phi * V_w / mm = 0.80 * 0.60 * 490 * 4.24 / 1000 = 1.00 kN/mm

Per CSA S16-24 Clause 13.13

phi_w * 0.67 * X_u * A_w

Where phi_w = 0.67 and X_u = ultimate strength of weld electrode. The combined factor phi_w * 0.67 = 0.449 makes CSA S16 the most conservative of the four codes for fillet weld design strength.

Strength Comparison

For a 6 mm fillet (throat = 4.24 mm) with matching electrode:

Weld Stress Distribution in Connections

Welds loaded parallel to their length (longitudinal, or side fillet welds) carry load primarily in shear along the throat plane. Welds loaded perpendicular to their length (transverse, or end fillet welds) carry load through a combination of shear and direct tension on the throat.

Transverse fillet welds are approximately 30 to 50 percent stronger than longitudinal welds of the same size because the load path is stiffer and the stress distribution is more uniform. The AISC approach using 0.60 * F_EXX is conservative for all loading directions. EN 1993 captures the transverse strength increase naturally through the directional method.

AISC 360 Section J2.2(b) also requires that the effective weld length be at least 4 times the weld leg size. A 6 mm fillet must be at least 24 mm long. This rarely governs in main connections but matters for small stiffener welds and tack welds.

Longitudinal Weld Shear Lag

When a longitudinal fillet weld is long relative to its throat thickness, the stress distribution becomes uneven. The ends carry a disproportionate share of the load because the connected parts deform elastically along the length.

AISC 360 Section J2.2(c)

For longitudinal fillet welds longer than 100a (where a = throat thickness), AISC 360 requires a strength reduction:

beta = 1.2 - 0.002 * (L / a)    where 0.6 <= beta <= 1.0

For L/a > 300, beta = 0.60 (cap).

For a 6 mm fillet (a = 4.24 mm), the critical length is 424 mm. Beyond this, the shear lag reduction applies. A 600 mm weld (L/a = 141) has beta = 1.2 - 0.002 * 141 = 0.918, an 8 percent reduction.

AS 4100 handles this differently by limiting the effective length to 150 times the leg length unless a more rigorous analysis is performed. EN 1993-1-8 requires the designer to consider elastic load distribution in long joints without a specific prescriptive formula.

Worked Example: Fillet Weld for Gusset Plate Connection (AISC 360)

Given: Gusset plate 12 mm thick, grade 300 (F_u = 400 MPa). Base plate 20 mm thick. E70XX electrode (F_EXX = 482 MPa). Required LRFD design strength: 400 kN tension. Configuration: two longitudinal fillet welds.

Step 1 -- Minimum weld size. Thicker part is 20 mm (over 19 mm). From Table J2.4, minimum leg size = 8 mm. The weld size need not exceed the thinner plate (12 mm). 8 mm is acceptable.

Step 2 -- Effective throat. a = 0.707 * 8 = 5.66 mm.

Step 3 -- Strength per unit length.

phi * R_n / L = 0.75 * 0.60 * 482 * 5.66 / 1000 = 1.23 kN/mm

Step 4 -- Required weld length. L_req = 400 / 1.23 = 326 mm. Two longitudinal welds: 163 mm each. Use 170 mm each.

Step 5 -- Minimum length check. 4 * w = 32 mm << 170 mm. OK.

Step 6 -- Maximum size on edge. For 12 mm plate: max = 12 - 2 = 10 mm. 8 mm <= 10 mm. OK.

Step 7 -- Shear lag. L/a = 170 / 5.66 = 30. Well below 100. No reduction needed.

Step 8 -- Verify 6 mm would not work. Minimum size per Table J2.4 is 8 mm for 20 mm thicker part. A 6 mm weld would not meet minimum size requirements regardless of strength.

Final design: 8 mm fillet weld, E70XX, two 170 mm longitudinal welds. Total capacity: 340 * 1.23 = 418 kN > 400 kN.

Worked Example: Fillet Weld for Lap Joint (EN 1993-1-8)

Given: Plates S355 (f_u = 510 MPa), both 10 mm thick. Factored load: 300 kN. Configuration: two transverse fillet welds across the lap.

Step 1 -- Minimum throat. Thinner part = 10 mm. From Table 4.2: a_min = 5 mm. Leg size = 5 / 0.707 = 7.1 mm. Use 8 mm leg (a = 5.66 mm).

Step 2 -- Simplified method. beta_w = 0.90 (S355).

f_vw_d = 510 / (sqrt(3) * 0.90 * 1.25) = 262 MPa
F_w,Rd = 262 * 5.66 / 1000 = 1.48 kN/mm per weld

Step 3 -- Capacity check. Two transverse welds, each 150 mm wide: total capacity = 2 _ 150 _ 1.48 = 444 kN > 300 kN. OK.

Step 4 -- Directional method verification. Throat area per weld = 150 * 5.66 = 849 mm2. Throat stress = 150,000 / 849 = 177 MPa. For a transverse weld at 45 deg: sigma_perp = tau_perp = 177 / sqrt(2) = 125 MPa, tau_para = 0.

Check: 125^2 + 3(125^2) = 62,500. Limit: 510 / (0.90 * 1.25) = 453 MPa. 62,500 <= 453^2 = 205,209. OK.

Common Fillet Weld Sizing Mistakes

Mistake 1: Specifying Undersized Welds

The most common error is specifying a fillet weld below the code minimum for the plate thickness. On a 25 mm thick plate, a 5 mm fillet weld is inadequate per all four codes -- the rapid cooling creates a brittle heat-affected zone prone to cracking.

Fix: Always check minimum weld size against the governing plate thickness before finalising.

Mistake 2: Confusing Leg Length with Throat Thickness

These dimensions differ by a factor of approximately 1.4. Using leg length instead of throat thickness in strength calculations overestimates capacity by 40 percent. Using throat thickness when specifying minimum size could result in a weld too small for cooling control.

Fix: Use leg length (w) for all size specifications, throat thickness (a) for all strength calculations. Never mix the two.

Mistake 3: Not Accounting for End Returns

AISC 360 Section J2.2(b) requires that fillet welds terminating at the ends of connected parts be returned around the corner for at least 2w. These returns prevent stress concentration at the weld termination point.

Fix: Add 2w returns to all fillet welds at member ends.

Mistake 4: Overlooking Minimum Weld Length

A 15 mm long fillet weld satisfies strength for a light connection but fails the 4w minimum length requirement. The stress concentration at start/stop points prevents the full throat area from developing.

Fix: Check minimum weld length = 4w per AISC regardless of strength calculations.

Mistake 5: Welds Too Large for Thin Base Metal

A 10 mm fillet weld on a 6 mm plate edge exceeds the maximum per AISC J2.2(b). The weld would burn through the edge or flow onto the plate surface.

Fix: Check maximum weld size against edge thickness.

Mistake 6: Neglecting Longitudinal Weld Shear Lag

For side fillet welds longer than 100a, the effective strength must be reduced. This is often overlooked in long gusset plate connections and stiffener welds on deep girders.

Fix: Check L/a ratio for all longitudinal welds. Apply the beta reduction per AISC J2.2(c).

Conclusion

Fillet weld design is more than picking a number from a table. The minimum size prevents cracking, the maximum size prevents edge burn-through, the throat thickness carries the load, and the effective length determines whether the weld actually develops its full strength. Each of the four major design codes addresses these considerations with slightly different approaches, but the underlying physics is the same.

When designing fillet welds, follow this checklist:

1. Determine the governing plate thickness
2. Find the minimum weld size from the applicable code table
3. Check the maximum weld size for edge conditions
4. Verify the thinner part exception
5. Calculate strength per unit length using the effective throat (0.707 * leg size)
6. Determine required weld length
7. Check minimum and effective length requirements
8. Apply shear lag reduction for long welds (L > 100a)
9. Verify the configuration is feasible for fabrication

Use the SteelCalculator.app Weld Capacity Calculator to verify your fillet weld designs quickly, or work through the full calculations using the formulas provided here. Always have your design independently verified by a licensed Professional Engineer.

References

• AISC 360-22: Specification for Structural Steel Buildings, Chapter J, Table J2.4
• AISC 360-22: Section J2.2(b) Maximum Fillet Weld Size, Section J2.2(c) Shear Lag
• AS 4100:2020: Steel Structures, Clause 9.7.3.10, Table 9.7.3.10
• EN 1993-1-8:2005: Eurocode 3, Part 1.8 (Design of Joints), Clause 4.5, Table 4.2
• CSA S16:24: Limit States Design of Steel Structures, Clause 13.13
• AWS D1.1/D1.1M: Structural Welding Code -- Steel
• SteelCalculator.app Weld Capacity Calculator