| t to 1/4 in. (t to 6 mm) | 1/8 in. (3 mm) | | Over 1/4 in. through 1/2 in. (6-13 mm) | 3/16 in. (5 mm) | | Over 1/2 in. through 3/4 in. (13-19 mm) | 1/4 in. (6 mm) | | Greater than 3/4 in. (greater than 19 mm) | 5/16 in. (8 mm) |

Critical note: The minimum does not increase beyond 5/16 in. regardless of part thickness. A 1 in. column flange and a 4 in. base plate both require a 5/16 in. minimum fillet weld.

Quick Lookup — Common Plate Thicknesses

Thicker Plate (in.) Minimum Fillet Weld (in.) Thicker Plate (mm) Minimum (mm)
3/16 1/8 4.8 3
1/4 1/8 6.4 3
5/16 3/16 7.9 5
3/8 3/16 9.5 5
7/16 3/16 11.1 5
1/2 3/16 12.7 5
9/16 1/4 14.3 6
5/8 1/4 15.9 6
3/4 1/4 19.1 6
7/8 5/16 22.2 8
1 5/16 25.4 8
1-1/4 5/16 31.8 8
2 5/16 50.8 8

The 1/2 in. boundary is the most important threshold: 1/2 in. thick requires 3/16 in. minimum. Anything thicker (even 9/16 in.) jumps to 1/4 in. minimum.

Thinner Part Exception

AISC 360 Section J2.2b: Where the minimum fillet weld size from Table J2.4 exceeds the thickness of the thinner part, the minimum shall be the thickness of the thinner part. You cannot specify a weld leg larger than the plate to which you are welding.

Example 1 — Exception applies: 1 in. column flange (thicker) to 3/16 in. gusset (thinner). Table J2.4 minimum: 5/16 in. Exception: 5/16 in. exceeds 3/16 in., so minimum = 3/16 in.

Example 2 — No exception: 1 in. column flange to 3/8 in. gusset. Table J2.4: 5/16 in. Not exceeding 3/8 in. No exception applies. Minimum = 5/16 in.

Example 3 — Same thickness: Both parts 3/16 in. Table J2.4: 1/8 in. Not exceeding 3/16 in. Minimum = 1/8 in.

Maximum Fillet Weld Size — Edge Condition (AISC J2.2b)

Plate Thickness at Edge (t) Maximum Fillet Weld Leg Along Edge
Less than 1/4 in. t (equal to plate thickness)
1/4 in. or greater t minus 1/16 in.

Examples:

This applies only along the edge of a plate. Flat-face welds are not subject to this limit.

The Heat Sink Principle

AISC is unique in basing minimum weld size on the thicker part. The reasoning:

  1. Larger heat sink: Thicker plates absorb heat faster; smaller welds cool too quickly.
  2. Fusion zone: Base metal surface must reach melting temperature for proper bonding.
  3. Microstructure: Rapid cooling on thick plates can produce brittle martensite. Larger welds increase heat input and slow cooling.

AS 4100 and EN 1993 base minimum on the thinner part, making AISC more conservative when joining thin to thick.

Comparison — AISC vs AS 4100 vs EN 1993

Material Thickness Range AISC 360 (thicker) AS 4100 (thinner) EN 1993-1-8 (thinner)
to 6 mm (to 1/4 in.) 3 mm 3 mm 3 mm
6-10 mm (1/4-3/8 in.) 5 mm 6 mm 3 mm
10-13 mm (3/8-1/2 in.) 5 mm 8 mm 4 mm
13-19 mm (1/2-3/4 in.) 6 mm 8 mm 5 mm
over 19 mm (over 3/4 in.) 8 mm 10 mm 6 mm

Effective Throat and Weld Capacity

Design capacity uses effective throat (E), not leg size (a): E = a x cos(45deg) = 0.707 x a

For E70XX electrodes (F_EXX = 70 ksi): phi*Rn per inch = 31.5 x E kips/in.

Leg Size a (in.) Effective Throat E (in.) phi*Rn per inch (kips/in.) phi*Rn per foot (kips/ft.)
1/8 0.088 2.78 33.4
3/16 0.133 4.18 50.1
1/4 0.177 5.57 66.9
5/16 0.221 6.96 83.6
3/8 0.265 8.36 100.3
7/16 0.309 9.75 117.0
1/2 0.354 11.14 133.7

For complete fillet weld capacity data covering all leg sizes, electrode strengths, and loading directions, see Fillet Weld Capacity Table.

Worked Example — Gusset-to-Column Weld

Problem: 3/8 in. gusset (A36) fillet-welded to 1 in. column flange (A992). Factored load = 45 kips. Weld length = 16 in. total (both sides). Determine required weld size.

Step 1: Thicker part = 1 in. Min = 5/16 in.

Step 2: Thinner part = 3/8 in. 5/16 in. is okay (does not exceed 3/8 in.). No exception.

Step 3: Max at edge: 3/8 minus 1/16 = 5/16 in.

Step 4: Required: 5/16 in. weld capacity = 6.96 x 16 = 111.4 kips, well above 45 kips. Minimum governs.

Result: Specify 5/16 in. fillet weld, both sides, 8 in. long each.

Common Mistakes

  1. Using the thinner part to determine minimum. AISC uses the thicker part.
  2. Forgetting the thinner part exception. Cannot specify a weld larger than the plate.
  3. Exceeding maximum weld at the plate edge. 1/2 in. weld on 1/2 in. plate edge exceeds 7/16 in. max.
  4. Confusing leg size with effective throat. Capacity uses 0.707 x leg size.
  5. Assuming minimum from Table J2.4 is always adequate for strength. The minimum ensures fusion quality, not strength.

Frequently Asked Questions

What is the minimum fillet weld size for 1/2 inch thick steel? Per AISC 360-22 Table J2.4, when the thicker part is 1/2 in., the minimum is 3/16 in. If the thicker part exceeds 1/2 in. (e.g., 5/8 in.), the minimum increases to 1/4 in.

Why does AISC Table J2.4 base minimum weld size on the thicker part? A thicker plate acts as a larger heat sink. A small weld on a thick plate may cool too quickly, preventing adequate fusion. The minimum weld size ensures sufficient heat input for metallurgical bonding.

What is the maximum fillet weld size along the edge of a plate? For plate thickness 1/4 in. or greater: maximum leg size = t minus 1/16 in. For plate less than 1/4 in.: maximum = t. This prevents the weld pool from notching the plate edge.

What is the thinner part exception? When the Table J2.4 minimum exceeds the thinner part thickness, the minimum is capped at the thinner part thickness. You cannot specify a weld larger than the plate being welded.

Does Table J2.4 require a larger minimum for very thick plates? No. The minimum caps at 5/16 in. for any thickness greater than 3/4 in. A 1 in. plate and a 6 in. plate both require a 5/16 in. minimum fillet weld.

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Related Pages

Educational reference only. Verify against AISC 360-22 Table J2.4 and governing project specification.