Minimum Fillet Weld Size per AWS

The minimum fillet weld size per AWS D1.1 (Structural Welding Code; Steel) is determined by the thickness of the thicker part being joined. AWS D1.1 Table 7.7 sets these minimums to ensure sufficient heat input during welding, which controls the cooling rate and prevents hydrogen-induced cracking in the heat-affected zone (HAZ). These requirements apply to all structural and piping fillet welds fabricated under AWS D1.1.

AWS D1.1 Minimum Fillet Weld Size Table

Thicker Part Joined (mm)	Thicker Part Joined (in)	Min. Fillet Weld Leg (mm)	Min. Fillet Weld Leg (in)	Min. Throat (mm)	Single-Pass Required
Up to 6	Up to 1/4	3	1/8	2.1	Yes
Over 6 to 13	Over 1/4 to 1/2	5	3/16	3.5	Yes
Over 13 to 19	Over 1/2 to 3/4	6	1/4	4.2	No
Over 19 to 38	Over 3/4 to 1-1/2	8	5/16	5.7	No
Over 38 to 57	Over 1-1/2 to 2-1/4	10	3/8	7.1	No
Over 57 to 150	Over 2-1/4 to 6	13	1/2	9.2	No
Over 150	Over 6	16	5/8	11.3	No

Key Rules for Minimum Fillet Weld Size

Single-pass rule: When the thicker part is 6 mm or less, the minimum fillet weld must be deposited in a single pass. Single-pass welds ensure sufficient heat input into the base metal to slow the cooling rate. Multi-pass welds on thin members can cause excessive distortion and underbead cracking.

Thicker part governs: The minimum is based on the thicker member, not the thinner one. For example, welding a 4 mm gusset plate to a 25 mm beam flange requires a minimum 8 mm fillet (based on the 25 mm member), not a 3 mm fillet based on the 4 mm plate.

Leg size, not throat: AWS D1.1 specifies minimums as leg sizes. The corresponding throat is the leg multiplied by 0.707.

Comparison with ASME B31.3 and ASME B31.1

Code	Minimum Fillet Weld Sizing Rule	Application
AWS D1.1	Based on thicker part thickness (Table 7.7)	Structural steel, pipe supports
ASME B31.3	1.09 x nominal pipe wall thickness (for socket welds)	Process piping
ASME B31.1	Equal to nominal pipe wall thickness or per design	Power piping
EN 1993-1-8	Min. throat 3 mm (structural)	European structural steel

The AWS D1.1 minimums are generally more conservative for thick base metals than ASME B31.3 socket weld rules. A 20 mm thick structural member requires an 8 mm minimum fillet per AWS D1.1, while a 5.54 mm wall NPS 2 socket weld pipe only requires a 6 mm fillet per ASME B31.3.

Why Minimum Sizes Exist

The minimum fillet weld size requirements are not driven by strength. They exist to:

1. Ensure adequate heat input to prevent rapid cooling and hydrogen cracking
2. Provide enough cross-section for stress transfer without creating a notch effect
3. Allow meaningful visual and surface inspection
4. Account for typical fabrication tolerances (undersized welds are the most common dimensional defect)

Pro Tip

When a project references both AWS D1.1 (for structural connections) and ASME B31.3 (for piping), apply each code’s minimum to its respective scope. Pipe supports welded to structural steel follow AWS D1.1; the same support welded to the pipe follows ASME B31.3. Mixing codes on the same joint creates inspection confusion.

Read the full guide to fillet welds