Fillet Weld Leg Size Chart
A fillet weld leg size chart provides the minimum weld dimensions required based on the thickness of the base metals being joined. The leg size is the distance from the root of a fillet weld to its toe along each member surface. Correct leg sizing ensures adequate heat input to prevent cracking, sufficient strength to carry the design load, and compliance with the applicable fabrication code.
When the Fillet Weld Leg Size Chart Is Used
Engineers and welders reference the leg size chart during design (to determine the minimum acceptable weld size), during fabrication (to set welding parameters), and during inspection (to verify the as-deposited weld meets the code minimum). The chart applies to all fillet weld applications in piping, structural steel, and pressure vessel construction.
Minimum Fillet Weld Leg Size per AWS D1.1
AWS D1.1 Table 7.7 specifies the minimum fillet weld size based on the thicker part joined:
| Thicker Part Joined (mm) | Thicker Part Joined (in) | Min. Leg Size (mm) | Min. Leg Size (in) | Throat (mm) |
|---|---|---|---|---|
| Up to 6 | Up to 1/4 | 3 | 1/8 | 2.1 |
| Over 6 to 13 | Over 1/4 to 1/2 | 5 | 3/16 | 3.5 |
| Over 13 to 19 | Over 1/2 to 3/4 | 6 | 1/4 | 4.2 |
| Over 19 to 38 | Over 3/4 to 1-1/2 | 8 | 5/16 | 5.7 |
| Over 38 to 57 | Over 1-1/2 to 2-1/4 | 10 | 3/8 | 7.1 |
| Over 57 to 150 | Over 2-1/4 to 6 | 13 | 1/2 | 9.2 |
| Over 150 | Over 6 | 16 | 5/8 | 11.3 |
Minimum Fillet Weld Size per ASME B31.3 (Socket Welds)
For socket weld connections on carbon steel pipes, ASME B31.3 specifies:
| Pipe NPS | SCH 80 Wall (mm) | Min. Fillet Leg = 1.09 x Tn (mm) |
|---|---|---|
| 1/2 | 3.73 | 4.1 |
| 3/4 | 3.91 | 4.3 |
| 1 | 4.55 | 5.0 |
| 1-1/2 | 5.08 | 5.5 |
| 2 | 5.54 | 6.0 |
Maximum Fillet Weld Size Rules
The maximum leg size is also limited by code:
| Rule | Requirement |
|---|---|
| Along edges (AWS D1.1) | Leg size shall not exceed the thickness of the thinner member |
| Material less than 6 mm | Max leg = material thickness |
| Material 6 mm and over | Max leg = material thickness minus 1.6 mm (1/16 in) unless the drawing specifies building the weld out to the full edge thickness |
Standard Fillet Weld Leg Sizes for Piping Attachments
| Application | Typical Leg Size | Code Reference |
|---|---|---|
| Socket weld fittings | 1.09 x nominal wall thickness | ASME B31.3 |
| Slip-on flange (inside and outside fillet) | Equal to pipe wall thickness | ASME B31.3 Fig. 328.5.2B |
| Pipe shoes and trunnion supports | 6-10 mm (per design) | Project specification |
| Reinforcing pads on branch connections | 6-10 mm (per design) | ASME B31.3 |
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