What Is Weld Leg? Fillet Dimensions
The weld leg is the distance from the root of a fillet weld to the toe, measured along either of the two surfaces being joined. In a standard equal-leg fillet weld, both legs are the same length and the weld cross-section forms an isosceles right triangle. The leg size is the primary dimension specified on engineering drawings and used to determine the weld throat thickness.
How Weld Legs Are Measured
Each fillet weld has two legs: one along the horizontal (flat) member and one along the vertical member. The leg dimension is measured from the weld root (the intersection of the two base metal surfaces) to the weld toe (the point where the weld face meets the base metal surface). For equal-leg fillets, a single number is specified (e.g., a 6 mm fillet). For unequal-leg fillets, both dimensions are stated (e.g., 6 mm x 10 mm).
| Term | Definition |
|---|---|
| Leg (L) | Distance from root to toe along each base metal surface |
| Equal-leg fillet | Both legs are the same length (most common) |
| Unequal-leg fillet | Legs differ in length; used to balance stress distribution or minimize distortion |
| Theoretical throat (a) | For equal legs: a = 0.707 x L |
| Weld size | Specified as leg size on drawings (AWS/ASME practice) |
Minimum Fillet Weld Sizes (AWS D1.1 / ASME B31.3)
| Thicker Part Joined (mm) | Min. Leg Size (mm) | Min. Leg Size (in) |
|---|---|---|
| Up to 6 | 3 | 1/8 |
| 6 to 13 | 5 | 3/16 |
| 13 to 19 | 6 | 1/4 |
| 19 to 38 | 8 | 5/16 |
| 38 to 57 | 10 | 3/8 |
| Over 57 | 13 | 1/2 |
These minimums ensure adequate heat input to prevent rapid cooling and hydrogen cracking in the heat-affected zone of carbon steel base materials.
Leg Size and Weld Strength
The strength of a fillet weld depends on its throat, not directly on the leg size. However, because throat = 0.707 x leg (for equal-leg fillets), increasing the leg size increases the throat proportionally. Doubling the leg size doubles the throat and the load capacity per unit length.
Measuring Leg Size in the Field
Fillet weld gauges (such as the Cambridge gauge or bridge cam gauge) are used during visual and dimensional inspection to verify that both legs meet the minimum specified size. The gauge is placed against the weld profile, and the legs are read directly. Undersized legs are the most common fillet weld dimensional defect.
For socket weld fittings and branch connections on piping, the minimum fillet weld leg size is defined by the applicable piping code (ASME B31.3, ASME B31.1) and the engineering line class specification.
Leave a Comment
Have a question or feedback? Send us a message.