What Is Weld Throat? Fillet Weld Sizing
The weld throat is the shortest distance from the root of a fillet weld to its face (hypotenuse). It is the critical dimension for calculating the load-carrying capacity of a fillet weld because the throat represents the minimum cross-section through which stress is transmitted. An undersized throat directly reduces the joint’s strength.
When Weld Throat Dimensions Apply
Weld throat measurements are used during design (to size the weld), during fabrication (to verify the welder is depositing sufficient metal), and during inspection (to confirm the as-built weld meets the specified minimum). The throat dimension governs acceptance in AWS D1.1, ASME Section IX, and EN ISO 5817.
Throat Definitions
| Term | Definition |
|---|---|
| Theoretical throat (a) | Shortest distance from the root to the hypotenuse of the largest right triangle inscribed in the weld cross-section. For equal-leg fillets: a = leg x 0.707. |
| Actual throat | Shortest distance from the root to the actual face of the weld (accounts for convexity or concavity) |
| Effective throat | The minimum throat used in strength calculations; equals theoretical throat for SMAW, FCAW; may include root penetration for SAW, GMAW per AWS D1.1 |
Minimum Throat Thickness (AWS D1.1)
AWS D1.1 specifies minimum fillet weld sizes based on the thickness of the thicker part joined:
| Base Metal Thickness (mm) | Minimum Fillet Weld Leg (mm) | Corresponding Min. Throat (mm) |
|---|---|---|
| Up to 6 | 3 | 2.1 |
| 6 to 13 | 5 | 3.5 |
| 13 to 19 | 6 | 4.2 |
| Over 19 | 8 | 5.7 |
These minimums prevent hydrogen cracking by ensuring enough heat input to slow the cooling rate of the heat-affected zone.
Calculating Weld Throat
For a standard 45-degree fillet weld with equal legs:
Theoretical throat = 0.707 x leg size
For example, a 10 mm leg fillet weld has a theoretical throat of 7.07 mm. The effective throat area per unit length is the throat dimension multiplied by the weld length. The allowable load equals the effective throat area multiplied by the allowable shear stress of the weld metal (typically 0.30 x tensile strength per AWS D1.1 or AISC 360).
For unequal-leg fillets, the throat is measured as the shortest distance from the root to the line connecting the two toes.
Inspection Methods
The actual throat is measured using fillet weld gauges (bridge cam gauge or Palmgren gauge) placed across the weld face. Undersized throats are a common NDT rejection cause. If the actual throat is less than the specified minimum, the weld must be built up with additional passes.
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