Fillet Weld Leg Size — Nominal Leg, Effective Throat, and the 0.707 Rule
The fillet weld leg size is the fundamental dimension specifying a fillet weld's size. It appears on every welding symbol — the number to the left of the fillet triangle (e.g., the "1/4" in a 1/4-inch fillet weld). Despite being a simple measurement taken parallel to the plate surfaces, it determines the weld's strength through the effective throat, not the leg itself.
Effective throat: t_e = leg × sin(45°) = leg × 0.707
Weld strength: φRn = φ × 0.60 × F_EXX × t_e × L_w
PRELIMINARY — NOT FOR CONSTRUCTION. All content is for educational and reference use only. Must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in any project.
The Geometry of a Fillet Weld
A fillet weld in cross-section is an approximately triangular deposit of weld metal joining two surfaces at (typically) 90 degrees. Three key dimensions define it:
- Leg size (w): The distance from the weld root to the weld toe measured along each fusion face. For an equal-leg fillet, both legs have the same length. The weld symbol specifies the leg size.
- Effective throat (t_e): The shortest distance from the weld root (the deepest point of joint penetration) to the weld face (the exposed surface of the deposit). For a flat-faced equal-leg fillet, t_e = w × sin(45°) = w × 0.707.
- Weld face: The exposed surface of the deposited weld metal. It may be flat, convex (bulging outward), or concave (dished inward). AISC 360 assumes a flat face at 45 degrees; convex profiles add metal without increasing effective throat, while concave profiles may reduce it.
The critical point is that the throat — not the leg — determines strength. The weld fails through the throat in shear. Doubling the leg size quadruples the throat area (area ∝ leg²), which is why a 5/16-inch fillet is worth far more than twice the strength of a 1/4-inch fillet — it's (0.3125/0.25)² ≈ 1.56 times stronger, not 1.25 times.
The 0.707 Rule — Where It Comes From
The 0.707 multiplier is sin(45°), based on a 45-degree right triangle. For a leg length w, the hypotenuse of the inscribed right triangle (the face) is at 45 degrees to each leg. The altitude of this triangle — the perpendicular distance from the right-angle vertex (the root) to the hypotenuse (the face) — is:
t_e = w × cos(45°) = w × sin(45°) = w × (√2/2) = w × 0.707106...
If the weld has unequal legs (a "skewed" fillet where the two parts meet at an angle other than 90 degrees), the effective throat computation changes. AWS D1.1 provides the general formula for any dihedral angle ψ:
t_e = w × sin(ψ/2) (for equal-leg fillets)
At ψ = 90 degrees: t_e = w × sin(45°) = w × 0.707 — the standard case. At ψ = 60 degrees (a steep junction): t_e = w × sin(30°) = w × 0.50 — significantly reduced. At ψ = 120 degrees (an obtuse junction): t_e = w × sin(60°) = w × 0.866 — increased throat per unit leg. Welders must be specifically instructed when dihedral angles deviate from 90 degrees because the visual leg size will differ from the effective throat controlling the design.
Fillet Weld Strength per AISC 360
AISC 360 J2.4 provides the nominal strength of a fillet weld as the shear strength of the effective throat:
Rn = 0.60 × F_EXX × t_e × L_w
φRn = 0.75 × 0.60 × 70 ksi × (w × 0.707) × L_w = 22.2 × w × L_w (kips, w in inches)
For E70XX electrodes (F_EXX = 70 ksi):
Strength per inch of 1/4" fillet: 5.54 kips/inch
Strength per inch of 5/16" fillet: 6.94 kips/inch
Strength per inch of 3/8" fillet: 8.33 kips/inch
Strength per inch of 1/2" fillet: 11.1 kips/inch
Where F_EXX is the electrode classification strength — 70 ksi for E70XX electrodes (the standard for structural steel), 60 ksi for E60XX. The 0.60 factor on F_EXX reflects that the weld throat fails in shear, and the shear strength of weld metal is approximately 60% of its tensile strength — the same von Mises relationship used for steel base metal (F_v = F_y/√3 ≈ 0.577F_y ≈ 0.60F_y).
Minimum and Maximum Fillet Weld Sizes — AISC J2.2b
Minimum Size (Table J2.4)
The minimum fillet weld size prevents the weld from cooling too fast and cracking (a function of heat input relative to base metal mass):
| Material Thickness of Thicker Part Joined (in) | Minimum Fillet Weld Size (in) |
|---|---|
| ≤ 1/4 | 1/8 |
| 1/4 to 1/2 | 3/16 |
| 1/2 to 3/4 | 1/4 |
| > 3/4 | 5/16 |
These minima are based on single-pass welds. Multi-pass welds (where one pass cools before the next is deposited) can violate the minimum because the individual passes are smaller than the minimum for the parent metal thickness.
Maximum Size Along Edges (J2.2b)
For material less than 1/4 inch thick: the weld leg shall not exceed the material thickness. A 1/4-inch fillet on a 3/16-inch plate would melt the plate edge away — the maximum is 3/16 inch.
For material 1/4 inch or thicker: the weld leg shall not exceed the material thickness minus 1/16 inch, unless the contract documents specifically require the edge to be built out to obtain full throat thickness. So a 3/8-inch fillet on a 3/8-inch plate requires that the drawing state "weld to build out to full thickness" — otherwise the maximum is 5/16 inch.
There is NO maximum fillet weld size for interior welds (away from edges). A 1-inch fillet weld joining two 1-inch-thick plates is perfectly acceptable — the limitation applies only when the weld runs along the edge of a plate where melting through must be prevented.
Measuring Fillet Welds in the Field
Field inspectors use fillet weld gages — small metal templates with notches corresponding to standard leg sizes — to verify the leg dimension. The gage measures each leg independently. A 1/4-inch fillet should measure at least 1/4 inch along both legs unless the drawing calls for an unequal-leg fillet (indicated on the weld symbol with leg sizes above and below the reference line: e.g., 1/4 on top for the vertical leg, 3/8 on bottom for the horizontal leg).
Convexity (excess weld metal beyond the flat triangular profile) does not increase effective throat — it's wasted metal that consumes electrode. Concavity (underfill) reduces throat and must be limited per AWS D1.1 Table 6.1. The throat dimension gage, which measures the perpendicular distance from root to face, provides a direct throat measurement irrespective of the face profile and is preferred over leg gages for critical welds.
Frequently Asked Questions
Can I use a larger fillet weld than the plate thickness?
Not along edges — the maximum size rule (t_plate − 1/16 inch) prevents melting away the edge. For interior welds (away from edges), there is no code limit on maximum fillet size. However, there is an economic limit: weld metal volume increases with the square of leg size, so doubling the leg quadruples the cost. A 5/8-inch fillet contains more than 6 times the metal of a 1/4-inch fillet. Beyond approximately 5/8 inch, groove welds become more economical than fillet welds.
Why is 0.707 used instead of 0.70?
0.707 is sin(45°) = √2/2 = 0.707106... It is exact for a 45-degree right triangle. Using 0.70 instead of 0.707 introduces a 1% error in weld strength — negligible for design but technically incorrect. Some simplified design aids use 0.70 for mental arithmetic (0.70 × 0.25 = 0.175 vs 0.707 × 0.25 = 0.177), but the AISC Specification Commentary uses the exact value.
What happens if the weld is not 45 degrees?
Effective throat decreases. A flatter fillet (legs at unequal angles) has a shorter perpendicular distance from root to face. AWS D1.1 requires that the effective throat be directly measured or computed from the actual dihedral angle. For skewed joints where the angle between parts differs from 90 degrees, the welding symbol qualified by the actual angle, and the weld size indicated is the effective throat, not the leg. The detailer must convert the required throat to the leg size for the actual angle.
International Code References
- AISC 360-22: Section J2.4 — Fillet weld strength. Table J2.4 — Minimum fillet weld sizes. Table J2.5 — Available strength of fillet welds per inch.
- AWS D1.1 (2020): Section 2 — Design of welded connections. Table 2.1 — Minimum fillet weld sizes (matches AISC Table J2.4). Section 6 — Inspection, including fillet weld gaging procedures.
- AS 4100: Section 9.7.3 — Fillet weld capacity. φVw = φ × 0.6 × fuw × tt × kr, where tt = effective throat thickness. Minimum and maximum sizes per AS/NZS 1554.1.
- EN 1993-1-8: Section 4.5.3 — Fillet weld design resistance. Fw,Rd = fvw,d × a, where a = effective throat throat. Minimum throat = 3 mm.
Educational reference only. Fillet weld sizes must be verified against AISC 360 Table J2.4 minimums and J2.2b maximums by a licensed Professional Engineer. All welding must comply with AWS D1.1 procedures and inspection requirements.
Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.