How to read a fillet weld symbol — leg size, throat, equal and unequal legs, intermittent fillet welds, weld-all-around. D1.1:2025 minimum size requirements from Table 7.7.
Fillet Weld Symbol — Anatomy
Basic fillet — 5/16" leg
Both sides — weld each face
Intermittent — 3" welds, 12" c/c
Key rule: The fillet weld symbol is always a right triangle with the vertical leg on the left. This orientation is fixed — it does not indicate direction of the weld or joint geometry.
Dimensions
Leg Size vs Throat — What Gets Specified
The leg size (shown on the symbol) is the distance from the root of the joint to the toe of the weld along each fusion face. For an equal-leg fillet weld, both legs are equal and this is the number on the symbol.
The theoretical throat is the perpendicular distance from the root to the hypotenuse of the largest right triangle inscribed within the weld cross-section. For a 45° equal-leg fillet weld:
Throat = Leg × 0.707
Per AWS D1.1, structural calculations use the effective throat to determine weld strength — not leg size. For prequalified fillet welds, effective throat = theoretical throat (no deduction). The symbol specifies leg size because that is what the welder measures and the inspector verifies with a fillet weld gauge.
Example
A 3/8" fillet weld symbol means: leg = 3/8 inch (9.5mm). Theoretical throat = 3/8 × 0.707 = 0.265" (6.7mm). This is the value used in the shear strength calculation Fw = 0.60 × FEXX × effective throat × length.
D1.1:2025 Table 7.7
Minimum Fillet Weld Size
AWS D1.1:2025 Table 7.7 specifies minimum fillet weld leg sizes based on the thicker part being joined. These minimums prevent underbead cracking from high restraint and insufficient heat input.
Thicker part joined
Min. fillet weld size
Notes
To 1/4" (6mm)
1/8" (3mm)
Thin material — avoid burn-through
Over 1/4" to 1/2" (6–12mm)
3/16" (5mm)
Most common in light structural
Over 1/2" to 3/4" (12–20mm)
1/4" (6mm)
Standard structural plate
Over 3/4" (20mm)
5/16" (8mm)
Heavy plate — preheat often required
Table 7.7 footnote a: For non-low-hydrogen processes without preheat, T = thickness of the thicker part joined (single-pass required). For non-low-hydrogen processes with cracking-prevention procedures per Clause 6.8.4, and for all low-hydrogen processes, T = thickness of the thinner part joined and single-pass is not required. Source: D1.1:2025 Table 7.7 Footnote a.
Table 7.7 footnote b: The minimum fillet weld size need not exceed the thickness of the thinner part joined. Source: D1.1:2025 Table 7.7 Footnote b.
Cyclically loaded structures (footnote c): For cyclically loaded structures, the maximum fillet weld size permitted by Table 7.7 is 3/16 in [5 mm], regardless of base metal thickness. Source: D1.1:2025 Table 7.7 Footnote c.
Maximum size on edges: Along edges of material 1/4" (6mm) or more thick, fillet weld size shall not exceed material thickness minus 1/16" (2mm) unless the drawing specifically requires full-thickness — D1.1:2025 Clause 4.5.2.9.
Common Questions
Fillet Weld Symbol FAQ
What is a fillet weld symbol?
A fillet weld symbol is a right triangle placed on the reference line of a welding symbol. The vertical leg is always on the left. The number to the left of the triangle is the fillet weld leg size. It is the most common weld type in structural steel fabrication — used for T-joints, lap joints, and corner joints.
How do you read fillet weld size from the symbol?
The fillet weld size appears to the left of the fillet weld triangle symbol. This is the leg length — e.g. 5/16" means a 5/16-inch (8mm) leg on both sides. For unequal-leg fillet welds, both leg sizes appear to the left in the format S1×S2 — e.g. 1/4×3/8. A drawing detail shows which leg applies to which member. The leg size is what the welder measures with a fillet gauge.
What is the difference between fillet weld leg and throat?
The leg is the distance from the root to the toe along each fusion face — this is what the symbol specifies and what the inspector measures. The throat is the perpendicular distance from the root to the weld face. For an equal-leg 45° fillet: throat = leg × 0.707. Throat governs structural strength calculations. AWS D1.1 specifies minimum legs via Table 7.7.
What does 3-12 mean on a fillet weld symbol?
The numbers to the right of the fillet symbol in the format length-pitch indicate an intermittent fillet weld: 3-12 means 3-inch welds with 12-inch center-to-center spacing. Used to reduce heat input and distortion on long joints. The length and pitch appear to the right of the weld symbol on the arrow side or other side as applicable.
What is the minimum fillet weld size per D1.1?
Per AWS D1.1:2025 Table 7.7, minimum fillet weld sizes by thicker part: ≤1/4" → 1/8" min; over 1/4"–1/2" → 3/16" min; over 1/2"–3/4" → 1/4" min; over 3/4" → 5/16" min. These minimums prevent underbead cracking. Always check Table 7.7 — not your memory — when specifying fillet sizes on thin material or thick plate.
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