What is the minimum fillet weld size per D1.1?
D1.1:2025 Table 7.7 defines four minimum fillet weld size thresholds. Material thickness T up to 1/4 in requires a 1/8 in minimum fillet. Over 1/4 in to 1/2 in requires 3/16 in. Over 1/2 in to 3/4 in requires 1/4 in. Over 3/4 in requires 5/16 in. Which thickness dimension T represents depends on your process per footnote (a): for low-hydrogen processes (E7018, SAW, GMAW, FCAW), T equals the thinner part joined and single-pass is not required. For non-low-hydrogen processes (E6010, E6011) without a qualified 6.8.4 preheat procedure, T equals the thicker part joined and single-pass welds are required. Footnote (b) states the weld size need not exceed the thickness of the thinner part joined. Footnote (c) sets a minimum fillet weld size of 3/16 in for cyclically loaded structures.
Does fillet weld size depend on the thinner or thicker plate?
It depends on your welding process per Table 7.7 footnote (a). For low-hydrogen processes (E7018, GMAW, FCAW, SAW), T equals the thickness of the thinner part joined. For non-low-hydrogen processes (E6010, E6011) without a qualified Clause 6.8.4 preheat procedure, T equals the thickness of the thicker part joined. This distinction matters because governing by the thicker plate produces a larger minimum weld size. Footnote (b) adds a practical cap: the required weld size need not exceed the thickness of the thinner part. For example, if a 1 in plate is fillet-welded to a 3/16 in plate using E6010 without 6.8.4 preheat, Table 7.7 would require a 5/16 in fillet based on the thicker plate, but footnote (b) allows a 3/16 in fillet since the thinner plate is only 3/16 in thick. This prevents overwelding thin members and potential burn-through or distortion.
Can you use a fillet weld smaller than the Table 7.7 minimum?
Generally no — Table 7.7 minimums are mandatory. The only code-permitted reduction is footnote (b), which states the weld size need not exceed the thickness of the thinner part joined. Note that when using non-low-hydrogen processes (E6010, E6011) without a qualified Clause 6.8.4 preheat procedure, the governing thickness is the thicker part and single-pass welds are required per footnote (a) — this typically results in a larger minimum size compared to low-hydrogen processes where the thinner part governs. Using a weld smaller than the applicable minimum is a code violation that would be rejected during visual inspection per Table 8.1 item (6) Undersized Fillet Welds and Clause 8.9. The Engineer may approve alternative sizes through WPS qualification per Clause 6, but this is uncommon for standard fillet welds.
What is the difference between fillet weld leg size and throat size?
Leg size is the distance from the weld root to the toe along either face of the joint. For an equal-leg fillet weld, both legs are the same dimension, and this is the number specified on the welding symbol per A2.4. Throat size is the shortest distance from the root to the face of the weld — for a flat or slightly convex equal-leg fillet at a 90-degree joint angle, the theoretical throat equals leg size multiplied by 0.707. D1.1 Table 7.7 specifies minimum leg size, not throat size. However, effective throat is what carries the load in design calculations per Clause 4.5.2.6. For deep-penetration GMAW or SAW processes, engineers may increase the effective throat beyond the theoretical value using the provisions in Clause 4.5.2.6, which addresses throat calculations for various joint geometries.
When is a single-pass fillet weld required?
Table 7.7 footnote (a) governs when single-pass fillet welds are required. For non-low-hydrogen processes (E6010, E6011) without a qualified Clause 6.8.4 preheat procedure, single-pass welds are mandatory at all thickness ranges — this is not optional. For low-hydrogen processes (E7018, GMAW, FCAW, SAW), the single-pass requirement does not apply, and multi-pass welds are permitted at any size. Footnote (a) makes no reference to welding position — the single-pass requirement applies regardless of whether the weld is in the flat, horizontal, vertical, or overhead position. In practice, single-pass fillets up to about 5/16 in are common in production because they reduce labor time and heat input. For larger fillets, multiple passes are typical to avoid lack of fusion at the root, excessive convexity, and poor toe tie-in — all rejectable conditions under Table 8.1.