Two limits bracket every fillet weld before strength is even considered: a minimum that keeps the weld from cooling too fast against thick material, and a maximum that keeps it from running off the edge of the part it is welded to. This page gives both, plus the minimum length that lets you use the size you specified and the effective throat that carries the load.
| Limit | Rule | Section |
|---|---|---|
| Minimum size | by the thickness of the thinner part joined: ≤ 1/4 → 1/8; over 1/4 to 1/2 → 3/16; over 1/2 to 3/4 → 1/4; over 3/4 → 5/16 | Table J2.4 |
| Maximum along an edge | material under 1/4" thick: the thickness itself. 1/4" or thicker: thickness − 1/16", unless the drawing calls for it built out to full throat | J2.2b |
| Minimum effective length | 4 × the nominal size, or the effective size is taken as 1/4 of the length. For intermittent fillet welds the minimum length is the larger of 4 × the size and 1 1/2 in. | J2.2b |
| Effective throat | 0.707 × leg, for an equal-leg fillet on surfaces meeting at 90° | J2.2a |
The minimum length on this page is the plain 4 × size rule. If the weld is intermittent, each segment must also be at least 1 1/2 in. long — the page does not know whether your weld is continuous, so it does not apply that one for you.
Both limits, the 4 × size length rule and the 0.707 throat, were confirmed in two independent published sources that agree line for line: a structural-connection software vendor's published code-check documentation, and a university-level AISC Specification J course handout. Neither is this page's author remembering a table. The minimum-size bands were also cross-checked against the metric companion table, which converts to the same four values.
3/16". AISC 360 Table J2.4 sets the minimum by the thickness of the thinner part joined: 1/8" up to 1/4"; 3/16" over 1/4" to 1/2"; 1/4" over 1/2" to 3/4"; 5/16" over 3/4".
If the edge is under 1/4" thick, the fillet may equal the thickness. If it is 1/4" or thicker, the maximum is the thickness minus 1/16", unless the drawing calls for the weld built out to full throat. That is AISC 360 J2.2b. A 1/2" edge takes at most a 7/16" fillet.
Four times the nominal size — otherwise the effective size is taken as one quarter of the length (J2.2b). A 3/16" fillet needs at least 3/4" of length. For an intermittent fillet each segment must also be at least 1 1/2" long.
0.707 times the leg, for an equal-leg fillet on surfaces meeting at 90° (J2.2a). A 3/16" leg gives a 0.133" throat; a 7/16" leg gives 0.309". A skewed joint has a different throat and the drawing has to say so.
A 1/2" plate welded to a 3/4" plate, with the weld running along the 1/2" edge: the range reads 3/16" to 7/16", minimum length at the minimum size 3/4" (4 × 3/16"), and the throat at that size 0.133". If this page says otherwise, do not use it — tell us.