How to calculate the size of a fillet weld?

The fillet weld is the most common weld in steel structures. Here is how its size is determined.

Geometry:

• Leg (weld size): distance from the root to the toe of the triangle, measured on the base metal
• Throat: height of the largest triangle inscribed in the weld — throat = leg × 0.707 for a 90° fillet weld, since sin 45° = 0.707 (geometry, not a code value)
• Effective area: throat × weld length

Strength:

The design strength of a fillet weld — allowable stress, safety factors and load combinations — is defined by the applicable code, CSA W59 for structural steel, and applied by the engineer. The platform publishes no shortcut formula: a formula stripped of its factors gives a wrong result.

Minimum size versus thickness:

Minimum fillet weld sizes as a function of the thickness of the parts joined are tabulated in CSA W59: that table, not a memory aid, is what applies to the drawing. On a real job, the size is shown on the drawing and verified by the engineer.