A fillet weld does not fail across the leg dimension written on the drawing. Shear cuts through the weld’s narrowest section — the throat — which runs from the root of the corner diagonally out to the weld face. For a standard 45-degree fillet that diagonal is the leg times sin 45°, giving the constant that runs through every fillet calculation: throat = 0.707 × leg. Confusing the two overstates a weld’s capacity by 41%, because the throat is always √2 smaller than the leg the welder gauges at the joint.
The throat is also why weld-group analysis produces answers in force per unit length rather than stress. Computing group properties per unit throat keeps the whole analysis independent of leg size: the result at the critical point is a shear flow in pounds per inch (or N/mm), and only the closing step — required leg = f_r / (0.707 × Fw) — brings the throat factor and the allowable weld stress in to supply enough throat area for the demand. AISC 360 frames fillet strength on the effective throat; the 0.707 conversion is the bridge back to the leg size that appears on the drawing callout.