Fillet Weld Strength Calculator

How to use this calculator

Select imperial or metric units, then enter weld leg size a and weld length L. Choose one or two weld lines, orientation (transverse or parallel), and electrode class. Optionally enter applied load F for a demand/capacity pass/fail check. Open Advanced to enter base metal thickness for AWS D1.1 minimum fillet size and adjust base metal yield strength F_y for the shear rupture check.

Understanding fillet weld strength

Effective throat: For an equal-leg fillet, effective throat t = 0.707 × a. This is the critical dimension for shear strength on the failure plane through the weld throat.

Allowable stress on weld metal: AWS D1.1 / AISC use allowable shear stress on the throat F_w = 0.30 × F_EXX, where F_EXX is the electrode classification strength (60, 70, 80 ksi, etc.).

Capacity: P = F_w × t × L × (number of welds). This is nominal weld metal strength in shear on the effective throat area.

Base metal shear rupture: Simplified LRFD check φR_n = 0.75 × 0.6 × F_y × t_bm × L × n. Governing strength is the lesser of weld metal capacity and base metal rupture when both apply.

Demand/capacity: When load F is entered, D/C = F / P_governing. PASS when D/C ≤ 1.0.

AWS D1.1 minimum weld size rules

Table 5.7 sets minimum fillet weld size from thicker connected part (base metal thickness):

• Up to 1/4 in. (6 mm): minimum 1/8 in. (3 mm)
• Over 1/4 to 1/2 in.: minimum 3/16 in. (5 mm)
• Over 1/2 to 3/4 in.: minimum 1/4 in. (6 mm)
• Over 3/4 to 1-1/2 in.: minimum 5/16 in. (8 mm)
• Over 1-1/2 to 2-1/2 in.: minimum 3/8 in. (10 mm)
• Over 2-1/2 to 6 in.: minimum 1/2 in. (13 mm)

Maximum fillet size is also limited by part thickness — do not exceed AWS maximums when detailing.

Pro tips for fillet weld sizing

• Size the fillet for load first, then verify it meets Table 5.7 minimums for the thicker part.
• Two-sided connections often use two weld lines — enter n = 2 when both sides carry shear.
• Transverse fillets are common in seated connections; parallel (longitudinal) fillets may govern differently in built-up members — confirm with connection design software for critical work.
• Undersized fillets fail inspection even when “strong enough” on paper — measure leg length after welding.
• Match electrode strength to base metal and WPS; E70XX is typical for A36/A992.

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