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BuyFillet Weld Strength Calculator
Load capacity and pass/fail check per AWS D1.1 and AISC
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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 Fy 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 Fw = 0.30 × FEXX, where FEXX is the electrode classification strength (60, 70, 80 ksi, etc.).
Capacity: P = Fw × 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 φRn = 0.75 × 0.6 × Fy × tbm × 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 / Pgoverning. 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.
Pro tip: When D/C is between 0.85 and 1.0, consider upsizing one fillet increment or adding a second weld line before fabrication — field rework is expensive.