How to use this calculator
- Choose the design basis. Use ASD for service loads or LRFD for factored loads.
- Enter the fillet legs. Use equal legs for a standard fillet or different legs for an unequal fillet.
- Enter length and fit-up. Enter effective weld length and any root-opening allowance.
- Select the electrode. Choose E60, E70 or E80 to set F_EXX.
- Check the result. Read effective throat, capacity, utilization and required equal-leg size.
How it works
A fillet weld is designed on the throat area, not the full visible leg. For a right-angle unequal-leg fillet, the theoretical throat is the perpendicular distance from the root to the weld face:
a = z1 · z2 / sqrt(z1² + z2²)
Equal legs reduce to the familiar a = 0.707 · z. The calculator
then subtracts the root-opening allowance:
a_eff = a - g
Capacity is the design weld stress times the effective throat area. Choose ASD for service loads or LRFD for factored loads:
R = Fw · a_eff · L
Worked example
Verified against the live calculator
Check a 6 mm × 6 mm E70 fillet, 100 mm long, with no
root-opening allowance under a 50 kN service load on the ASD basis.
The theoretical and effective throat are 6 / sqrt(2) = 4.24 mm.
ASD design stress is 0.30 × 483 = 144.9 MPa. Capacity is
144.9 × 4.24 × 100 = 61.5 kN, so utilization is
0.81. The required equal-leg size for the same load is
4.88 mm.
Frequently asked questions
How do you calculate weld throat?
For a right-angle fillet weld with unequal legs z1 and z2, the theoretical throat is z1*z2 / sqrt(z1^2 + z2^2). For equal legs, that reduces to 0.707 times the leg size. The calculator subtracts any entered root-opening allowance to get effective throat.
What is the difference between leg size and throat size?
Leg size is the visible fillet dimension along the connected parts. Throat size is the shortest design distance from the weld root to the face. Strength is normally based on throat area, not the full leg size.
How do ASD and LRFD change the result?
ASD uses 0.30*F_EXX and should be checked against service loads. LRFD uses phi*0.60*F_EXX with phi = 0.75, so the design stress is 0.45*F_EXX and should be checked against factored loads.
Does root opening reduce effective throat?
A root opening or poor fit-up can reduce the effective throat unless the weld size is increased or the required throat is demonstrated. The root-opening field is a conservative reduction for that case.
When should I use the weld group calculator instead?
Use the weld group calculator when the weld is part of a bracket or rectangular weld layout under eccentric loading. This page is for throat area and straight-line weld capacity only.
Does this replace AWS or AISC design?
No. It is a first-pass weld-metal throat check. Minimum and maximum weld sizes, base metal, fatigue, joint details, WPS/PQR requirements and inspection acceptance must still follow the governing code and project specification.
Method & assumptions
- Right-angle fillet weld geometry. For non-90° skewed T-joints, qualified effective-throat rules and Z-loss allowances may apply.
- ASD capacity uses
0.30 × F_EXX. LRFD capacity uses0.75 × 0.60 × F_EXX. Match service/factored loads to the selected basis. - Root opening is treated as a conservative throat reduction. If the weld size is increased to compensate or the required throat is demonstrated, use zero.
- Use the fillet weld size calculator for the simpler equal-leg capacity workflow, or the weld group calculator for eccentric rectangular weld groups.
- Base metal, minimum/maximum fillet size, edge melting, intermittent welds, weld returns, fatigue, inspection acceptance and WPS/PQR requirements are not included.