Required leg size
-At the allowable throat stress, no safety factor addedLoad per unit length
-Resultant of shear and bendingChosen weld
-Enter a leg sizeResults
| Total weld length A | - |
| Section modulus Sw (per unit throat) | - |
| Moment M = V·e | - |
| Shear per length V/A | - |
| Bending per length M/Sw | - |
| Allowable throat shear 0.30·FEXX | - |
| Capacity per mm (in) of leg per unit length | - |
| Minimum / maximum size (AISC) | - |
Checks
How the calculator works
The weld group is treated as a line of unit thickness (the method in Blodgett and in Shigley), so the geometry properties are independent of the weld size. The weld size is found afterwards from the force per unit length.
- Throat. For an equal-leg fillet the throat is a = 0.707·w.
- Group properties. Single line: A = d, Sw = d²/6. Two parallel welds: A = 2d, Sw = d²/3. Rectangle: A = 2(b+d), Sw = d(3b+d)/3. Circle: A = πd, Sw = πd²/4.
- Force per unit length. fv = V/A, fb = M/Sw with M = V·e, and f = √(fv² + fb²). The vector sum is a conservative simplification.
- Size. w = f / (0.707 · 0.30 · FEXX). The 0.30 FEXX allowable throat shear is the AISC 360 allowable stress design value (0.60 FEXX divided by 2.00). The safety factor for a chosen weld is the weld capacity over f, relative to that allowable.
- Size limits. AISC minimum leg from the thicker plate (1/8 in to 5/16 in) and maximum along an edge (t − 1/16 in for plate 1/4 in and thicker).
Electrode strengths: E60 = 60 ksi (414 MPa), E70 = 70 ksi (483 MPa), E80 = 80 ksi (552 MPa) minimum tensile. The model is static, in-plane, and takes no credit for the extra strength of transverse welds or for penetration.
Worked example: bracket on two vertical welds
A bracket is welded with two vertical fillets, each d = 100 mm long, and carries V = 10 kN at e = 150 mm. A = 200 mm, Sw = 100²/3 = 3333 mm², M = 1.5×106 N·mm. fv = 50 N/mm, fb = 450 N/mm, f = 452.8 N/mm. With E70 the allowable is 0.30×482.6 = 144.8 MPa, so each mm of leg carries 0.707×144.8 = 102.4 N/mm and w = 4.42 mm: use a 5 mm fillet.
Questions
How do you calculate fillet weld size?
The effective throat of an equal-leg fillet is a = 0.707 w, where w is the leg size. The load per unit length of weld is f = sqrt((V/A)² + (M/Sw)²), with A the total weld length and Sw the weld section modulus treating the weld as a line. The required leg is w = f / (0.707 × allowable shear stress). With AISC ASD the allowable is 0.30 × FEXX, which is 145 MPa (21 ksi) for E70 electrodes. Example: 10 kN at 150 mm eccentricity on two 100 mm vertical welds gives f = 453 N/mm and a required leg of 4.4 mm, so use a 5 mm weld.
What is the strength of a 6 mm fillet weld?
With E70 electrodes and the AISC ASD allowable of 0.30 × 483 MPa = 145 MPa on the throat, a 6 mm fillet (throat 4.24 mm) carries about 615 N per mm of weld length, or 0.61 kN/mm. Multiply by the weld length for the capacity. This is the shear capacity on the throat; the base metal must be checked separately.
What is the minimum fillet weld size?
The AISC minimum depends on the thicker plate joined: 1/8 in (3 mm) up to 1/4 in, 3/16 in (5 mm) up to 1/2 in, 1/4 in (6 mm) up to 3/4 in and 5/16 in (8 mm) above that. The calculator shows this minimum and the maximum size along a plate edge. Check the code that applies to your product.
Is the throat of a fillet weld 0.707 times the leg?
Yes for an equal-leg, flat-face fillet with a 90 degree joint: the throat is the shortest distance from the root to the face, w·cos 45° = 0.707 w. Convex or concave profiles and unequal legs change it, and some processes (submerged arc, deep penetration) allow extra credit for penetration that this calculator does not take.
Does this calculator replace a welding code check?
No. It is a quick static sizing aid for a weld group under in-plane load. Fatigue, out-of-plane loads, base metal strength, joint prep and the rules of AWS D1.1, EN 1993-1-8 or ISO 5817 are outside its scope. Verify critical welds against your code.
Engineering aid for guidance. Static, in-plane sizing of fillet welds only; not fatigue, out-of-plane or eccentric torsional groups, base metal, or code compliance. Verify critical welds against AWS D1.1, AISC 360, EN 1993-1-8 or your applicable code; you remain responsible for the design. Nothing you enter leaves your browser except through a share link you copy yourself.