About the Fillet Weld Calculator
This fillet weld calculator works out the effective throat of an equal-leg fillet weld, the effective weld area, and the shear strength of the weld metal using the AISC 360 approach. Enter the leg size, the weld length, how many weld lines share the load and the electrode classification (E60XX, E70XX and so on), then choose LRFD or ASD to get the design or allowable strength. Add the applied load to see how much of the weld capacity it uses.
It is meant for fabricators, structural and mechanical engineers, welding students and inspectors who need a fast check on a lap joint, a stiffener, a bracket or a gusset plate. Results are shown in inches, ksi and kips or in millimetres, MPa and kN.
The calculation covers the weld metal only. A complete connection check also needs the base metal shear and rupture strength, minimum and maximum weld sizes, and end-return and length limits from the governing code. The optional load angle applies the AISC directional strength increase for welds loaded transversely; leave it at 0° for welds loaded along their length.
How to use the fillet weld calculator
- 1Choose imperial or metric units.
- 2Enter the fillet weld leg size and the effective length of one weld line.
- 3Enter how many weld lines carry the load and pick the electrode grade.
- 4Choose LRFD or ASD and, if the weld is loaded across its axis, enter the load angle.
- 5Enter your factored (LRFD) or service (ASD) load to see the utilization.
Formula and method
A fillet weld fails in shear through its throat, the shortest distance from the root to the face. For an equal-leg 45° fillet the effective throat is the leg size times sin 45°, or 0.707w. The effective area is the throat times the total effective length of all weld lines that share the load.
AISC 360 (Section J2) takes the nominal stress of the weld metal as 0.60 F_EXX, where F_EXX is the electrode tensile strength. For a linear weld group loaded in-plane, that stress may be multiplied by 1.0 + 0.50 sin^1.5 θ, where θ is the angle between the load and the weld axis, giving up to 50% more strength for transverse welds. The design strength is φRn with φ = 0.75 (LRFD) or the allowable strength Rn/Ω with Ω = 2.00 (ASD). Metric results use the same formula with F_EXX converted to MPa (70 ksi ≈ 482.6 MPa).
- w
- Fillet weld leg size
- t
- Effective throat (0.707w for equal legs)
- L, n
- Length of each weld line and number of lines
- F_EXX
- Electrode classification strength (e.g. 70 ksi for E70XX)
- θ
- Angle between the load direction and the weld axis
- φ, Ω
- Resistance factor (0.75) and safety factor (2.00)
Worked examples
1/4" E70 fillet, two 6" welds, LRFD
The throat is 0.707 × 0.25 = 0.1768 in, so two 6-inch welds give 2.121 in² of throat area. Nominal strength = 0.60 × 70 × 2.121 = 89.1 kips and φRn = 0.75 × 89.1 = 66.8 kips. A 50-kip factored load uses about 75% of the capacity. Per inch, that is 5.57 kips/in — the familiar 1.392 kips/in per sixteenth of an inch.
5/16" E70 fillet, one 4" weld, ASD
Throat = 0.707 × 0.3125 = 0.221 in and area = 0.884 in². Rn = 0.60 × 70 × 0.884 = 37.1 kips, and the allowable strength is 37.1 ÷ 2.00 = 18.6 kips, so a 10-kip service load uses about 54%.
Metric: 8 mm E70 fillet, two 150 mm transverse welds
The throat is 5.657 mm and the area 1,697 mm². With θ = 90° the directional factor is 1.5, so Rn = 0.60 × 482.6 MPa × 1.5 × 1,697 mm² ≈ 737 kN and φRn ≈ 553 kN; a 400 kN factored load uses about 72%.
Frequently asked questions
What is the throat of a fillet weld?+
The throat is the shortest distance from the weld root to the face of the weld. For an equal-leg fillet with a flat face it is the leg size × 0.707, so a 1/4-inch fillet has a throat of about 0.177 inch. Strength is calculated on this throat area.
How strong is a 1/4 inch fillet weld per inch?+
With E70 electrodes under AISC LRFD, a fillet weld carries 1.392 kips per inch for every 1/16 inch of leg, so a 1/4-inch fillet gives about 5.57 kips per inch (3.71 kips/in under ASD) when loaded along its length.
Why is a transverse fillet weld stronger?+
Tests show a fillet weld loaded perpendicular to its axis is stiffer and up to 50% stronger than one loaded along its length. AISC allows the factor 1.0 + 0.50 sin^1.5 θ for welds in a linear group; it does not apply to every weld group configuration.
Does this check the base metal?+
No. It checks only the weld metal. The connected parts must also be checked for shear yielding and shear rupture along the weld line, and the weld size must respect the minimum size for the thinner part and the maximum size along plate edges.
What is the minimum fillet weld size?+
AISC Table J2.4 sets minimum sizes by the thinner part joined: 1/8 in up to 1/4 in thick, 3/16 in over 1/4 to 1/2 in, 1/4 in over 1/2 to 3/4 in, and 5/16 in over 3/4 in. Always confirm with the code edition you are designing to.
What does E70XX mean?+
E stands for electrode and 70 is the minimum tensile strength of the deposited weld metal in ksi (70 ksi ≈ 483 MPa). The last digits describe welding position and coating or current type, which do not change the strength calculation.