Fillet Weld Size and Strength Calculator

Strength of a fillet weld, or the weld length needed for a load, from the leg size, electrode and load direction, by the allowable (ASD) or design (LRFD) method.

How it works

A fillet weld fails by shear through its throat, the shortest distance from the root to the face of the weld: for an equal-leg 45° fillet of leg w the throat is 0.707 w. The nominal strength of the weld metal in shear is 0.6 FEXX (FEXX is the electrode classification strength), and a load across the weld is stronger than one along it.

Rn = 0.6 FEXX × 0.707 w × (1 + 0.5 sin1.5 θ) per unit length

The smallest permitted fillet depends on the thickness of the thinner part (AWS D1.1 Table 5.8): 3 mm up to 6 mm thick, 5 mm up to 12.5 mm, 6 mm up to 19 mm and 8 mm above. The weld may not be wider than the plate edge allows. The stresses in joined members and bolted joints are in Bolted and Welded Joints.

Worked example

An E70 fillet weld of 6 mm leg joins two 10 mm plates; the load acts along the welds; the total weld length (both sides) is 300 mm; design by ASD.

  • Throat a = 0.7071 × 6 = 4.24 mm. Directional factor = 1 (longitudinal).
  • Rn = 0.6 × 482.6 × 4.24 = 1 228.5 N per mm of weld; the allowable is 1 228.5 ÷ 2 = 614.2 N/mm.
  • Strength = 614.2 × 300 = 184.3 kN.
  • If the load were across the weld (θ = 90°) the factor would be 1.5 and the allowable 921.4 N/mm. With the LRFD method the design strength per mm is 0.75 × 1 228.5 = 921.4 N/mm.
  • For a 120 kN load the weld length needed is 120 000 ÷ 614.2 = 195 mm. The minimum fillet for 10 mm plate is 5 mm, so a 6 mm weld is acceptable.

Things to know

The strength of the base metal at the fusion face (shear on the plate: 0.6 Fy × plate thickness × length) can govern for thin plates and strong welds: check it as well.

Welds loaded in bending or torsion (eccentric loads) need the weld group treated as a line section: the stress is the vector sum of the direct shear and the moment-induced shear at the farthest point.

Fatigue governs welds in cyclic service (AWS D1.1 Chapter 2, Eurocode 3 Part 1-9): the allowable stress range is much lower than the static strength.

The effective length of a longitudinal fillet weld is reduced if it is longer than 100 times the leg size. End returns do not count.

Eurocode 3 and AWS differ in method: this calculator follows the US practice (AISC 360 and AWS D1.1). Always follow the code that applies to your design and have critical welds inspected.

Common questions

What is the throat of a fillet weld?

The shortest distance from the root of the weld to its outer face. For an equal-leg fillet at 90° between the plates it is 0.707 times the leg size.

Why is a transverse weld stronger than a longitudinal one?

The shear is shared over a larger plane and the weld metal is loaded at an angle of higher strength, so AISC gives up to 50 percent more strength for a load at 90° to the weld axis.

What does E70 mean?

An electrode with a minimum tensile strength of 70 ksi (483 MPa) for the weld metal. E60 is 60 ksi and E80 is 80 ksi.

Sources

  • AISC 360-22, Specification for Structural Steel Buildings, Section J2.4 and Table J2.5; AWS D1.1/D1.1M, Structural Welding Code, Steel, Table 5.8.
  • Blodgett, O. W., Design of Welded Structures, The James F. Lincoln Arc Welding Foundation; Budynas and Nisbett, Shigley’s Mechanical Engineering Design, Chapter 9.

Last reviewed: November 2, 2026