Fillet Weld Strength Calculator

What this tool does

Gives the strength of a fillet weld by leg size and electrode, per inch of weld and for the full length you specify. E60 through E120 are covered, with E70 the usual default.

Strength is referenced to the 45° effective throat. The nominal value is divided by Ω = 2.00 to give the ASD allowable. The weld metal is assumed to govern.

Formula

A fillet weld is checked on the throat, not the leg, and the throat of a standard 45° fillet is 0.707 times the leg. Leg w, throat t_e and length L in inches; F_EXX in ksi; R_n and the allowable in kip (kip/in where L = 1 in).

Assumptions

Fillet welds are checked by ASD with Ω = 2.00. Per-inch strengths come from the electrode grade (FEXX) and weld size using the effective throat = 0.707 × leg. The weld metal is assumed to govern.

• Electrodes, FEXX 60–120 ksi; E70 is the common default for typical structural steel.
• Throat, strength is referenced to the 45° effective throat (0.707 × leg) of a standard fillet.
• Nominal against allowable, R_n = 0.60·F_EXX·t_e·L is the nominal strength. The allowable is that divided by Ω = 2.00. The two are not interchangeable; never size a weld off the nominal figure.
• Connected material, the weld is checked on its throat only. The connected materials (base-metal shear yielding and rupture along the weld, and the plate or member itself) require a separate check (AISC J2, J4).
• Load direction, a fillet weld is checked in shear on the effective throat whatever direction the load comes from. There is no separate tension check on a fillet: the tool resolves a resultant into components so you can see them, then checks the resultant itself against the one allowable.
• No directional increase taken, §J2.4(a) permits (1.0 + 0.50·sin^1.5 θ) on a fillet loaded at an angle to its axis, worth up to 50% on a transverse weld. The tool does not take it. Results are conservative for any load that is not purely longitudinal.
• Overhead lifting, Ω = 2.00 is a structural design factor, not a lifting one. Welds in a lifting lug, spreader beam or any below-the-hook assembly are designed to a 5:1 design factor on ultimate; see ASME BTH-1 and the applicable lifting standard. Do not use these allowables for a lift.
• Dynamic loading, these are static allowables. Impact, vibration, swinging, cyclic or fatigue loading all require an additional factor above what is shown here.
• Weld quality, the calculated strength assumes a weld that is actually made to size and is sound. Confirm procedure, electrode, prequalification, inspection and acceptance criteria against AISC 360 Chapter J and AWS D1.1.

How capacity is derived

Nominal weld strength scales with electrode strength, throat and length; the allowable divides the nominal by Ω = 2.00. The equations are in the Formula block above.

Worked example

Applications & rules of thumb

• Develop the part, size the weld to carry the connected element's force, no more than it can deliver.
• Min / max fillet, minimum size is set by the thinner part joined (AISC 360 Table J2.4); maximum along an edge is the part thickness (− 1/16" for material ≥ 1/4").
• Balance the welds, place welds symmetrically about the load line to avoid eccentric prying, and return them around corners.

Field notes

• Minimum fillet size is set by the thinner part joined (AISC 360 Table J2.4), not the load. A weld can be strong enough on paper and still be undersized by code.
• Ω = 2.00 is for structure, not for lifting. A lug, spreader or any below-the-hook weldment is designed to 5:1 on ultimate per ASME BTH-1, roughly 2.5 times more conservative than what this page shows.
• Add margin for anything that moves. Impact, swing, vibration and fatigue are all outside these static numbers.
• A calculated weld and a sound weld are two different things. Confirm the procedure, electrode, fit-up and inspection against AISC 360 Chapter J and AWS D1.1 before you count on the number.