Fillet Weld Size and Strength Calculator
Size a fillet weld or check its strength with AISC 360 LRFD and ASD rules, including load angle, and estimate the weld metal and filler wire or rod needed.
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About this toolWhat is the Weld Size Calculator?
A fillet weld carries load through its throat, the shortest distance from the root to the face. For the usual equal-leg fillet with a flat face that is 0.707 times the leg size, and the weld’s strength is the throat area times the shear strength of the weld metal. This calculator applies that idea the way the AISC Specification (ANSI/AISC 360) does, for both LRFD and ASD design.
Choose whether you want the strength of a given weld or the leg size for a given load. Enter the leg, the length of each weld line and how many lines share the load, pick the electrode classification and give the angle between the load and the weld axis. The tool returns the design or allowable strength, strength per unit length, effective throat and the required leg rounded up to the next 1/16 in or millimetre. A table compares common leg sizes, and the filler section estimates deposited weld metal and how much rod or wire to buy for your process.
- Strength check or required leg size for a load
- AISC 360 LRFD (φ 0.75) and ASD (Ω 2.00), E60 to E100 electrodes
- Directional strength increase for load angle
- AISC minimum and maximum fillet size check
- Weld metal and filler weight by process, leg size table, inch and metric
How to use the Weld Size Calculator
- Pick inch or metric units and whether to check a weld’s strength or size a weld for a load.
- Enter the leg size (or the load), the length of each weld line and the number of lines.
- Select the electrode strength and the load angle: 0 for load along the weld, 90 for load across it.
- Choose LRFD with factored loads or ASD with service loads, and optionally enter the thinner plate thickness to check AISC size limits.
- Pick the welding process and overfill allowance for the filler estimate, then read the results and the leg size table.
Worked examples
Default: two 6 in lines of 1/4 in E70 fillet
Input
Leg 1/4 in, 2 x 6 in, E70, θ 0°, LRFD
Output
φRn 66.82 kip, 5.568 kip/in per line, throat 0.1768 in
This is 4 sixteenths x 1.392 kip/in x 12 in. In ASD the same welds are good for 44.55 kip.
Size a weld for a 30 kip factored load
Input
Load 30 kip, 2 x 5 in, E70, LRFD
Output
Required leg 0.135 in, use 3/16 in
Check the minimum size too: on a 1/2 in plate AISC Table J2.4 requires at least 3/16 in anyway.
Metric bracket and filler
Input
Leg 6 mm, 2 x 150 mm, E70 (483 MPa), LRFD; stick electrode at 60%, 10% overfill
Output
276.4 kN design strength; about 0.155 kg of weld metal and 0.26 kg of electrodes per metre of weld
Stick welding wastes about 40% of the rod weight to stubs, slag and spatter, so buy nearly twice the deposited weight.
Background and insight
AISC 360 Section J2 gives the nominal strength of a fillet weld as Rn = Fnw x Awe, with Fnw = 0.60 Fexx for welds loaded along their length. LRFD multiplies by φ = 0.75, and ASD divides by Ω = 2.00, which is the familiar 0.30 Fexx allowable stress on the throat. For E70 electrodes that gives 1.392 kip per inch for each sixteenth of an inch of leg in LRFD and 0.928 kip per inch in ASD, the constants printed in the AISC Steel Construction Manual.
Welds loaded across their axis are stronger. Equation J2-5 multiplies the strength by 1.0 + 0.50 sin^1.5 θ, so a transverse weld (θ = 90°) is 1.5 times as strong as a longitudinal one. The code limits where this increase may be used, for example in groups that mix longitudinal and transverse welds, so leave the angle at 0 unless you know the rule applies to your joint.
Filler metal estimates use the cross-section of the weld: w²/2 for a flat-faced equal-leg fillet, plus an allowance for convexity and overfill. Multiplied by length and the density of steel this gives the deposited weight; dividing by the deposition efficiency gives the electrode or wire to buy. Typical efficiencies are about 60% for stick electrodes with a 2 in stub, 95% for solid MIG wire, 82 to 85% for flux-cored wire and close to 100% for submerged arc.
Common use cases
Connection checks
Check fillet welds on brackets, clip angles, gussets and base plates against a design load.
Specifying weld sizes
Find the smallest practical leg size for a load, then check it against the AISC minimum for the plate thickness.
Estimating consumables
Work out how many kilograms of wire or rod a job needs for costing and ordering.
Training and exam prep
Reproduce the throat, LRFD and ASD numbers from the AISC Manual by hand and check your working.
Tips and limitations
- This checks the weld metal only. Also check the base metal at the fusion face and the connected elements, as AISC J4 requires.
- Use factored loads with LRFD and service loads with ASD. Mixing them is a common error.
- Bigger is not better: weld volume grows with the square of the leg, so two passes of a larger weld cost far more than a longer small weld.
- Very short welds and end returns have extra rules in AISC J2.2b; keep each fillet at least four times its leg size long.
- Undercut, porosity and lack of fusion reduce strength. Inspection matters as much as the size calculation.
Frequently asked questions
How do you calculate fillet weld strength?
Multiply the effective throat (0.707 x leg) by the weld length and by 0.60 Fexx to get the nominal strength. In AISC LRFD multiply by 0.75; in ASD divide by 2.00.
What is the strength of a 1/4 inch fillet weld per inch?
With E70 electrodes, 5.57 kip per inch in LRFD and 3.71 kip per inch in ASD for load along the weld, before any directional increase.
What is the throat of a fillet weld?
For an equal-leg fillet with a flat or slightly convex face, the effective throat is the leg times cos 45°, 0.707 x leg. A 6 mm fillet has a 4.24 mm throat.
What is the minimum fillet weld size?
AISC Table J2.4 bases it on 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 above that.
How much welding wire do I need per foot of fillet weld?
A flat 1/4 in fillet holds about 0.106 lb of weld metal per foot. Divide by the deposition efficiency, about 0.95 for solid MIG wire or 0.6 for stick rod, and add an allowance for convexity.
Are transverse fillet welds stronger than longitudinal ones?
Yes. AISC allows up to 1.5 times the strength for a weld loaded at 90° to its axis, through the factor 1 + 0.5 sin^1.5 θ, subject to the code’s conditions.
Is there an in-depth guide to the Weld Size Calculator?
Yes. “Fillet Weld Size and Strength: Throat, AISC LRFD and ASD” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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