Fillet Weld Size & Strength Calculator
Work out the strength of a fillet weld to AISC 360-22 Section J2.4, or the leg size or length it needs for a load, in LRFD or ASD and in inches or millimetres. It finds the effective throat for equal or unequal legs and checks the AISC minimum and maximum sizes.
How to Use
- Pick what to find: the strength of a weld you have, the leg size a load needs, or the length a load needs.
- Enter the leg size (and a second leg only if the legs are unequal), the length of each weld and whether there is one weld or one on each side.
- Choose the electrode strength (E70 is 70 ksi; E49 is 490 MPa), LRFD or ASD, and the load with its unit.
- Enter the thickness of the thinner part to check the AISC Table J2.4 minimum size and the maximum size along an edge.
- Turn on the direction factor kds only for a linear weld group loaded through its centre, and mark end-loaded welds so long welds get the J2.2b(d) reduction. Show Work lists each clause used.
Worked Example
A 1/4 in E70 fillet, 6 in long. Throat = 0.7071 × 0.25 = 0.1768 in; effective area = 0.1768 × 6 = 1.061 in². Nominal strength Rn = 0.60 × 70 × 1.061 = 44.55 kip. LRFD design strength φRn = 0.75 × 44.55 = 33.41 kip; ASD allowable strength Rn ÷ 2.00 = 22.27 kip.
The leg for a 50 kip load. Two 8 in E70 welds, LRFD: Rn needed = 50 ÷ 0.75 = 66.67 kip, so w = 66.67 ÷ (0.60 × 70 × 0.7071 × 8 × 2) = 0.1403 in. Round up to 3/16 in, which is also the Table J2.4 minimum when the thinner part is 1/2 in thick; it carries 66.82 kip.
The common mistake: using the leg as the throat. Working the 1/4 in, 6 in weld with the leg in place of the throat gives 0.75 × 0.60 × 70 × 0.25 × 6 = 47.25 kip, 41% more than the real 33.41 kip. The weld fails across its throat, which is only 0.7071 of the leg.
Show Work
Formulas
Why the Throat, and Where the Rules Come From
A fillet weld is triangular in section, and the narrowest path through it runs from the root straight out to the face: the throat. Loaded along its length, the weld shears on that plane. The AISC Commentary notes that tests showed the stress on the fusion faces between weld and base metal does not decide the shear strength (Preece, 1968), so the strength is the throat area times the shear strength of the weld metal, taken as 0.60 times the electrode’s classification strength FEXX.
The rules here are from ANSI/AISC 360-22, the Specification for Structural Steel Buildings dated August 1, 2022, which superseded the June 7, 2016 edition. AISC publishes it on aisc.org. Section J2.2 covers effective area and the size and length limits, and J2.4 and Table J2.5 give the strength with φ = 0.75 for LRFD and Ω = 2.00 for ASD. Welds loaded across their axis are stronger, which the directional factor kds rewards, but the Commentary explains it must be set to 1.0 for fillet welds at the ends of square and rectangular HSS loaded in tension, where tests fell short of the target reliability.
Matching filler metal is set by AWS D1.1, which Table J2.5 refers to. The base metal next to the weld must be checked separately under Section J4 where it could govern.
About This Tool
This calculator applies AISC 360-22 Section J2.4 to straight fillet welds: the strength of a weld you have, or the leg or length a load needs, with the effective throat for equal or unequal legs, the direction factor, the end-loaded length reduction and the 4 × size minimum length. It also checks the leg against the Table J2.4 minimum and the J2.2b(b) maximum along an edge. It does not analyse eccentric weld groups (the instantaneous centre method in the AISC Manual) or check the base metal.
Use it for sizing and checking; a connection on a real structure is designed and signed off by the engineer of record to the code that applies. Everything runs in your browser; nothing you enter is sent anywhere.
Related tools: Weld Metal & Filler Consumption Calculator, Weld Symbol Reference, and Weld Heat Input Calculator.
Frequently Asked Questions
How strong is a 1/4 inch fillet weld?
With E70 electrodes, AISC 360 gives 0.75 × 0.60 × 70 ksi × 0.7071 × 0.25 in = 5.568 kip per inch of weld (LRFD), which is the familiar 1.392 kip per inch for each sixteenth of leg. In ASD it is 0.60 × 70 × 0.1768 ÷ 2.00 = 3.712 kip per inch. A 6 in weld carries 33.41 kip LRFD or 22.27 kip ASD.
What is the throat of a fillet weld?
The shortest distance from the root to the face. For equal legs it is 0.7071 × the leg, so a 1/4 in fillet has a 0.1768 in throat. For unequal legs a and b it is a × b ÷ √(a² + b²): a 1/4 by 3/8 in fillet has a 0.208 in throat.
What is the minimum fillet weld size?
AISC Table J2.4 sets it by the thinner part joined: up to 1/4 in (6 mm) thick, 1/8 in (3 mm); over 1/4 to 1/2 in (13 mm), 3/16 in (5 mm); over 1/2 to 3/4 in (19 mm), 1/4 in (6 mm); over 3/4 in, 5/16 in (8 mm). The weld must also be big enough for the load.
What is the largest fillet weld allowed along a plate edge?
AISC J2.2b(b): along the edge of material under 1/4 in (6 mm) thick, no more than the thickness; at 1/4 in or more, the thickness minus 1/16 in (2 mm), unless the drawings call for building it out to full throat. Along the edge of a 3/8 in plate that is 5/16 in.
Why is a transverse fillet weld stronger?
Fillet welds loaded across their axis are stronger than ones loaded along it. AISC lets you multiply by kds = 1.0 + 0.50 sin^1.5 θ, which is 1.5 at 90°, where strain compatibility is considered (for example a linear weld group loaded through its centre). A 5/16 in E70 fillet 4 in long goes from 27.84 kip to 41.76 kip LRFD.
How do I use the Fillet Weld Size & Strength Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Sizing a connection
A 50 kip LRFD load on two 8 in E70 welds needs a 0.1403 in leg, so specify 3/16 in, which is also the Table J2.4 minimum for 1/2 in plate, and carries 66.82 kip.
Metric brackets
A 6 mm E49 fillet 100 mm long carries 0.75 × 0.60 × 490 × 4.243 × 100 = 93.55 kN, so an 80 kN load uses 85.5% of it.
Weld length for a load
Two 8 mm E49 fillets carrying 120 kN need 48.1 mm each, about 96.2 mm of weld in all.
Checking drawings
A 1/2 in fillet along the edge of a 3/8 in plate breaks J2.2b(b), which caps it at 5/16 in there.
Long end-loaded welds
A 1/4 in fillet 40 in long in an end connection is 160 times its leg, so β = 1.2 − 0.002 × 160 = 0.88 and it carries 196 kip, not 222.7 kip.
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