Fillet Weld Size and Strength Calculator
A fillet weld is sized by its leg, but it carries load through its throat. This calculator converts leg and length to throat area and then to weld metal capacity for common electrode classes, with both LRFD and ASD values.
Your numbers
Leg of an equal-leg fillet.
Length of the weld that carries load, minus crater lengths.
A lap joint with welds on both sides has 2 lines.
Your results
Weld metal check only, using the AISC style 0.6 FEXX shear basis. Base metal shear, load direction effects, minimum sizes and fatigue must be checked by the responsible engineer.
Design strength (kN) vs leg size
Sweeps Fillet leg size from 3 to 16, holding your other inputs fixed.
Weld size to estimate hand-off
Tell us how weld sizes reach your estimators and we will suggest a way to connect them.
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How to use it
Enter the leg of the fillet, the effective length that actually carries load, the filler metal class and how many weld lines share the load. The tool returns the effective throat, nominal strength and both design bases. Use the chart to see how quickly capacity grows with size: strength is linear in leg, while the weld volume and cost grow with the square of the leg, so a bigger weld is not always the efficient answer.
- Subtract crater lengths from the length
- Count each side of a lap joint as a line
- Compare against the required load in the same units
Method and assumptions
The throat of an equal-leg fillet is 0.707 times the leg. Nominal shear capacity of the weld metal is taken as 0.6 times the filler metal tensile strength times the throat area, which is the common AISC style basis. Design strength applies a resistance factor of 0.75 for LRFD or a safety factor of 2.00 for ASD. Directional strength increases for loads transverse to the weld are ignored here, which is conservative.
Worked example
A 6 mm leg gives a 4.24 mm throat. With E70xx filler and two lines of 200 mm, the nominal strength is about 0.6 x 483 x 4.24 x 400 divided by 1000, close to 491 kN. LRFD design strength is about 368 kN and ASD allowable about 246 kN. Then check the base metal: the connected plate may govern in shear or tension before the weld does.
Common mistakes
Do not use the leg as if it were the throat, and do not forget the weld metal is only one check. Minimum fillet sizes depend on the thicker part joined, and maximum sizes depend on edge thickness. Intermittent welds, eccentric loads, and fatigue-governed details need separate treatment. Use this as a quick screen, not as the design record.
Frequently Asked Questions
How do I convert fillet leg to throat?
For an equal-leg fillet with flat face, throat equals 0.707 times the leg. A 6 mm leg gives a throat of about 4.24 mm. Convex or concave profiles and unequal legs change the effective throat.
What is the strength of an E70 fillet weld?
Using the AISC style basis, nominal shear is 0.6 times 70 ksi, or about 42 ksi (290 MPa), applied to the throat area. Apply phi 0.75 for LRFD or divide by 2.00 for ASD.
Is the base metal checked here?
No. Only the weld metal is checked. Base metal shear rupture, yielding and tear-out must be verified separately, and they sometimes govern for thin plates and large fillets.
Why does the chart rise in a straight line?
Weld capacity is proportional to throat, which is proportional to leg. The volume of weld metal, however, grows with the square of the leg, which is why continuous small fillets are often cheaper than one large fillet.
If your drawings and weld symbols live in a PLM or ERP, Netray can help route weld sizes into BOM and routing data so shop estimates match the design.
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