Fillet Weld Strength Calculator

This calculator uses a common fillet weld shear strength model:

• Effective throat: t = 0.707 · w
• Effective weld area: A_w = t · L · (number of weld lines)
• Nominal strength: R_n = 0.6 · F_EXX · A_w
• Reported capacity: LRFD: φR_n   |   ASD: R_n/Ω

Always confirm the correct factors, limits, and detailing rules for your standard.

1. Select the unit system and the design method.
2. Enter weld size (leg), effective length, and number of weld lines.
3. Enter electrode strength (FEXX) and the applicable φ or Ω values.
4. Optionally enter an applied load to get utilization and a status check.
5. Press Calculate Strength; results appear above the form.
6. Use Download CSV or Download PDF for records.

1) Why fillet weld strength matters on site

Fillet welds tie angles, plates, stiffeners, and brackets together. A strength check confirms the specified weld size and length can transfer shear without overstressing the weld metal. This calculator reports nominal strength and design capacity in LRFD or ASD format for clear submittals and safer decisions during fabrication and erection.

2) Inputs that control capacity

The key inputs are weld leg size, effective weld length, number of weld lines, and electrode strength (FEXX). Verify continuity where assumed and exclude defective start/stop regions from the effective length. Common electrode strengths include 60 ksi (≈410 MPa) and 70 ksi (≈490 MPa). When using inch units, keep FEXX in ksi and lengths in inches. In metric, use MPa and millimeters; results are summarized in kN.

3) Effective throat and weld area

For an equal-leg fillet weld, the effective throat is t = 0.707·w. The effective area is A_w = t·L·n, where L is effective length and n is the number of resisting weld lines. Length increases capacity linearly and often improves economy when detailing allows.

4) Nominal strength model and worked example

A widely used weld-metal shear model is R_n = 0.6·F_EXX·A_w. With metric inputs, MPa equals N/mm². Example: w = 6 mm, L = 200 mm, n = 2, FEXX = 490 MPa → A_w ≈ 1697 mm² and R_n ≈ 499 kN; with φ = 0.75, capacity ≈ 374 kN.

5) LRFD versus ASD reporting

LRFD multiplies nominal strength by φ to get a factored resistance. ASD divides nominal strength by Ω to get an allowable capacity. Match the method to your load combinations so demand and capacity are compared on the same basis.

6) Load direction and weld layout

This calculator targets direct shear on the weld metal. If loads are eccentric, weld groups see nonuniform demand along the welds. For brackets, copes, and seat connections with offset loads, use a weld-group analysis to capture torsion and combined effects, and consider direction parallel or transverse to the weld.

7) Detailing, base-metal checks, and documentation

Confirm minimum and maximum weld sizes for the joined thicknesses, access for welding, edge distances, and intermittent weld rules. Also check base-metal limit states; thin plates or heat-affected zones can govern first. Document electrode class, measured welds, inspection notes, and utilization for traceable QA/QC.

Q1. What is the fillet weld leg size in this tool?

It is the visible weld “leg” dimension measured from the root to each toe. For equal-leg fillets, the calculator converts leg size to effective throat using the 0.707 factor.

Q2. Why does the calculator use 0.707 times the weld size?

For a 45° fillet, the effective throat is the shortest distance from root to face. Geometry gives t = w/√2, which is approximately 0.707·w for equal-leg fillets.

Q3. Should I enter actual length or effective length?

Enter effective length: the portion that reliably carries load. Exclude end craters, poor tie-ins, and discontinuities. If the weld is intermittent, enter the total effective sum length of the segments.

Q4. How do multiple weld lines affect the result?

If welds act in parallel resisting the same shear, capacity scales with the number of weld lines through A_w = t·L·n. Ensure the load path truly shares force between lines.

Q5. Which φ and Ω values should I use?

Use the values required by your governing design standard and project specifications. Many structural steel workflows use different factors for LRFD and ASD. If you are unsure, confirm with the project engineer.

Q6. Does this cover fatigue, seismic, or fracture checks?

No. It reports weld-metal strength in shear and a utilization check against an applied load. Fatigue, seismic detailing, fracture-critical requirements, and base-metal limit states may require separate evaluation.

Q7. Why is my utilization high even with a large weld size?

Capacity may be limited by short effective length, too few weld lines, low electrode strength, or conservative factors. Also check eccentric loading; weld-group effects can increase demand above the direct shear assumption.