BIZOMAT MAKEOS · Fabrication
Fillet Weld Strength
Can these fillet welds carry the load, and what leg size is needed?
Throat thickness, load capacity of fillet welds on the throat with an allowable shear stress, utilisation under a load and the minimum leg size required.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Leg size | mm | Leg length of an equal-leg fillet weld. |
| Effective length of each weld | mm | Effective length of each weld, excluding craters and start-stops. |
| Number of welds sharing the load | welds | Number of welds of this size carrying the load. |
| Allowable shear stress on the throat | MPa | Allowable or design shear stress on the weld throat from your design code and electrode (e.g. AS 4100 / AS/NZS 1554 or AWS D1.1). |
| Applied load | kN | Load on the weld group, assumed shared uniformly in shear. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Leg size
- What it is
- The length of each leg of an equal-leg fillet weld, measured from the root along each plate surface to the weld toe.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Weld capacity rises in direct proportion.
- Get it right
- Leg size is not throat size; the tool converts leg to throat for you.
What this means: Effective length of each weld
- What it is
- The length of each weld that carries load at full size, leaving out craters and start and stop regions.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Weld capacity rises in direct proportion.
- Get it right
- Deduct start and stop lengths if your code requires it; enter the length of one weld, not the total.
What this means: Number of welds sharing the load
- What it is
- How many separate fillet welds of this size and length share the applied load, for example one each side of a plate.
- Units and format
- Enter a number in welds: welds. The form accepts 1 welds to 100 welds. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 welds as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Weld capacity rises in direct proportion.
- Get it right
- Count only welds that genuinely share this load; the tool assumes they share it equally.
What this means: Allowable shear stress on the throat
- What it is
- The shear stress the weld throat is allowed to carry, set by your design code, weld category and electrode strength.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 10000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 145 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Weld capacity rises in direct proportion.
- Get it right
- Take it from AS 4100, AS/NZS 1554 or your governing code for the actual electrode; do not use the plate strength.
What this means: Applied load
- What it is
- The total load on the whole weld group, assumed to act in shear and shared evenly between the welds.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0 kN to 10000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 120 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Utilisation and the minimum leg size rise in direct proportion.
- Get it right
- Enter the total load in kilonewtons, not the load per weld.
Method and formulas
- Throat a = 0.7071 × leg (equal-leg, 90° fillet).
- Capacity = a × L × number of welds × allowable shear.
- Minimum leg = F ÷ (0.7071 × L × number × allowable shear).
- Example: two 6 mm fillets 100 mm long carry 123.04 kN at 145 MPa.
Limitations
- Uniform shear on the throat; eccentric loads add bending and torsion to the weld group.
- Base metal capacity, fatigue category and weld quality must be checked to the code.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size fillet welds for a bracket or lug
- Check an existing weld after a load change
- Compare weld length and leg size
Who it is for
- Structural and mechanical engineers
- Welding supervisors
- Fabrication estimators
What you get
- Throat thickness
- Weld capacity
- Utilisation
- Minimum leg size for the load
Interpreting the result
- Use code-specified design strengths and capacity factors for structural work.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1476–1486, 1469–1475. Topic reference; no source tables or recommended settings are reproduced.
- AS/NZS 1554.1 Structural steel welding — Welding of steel structures; AS 4100 Steel structures
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Weld metal in fillet welds
What it covers: Calculate the throat, cross-section and mass of fillet welds, see how fast weld metal grows with leg size and gaps, and estimate the weld metal in a conveyor frame.
Method explained here · BIZOMAT original book
Fillet welds: throat, length and capacity
In Designing Welded Joints by the Numbers
What it covers: Size fillet welds from the throat, calculate the load a millimetre of weld can carry at each size, find the length a joint needs, and see why doubling the leg nearly quadruples the weld metal.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.