A fillet weld fails through its throat, the shortest distance from the root to the face – about 0.707 times the leg size for an equal-leg weld. Transverse fillets (perpendicular to the load) are checked against the allowable tensile stress, parallel fillets against the allowable shear stress. A full-penetration butt weld carries the load across the full plate thickness.
How to use the Welding Calculator
- Weld type
- Leg size / plate thickness
- Weld length
- Number of welds
- Allowable tensile stress
- Allowable shear stress
- Every result field is editable: type into Weld load capacity and Weld length plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the weld type, enter the leg size (or plate thickness for a butt weld), the weld length, the number of welds and the allowable stresses. Type the load you need into the result to find the weld length required.
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Weld type | Choose one: Transverse fillet, Parallel fillet, Butt weld (full penetration) |
| Leg size / plate thickness | Units: mm, in, cm |
| Weld length | Units: mm, in, cm |
| Number of welds | Choose one: Single, Double (both sides) |
| Allowable tensile stress | Units: MPa, psi, ksi, kPa, Pa |
| Allowable shear stress | Units: MPa, psi, ksi, kPa, Pa |
Example values
How the main result changes when Weld length changes and everything else stays at the example values above:
| Weld length (mm) | Weld load capacity (kN) |
|---|---|
| 50 | 79.18 |
| 75 | 118.78 |
| 100 | 158.37 |
| 125 | 197.96 |
| 150 | 237.55 |
| 200 | 316.74 |
Formula
Transverse fillet: P = 0.707 × s × l × σt × nParallel fillet: P = 0.707 × s × l × τ × nButt weld: P = t × l × σts = leg size, 0.707 s = throat, n = number of weldsWorked example
Inputs — Weld type: Transverse fillet; Leg size / plate thickness: 8 mm; Weld length: 100 mm; Number of welds: Double (both sides); Allowable tensile stress: 140 MPa; Allowable shear stress: 95 MPa.
Results — Throat thickness = 5.66 mm; Weld load capacity = 158.37 kN.
Key terms
- Throat
- The minimum distance from the weld root to its face.
Frequently asked questions
What is the throat of a fillet weld?
For an equal-leg 45° fillet, the throat is 0.707 × leg size.
What allowable stress should I use?
AWS D1.1 uses 0.30 × the electrode tensile strength for fillet welds – about 21 ksi (145 MPa) for E70 electrodes.
Why do parallel and transverse fillets differ?
Parallel fillets are loaded in shear along their length; transverse fillets see combined tension and shear and are somewhat stronger.
What formula does the Welding Calculator use?
Transverse fillet: P = 0.707 × s × l × σt × n.