Fillet Weld Calculator: Throat Stress and Length

The throat stress in a fillet weld carrying a force, and the length or leg size needed.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

A fillet weld fails across its throat, the shortest distance through the weld from the root to the face. For an equal-leg weld with leg size s the throat is 0.707 s. The simple method divides the force by the throat area (throat times length) to give an average stress.

Compare that stress with the allowable stress from your welding standard, the electrode strength and the type of loading. The calculator also gives the shortest length at your leg size and the smallest leg size at your length for the factor you want.

Formulas used

  • a = 0.707\,s
  • \tau = \frac{F}{a L}
  • L_{req} = \frac{F}{a\,\tau_{allow}}

Worked example

With the default numbers in the calculator above:

  1. Throat thickness of an equal-leg fillet: 4.243 mm
  2. Throat stress: 70.71 MPa
  3. Factor against the allowable stress: 1.697
  4. Shortest weld length at this leg size: 117.9 mm
  5. Smallest leg size at this length: 3.536 mm

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What is the throat?

The thickness of the weld measured across its smallest section. For an equal-leg fillet it is 0.707 times the leg.

Which allowable stress do I use?

The one from your welding standard for the electrode, the loading and the safety level. The calculator does not choose it for you.

Does it include fatigue?

No. Welds under repeated loading need a fatigue check, which usually governs.

Related calculators

Further reading: Shigley, Mechanical Engineering Design (welded joints); AWS and AS/NZS 1554 are the standards themselves (licensed).

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