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How do you determine the minimum size of a fillet weld?


Frequently Asked Questions

The size of a fillet weld should be calculated by the designer of the welded structure, based on the nature and magnitude of applied loads, on the material and on the design basis. Methods for the calculation of fillet weld sizes are explained here.

Once a fillet weld size has been calculated, it should be reported on the technical drawing(s), using the appropriate welding symbols or provided in a technical specification.

Unfortunately, there are many instances in which a fillet weld sizes is unclear on the available drawings, not reported, or simply the information has been lost. In such cases, one should get back to the designer. Failing this, as a rule-of-thumb at least two passes should be deposited, with a minimum leg length of 6mm (1/4in), considering that the first run is likely to be defective.

International standards do not generally provide requirements or guidance on minimum fillet weld sizes. The only example of such guidance is AWS D1.1, which includes the following table (size=leg length):

Table. Minimum fillet weld sizes

Base metal thickness (T)a

Minimum size of fillet weldb





T < 1/4

T < 6



1/4 < T < 1/2

6 < T < 12



1/2 < T < 3/4

12 < T < 20



¾ < T

20 < T




  1. For non low-hydrogen processes without preheat calculated in conformance with 4.8.4, T equals thickness of thicker part joined; single pass welds shall be used.

For non low-hydrogen processes using procedures established to prevent cracking in conformance with 4.8.4 and for low-hydrogen processes, T equals thickness of thinner part joined; single pass requirement shall not apply.

  1. Except that the weld size need not exceed the thickness of the thinner part joined.
  2. Minimum size for cyclically loaded structure shall be 3/16in [5mm].

DISCLAIMER: the advice provided above cannot substitute appropriate design of the welded structure and consultation of the applicable standards and specifications. It is an opinion only, and cannot be taken as a definitive answer. No liability rests with TWI for any damages arising from the information provided in this web page.

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