What is the minimum effective length of a fillet weld?

• A. 38 mm (1 1/4 in) or 4 times the size of the fillet
• B. 25 mm (1 in)
• C. 50 mm (2 in)
• D. 6 mm (1/4 in)

Answer: (A)

The key idea is that a fillet weld must have enough length to develop its strength relative to the weld’s thickness. The minimum effective length is tied to the leg size, but there’s also a practical floor to ensure short welds still behave properly under load. The standard rule is that the minimum effective length equals the greater of four times the fillet leg size or 1 1/4 inches (38 mm).

So you evaluate the weld by comparing 4 times the leg size to 1 1/4 in. If the leg is small, 4 × leg might be less than 1 1/4 in, and you must use 1 1/4 in. If the leg is larger, 4 × leg will exceed 1 1/4 in, and that becomes the minimum length. This guarantees the weld has a sufficient load path and throat area.

That’s why the answer matches: it states either 38 mm or 4 times the fillet size, whichever is larger. The fixed lengths like 25 mm, 50 mm, or 6 mm don’t properly reflect the relationship between leg size and required length.