Fillet weld size
Read the size and length of fillet welds. In AWS mode the sizes are in inches, in ISO mode in millimetres.
What leg size does this fillet weld need?
Size on the left, length on the right
The number to the left of the symbol is the size of the weld. The number to the right is the length. No length means the weld runs the full length of the joint.
AWS A2.4: the size is the leg
An AWS fillet size has no letter in front. It is always the leg size: how far the weld reaches along each part from the root of the joint.
ISO 2553: the letter tells you what was measured
- a = design throat thickness, the depth of the weld from the root to the face.
- z = leg length, the same thing AWS calls the leg size.
The throat is shorter than the leg. On an equal-leg fillet: leg = throat x 1.414.
| Marked | Throat | Leg needed |
|---|---|---|
| a3 | 3 mm | 4.2 mm, about z4 |
| a4 | 4 mm | 5.7 mm, about z6 |
| a5 | 5 mm | 7.1 mm, about z7 |
| a6 | 6 mm | 8.5 mm, about z8 |
| a7 | 7 mm | 9.9 mm, about z10 |
| a8 | 8 mm | 11.3 mm, about z11 |
The most common mistake: reading a5 as 5 mm legs. That weld ends up with a 3.5 mm throat, about 30% short of what the designer asked for.
Questions
- What does the number next to a fillet weld symbol mean?
- On an AWS drawing it is the leg size. On an ISO 2553 drawing it always has a letter in front: "a" means the design throat thickness and "z" means the leg length.
- Is a5 the same as a 5 mm fillet?
- No. a5 is a 5 mm throat. On an equal-leg fillet that needs legs of about 7 mm. Welding a5 with 5 mm legs gives a weld that is too small.
- How do I convert throat to leg size?
- For an equal-leg fillet weld, leg = throat x 1.414 and throat = leg x 0.707.
- What if a fillet weld has two sizes, like 1/4 x 3/8?
- That is an unequal-leg fillet in AWS. The drawing shows which leg goes on which part. ISO 2553 does not use "a" for unequal legs.
Explained in our own words. For the full rules, see AWS A2.4:2020 and ISO 2553:2019.