Fillet Weld a-Dimension vs z-Dimension (ISO 2553)
The a-dimension is the design throat thickness of a fillet weld. The z-dimension is the leg length. They describe the same weld but are not the same number — mixing them up either wastes weld metal or leaves the joint under-strength.
Definitions
For an equal-leg fillet weld with a 90° joint angle, the throat and the leg are linked by a fixed factor.
- a = design throat thickness, measured from the theoretical root to the face of the weld.
- z = leg length, measured along the fusion face on each plate.
- a = z × 0.7 (more exactly z / √2), so z = a × 1.41.
- ISO 2553 writes the value in front of the fillet triangle: a4 or z6.
- Always write the letter. A bare 4 in front of the triangle is ambiguous.
Conversion table
Common equal-leg fillet sizes, rounded the way shops normally specify them.
| a (mm) | z (mm) | Typical plate thickness (mm) |
|---|---|---|
| 2 | 3 | 3–4 |
| 2.5 | 3.5 | 4–5 |
| 3 | 4 | 5–6 |
| 3.5 | 5 | 6–8 |
| 4 | 6 | 8–10 |
| 5 | 7 | 10–12 |
| 6 | 8.5 | 12–16 |
| 7 | 10 | 16–20 |
| 8 | 11 | 20–25 |
How to pick a size
A fillet weld rarely needs to be larger than the thinner part it joins.
- Rule of thumb: a ≈ 0.7 × t for a full-strength fillet on a plate of thickness t in mild steel.
- Practical maximum: a ≤ 0.7 × thinnest plate. A bigger weld adds heat, distortion and cost without extra strength.
- Practical minimum: a 3 mm on structural steel; thinner welds cool too fast and crack risk rises.
- Multi-pass welds are needed above roughly a 5 mm in the flat position.
AWS A2.4 does it differently
US drawings normally give the leg length only, written to the left of the fillet triangle in inches (for example 1/4). Where a throat is meant, AWS writes (E) for effective throat. If you receive a mixed ISO/AWS drawing set, convert once and note the convention in the title block.
Common mistakes
Most fillet weld disputes come from a handful of repeatable errors.
- Reading a4 as a 4 mm leg — that is a 43 % undersized weld.
- Reading z6 as a 6 mm throat — that is an oversized weld and heavy distortion.
- Specifying a large fillet on thin sheet instead of a smaller continuous weld.
- Forgetting that a and z assume a 90° joint; for other angles the throat must be calculated.
Frequently asked questions
What does a4 mean on a welding drawing?
a4 means a fillet weld with a 4 mm design throat thickness per ISO 2553. The equivalent leg length is about 5.7 mm, usually rounded to z6.
What does z6 mean on a welding drawing?
z6 means a fillet weld with a 6 mm leg length. The corresponding throat is about 4.2 mm, so z6 is roughly equal to a4.
How do I convert a to z?
Multiply by 1.41: z = a × 1.41. To go the other way, multiply by 0.7: a = z × 0.7.
Which one should I use on my drawing?
Use whichever your welding procedure and inspection method measure. Throat (a) is the strength-critical dimension; leg (z) is easier to gauge on the shop floor. Never put both on the same weld.
Does AWS A2.4 use a-dimension?
No. AWS A2.4 normally specifies the leg length in inches and writes (E) when an effective throat is meant instead.
Related pages
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ISO 2553 System A vs System B
The two systems inside one standard, and how to tell which a drawing uses.
ISO 2553 vs AWS A2.4
Full comparison of the European and US welding symbol conventions.
Form and position tolerances
ISO 1101 GD&T symbols explained visually, plus ISO 2768 and 13920.