AWS A2.4 vs ISO 2553 welding symbols
The same weld can look different on a US drawing and a European drawing. These are the differences that cause wrong welds, side by side.
| AWS A2.4 (US) | ISO 2553 | |
|---|---|---|
| Used mostly in | USA, Canada | Europe, UK, Australia and much of the rest of the world |
| Reference line | One solid line | System A: solid line plus dashed line. System B: one solid line |
| Arrow side | Symbol below the line | System A: symbol on the solid line. System B: below the line |
| Other side | Symbol above the line | System A: symbol on the dashed line. System B: above the line |
| Fillet size | Number only, always the leg size | Letter first: a = throat thickness, z = leg length |
| Units | Inches or millimetres | Millimetres |
| Intermittent welds | Length-pitch, e.g. 2-6, pitch center to center | n × l (e), e.g. 3 × 50 (100) |
| Weld names | Groove welds: V-groove, bevel-groove | Butt welds: single-V butt, single-bevel butt |
| Welding process | Letters in the tail, e.g. GMAW | Number in the tail, e.g. 135 for MAG welding |
| Weld all around, field weld | Circle, flag | Circle, flag |
The same fillet weld, three ways
A fillet weld on the other side of the joint, with legs of about 7 mm:
The two mistakes that cost the most
- Reading System A like AWS. On a System A drawing, a symbol above the line can still be the arrow side, if the dashed line is below it. Always find the dashed line first.
- Reading a5 as a 5 mm leg. a5 is a 5 mm throat, which needs about 7 mm legs. The weld comes out about 30% too small.
How to tell which system a drawing uses
- A dashed line next to the reference line: ISO 2553 System A.
- Fillet sizes with a or z in front: ISO 2553.
- Fractions of an inch and length-pitch like 2-6: AWS A2.4.
- The title block or the welding notes should name the standard. When in doubt, ask.
Explained in our own words. For the full rules, see AWS A2.4:2020 and ISO 2553:2019.