AWS A2.4 vs ISO 2553
Two standards, one goal: telling a welder exactly what to do. AWS A2.4 dominates in North America, while ISO 2553 is used across the UK, Europe, Asia, and most of the rest of the world. The symbols look different, but the resulting weld is the same. This page covers every major difference, side by side.
1. Quick comparison table
The headline differences at a glance. For the full explanation of each row, read the sections below.
| Feature | AWS A2.4 | ISO 2553 |
|---|---|---|
| Reference line | Single solid line | Dual line (solid + dashed) in System A; single line in System B |
| Arrow side convention | Below the line | On the solid line (System A); below the line (System B) |
| Other side convention | Above the line | On the dashed line (System A); above the line (System B) |
| Fillet weld sizing | Bare number = leg size | a prefix = throat; z prefix = leg |
| Groove angle placement | Outside the weld symbol | Inside the weld symbol (within the V or bevel) |
| Weld process notation | Letter codes in tail (SMAW, GMAW, etc.) | ISO 4063 numeric codes in tail (111, 135, etc.) |
| Geographical usage | USA, Canada | UK, EU, Australia, Asia, most of world |
| Current edition | AWS A2.4:2020 | ISO 2553:2019 |
2. Reference line
The reference line is the horizontal backbone of every welding symbol. It is the most visible difference between the two standards.
Single reference line
One solid horizontal line. The weld symbol's position above or below this line determines arrow side vs other side.
Dual reference line
Two parallel lines: a solid line and a dashed line. The dashed line marks the "other side". Either line may be on top.
What about ISO System B?
ISO 2553 defines two systems. System A (dual line) is the most widely used internationally and the most distinctively "ISO". System B uses a single reference line and follows the same above/below convention as AWS A2.4. If a drawing uses ISO System B, reading it is effectively the same as reading an AWS symbol.
3. Arrow side / other side convention
This is the number-one source of confusion when reading welding symbols, especially for people switching between standards. The core concept is simple: the arrow points to a joint, and each side of that joint can be independently specified. The two systems just use different visual cues.
Same weld, different symbol
Fillet weld on the arrow side of a T-joint
AWS A2.4 / ISO System B
ISO 2553 System A
Both symbols instruct the welder to place a fillet weld on the arrow side. The result on the metal is identical.
Other side example
Fillet weld on the other side (away from the arrow)
AWS A2.4 / ISO System B
ISO 2553 System A
Both sides
When a weld is required on both sides of the joint, AWS places the symbol both above and below the single reference line. ISO System A places the symbol on both the solid and dashed lines. The dimensions may differ per side if the weld is asymmetric.
For a step-by-step walkthrough of reading arrow side vs other side on a real symbol, see the how to read welding symbols guide.
4. Fillet weld sizing
Fillet welds are the most common weld type, and the two standards dimension them differently. This is a frequent source of error when converting between standards.
Explicit prefix required
ISO mandates a prefix letter before the dimension number:
a5 = 5 mm throat thickness z7 = 7 mm leg length No prefix, assumes leg
AWS does not use a prefix. The number to the left of the fillet symbol is always the leg size.
5/16 = 5/16" leg size (imperial) 7 = 7 mm leg size (metric)
AWS can also show two unequal leg sizes for asymmetric fillets, e.g. 3/8 × 1/4.
Converting between throat and leg
For an equal-leg fillet weld, the relationship is:
a = z × 0.707 or equivalently z = a × 1.414 | ISO throat (a) | ISO leg (z) / AWS leg | Note |
|---|---|---|
| a3 | z4 / 4 | 3 × 1.414 = 4.24, rounded to 4 |
| a5 | z7 / 7 | 5 × 1.414 = 7.07, rounded to 7 |
| a6 | z8 / 8 | 6 × 1.414 = 8.49, rounded to 8 |
| a8 | z11 / 11 | 8 × 1.414 = 11.31, rounded to 11 |
5. Where each standard is used
The geographical split is broadly America vs everywhere else, but the reality is more nuanced. Many multinational projects use both, and ISO explicitly accommodates this by defining two systems.
| Region | Primary standard | Notes |
|---|---|---|
| United States | AWS A2.4 | Dominant in structural, piping, and general fabrication |
| Canada | AWS A2.4 | CSA W59 references AWS conventions |
| United Kingdom | ISO 2553 | BS EN ISO 2553 (identical adoption). Replaced BS 499 |
| European Union | ISO 2553 | Adopted as EN ISO 2553 across all member states |
| Australia & New Zealand | ISO 2553 | AS/NZS ISO 2553. Replaced AS 1101.3 |
| Japan | ISO 2553 | JIS Z 3021 aligns with ISO 2553 |
| China | ISO 2553 | GB/T 324 based on ISO 2553 |
| Multinational projects | Varies | Often specify which system in the project welding specification. Both accepted on many global projects |
Important: ISO 2553 recognises both System A and System B. Since System B closely mirrors AWS conventions, it effectively means ISO-compliant drawings can look very similar to AWS drawings. The key is to always check the drawing title block or welding specification for which system is in use.
6. Key takeaways
The most important differences to remember when switching between AWS and ISO:
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The resulting weld is the same. Both standards describe the same physical welds. Only the notation differs.
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The dashed line is the giveaway. If you see a dashed line running parallel to the reference line, you are reading ISO System A. No dashed line means AWS or ISO System B.
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Arrow side convention is the biggest difference. ISO System A: solid line = arrow side. AWS / ISO System B: below the line = arrow side. Get this right and the rest follows.
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Fillet sizes need converting. ISO uses
a(throat) orz(leg). AWS uses bare numbers and assumes leg. When converting ISO throat to AWS leg, multiply by 1.414. -
Always check the title block. The drawing should specify which standard (and which ISO system) is in use. Do not guess.
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ISO System B exists. It uses a single reference line, just like AWS. This means an ISO-compliant drawing does not necessarily have a dashed line.
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