AWS A2.4 Annex F · ISO 2553
ISO 2553 vs AWS A2.4 — Welding Symbol Differences
ISO 2553 and AWS A2.4 both define welding symbols, but they differ in three critical areas: arrow-side indication (dashed line vs position), fillet weld dimensioning (a throat vs z leg modifiers), and intermittent weld notation. This guide covers every difference referenced in A2.4:2020 Annex F with side-by-side diagrams.
Ask Flux
Reference Line Convention — The Core Difference
Single line — position determines side
Dual line — line type determines side
Key difference: In AWS A2.4, below = arrow side, above = other side. In ISO 2553 System A, the solid line = arrow side and the dashed line = other side, regardless of which line is on top. The dashed identification line may be drawn above or below the solid reference line without changing meaning.
A2.4 Annex F · System A vs System B
Two Systems in ISO 2553
ISO 2553 provides two systems for representing welding symbols:
| Feature | System A | System B | AWS A2.4 |
|---|---|---|---|
| Reference line | Solid + dashed (dual) | Single continuous | Single continuous |
| Arrow side indicated by | Solid line (always) | Below reference line | Below reference line |
| Other side indicated by | Dashed line (always) | Above reference line | Above reference line |
| Fillet weld size | a (throat) or z (leg) | a (throat) or z (leg) | Leg only (no modifier) |
| Units | Metric required | Metric required | U.S. Customary or metric |
Per A2.4:2020 Annex F, System B functions similarly to the AWS A2.4 welding symbol. However, differences remain in fillet weld dimensioning and intermittent weld notation even in System B. For AWS arrow-side rules, see Clause 6.1 and Figure 4.3. For D1.1 applicability, Table 1.1 specifies which standard governs symbol usage on structural steel drawings.
A2.4 Annex F · Figure F2
Fillet Weld Modifiers — a vs z
This is the most practically significant difference between ISO 2553 and AWS A2.4 for fabricators. A misread can result in an undersized weld.
ISO a/z Modifier vs AWS Leg Dimension
a = throat dimension
z = leg dimension
AWS = leg, no modifier
Conversion (A2.4 Annex F): For equal-leg 45° fillet welds, z = a × √2 (approximately a × 1.414). An a4 fillet (4 mm throat) has a leg of approximately z5.7 (5.7 mm). A fabricator reading a4 as a 4 mm leg would produce an undersized weld.
A2.4 Annex F · Intermittent Welds
Intermittent Weld Notation Differences
ISO 2553 and AWS A2.4 specify intermittent welds using different notation systems:
| Feature | ISO 2553 | AWS A2.4 |
|---|---|---|
| Format | n × l (e) | length-pitch |
| n | Number of weld elements | Not used |
| l | Length of each element | Length (to left of hyphen) |
| (e) or pitch | Distance between elements (edge-to-edge) | Center-to-center pitch |
| Staggered indication | Z symbol between lines | Symbols offset on reference line |
Critical note: ISO 2553 uses edge-to-edge distance between weld elements (e), while AWS A2.4 uses center-to-center pitch. Converting between the two: pitch = length + distance. For a notation of 2-6 (AWS A2.4), the equivalent ISO distance would be 4 in (6 minus 2). Always verify which standard governs before interpreting intermittent weld spacing.
A2.4 Annex F4
Weld Types with No AWS Equivalent
ISO 2553 defines several weld types that have no direct AWS A2.4 symbol equivalent. Per A2.4:2020 Annex F, the comparable AWS symbol should be used with dimension information where possible.
| ISO 2553 Type | AWS A2.4 Equivalent |
|---|---|
| Single-V butt weld with broad face root | No symbol |
| Single bevel butt weld with broad face root | No symbol |
| Steep-flanked single-V butt weld | Use comparable V-groove with dimensions |
| Steep-flanked single bevel butt weld | Use comparable bevel-groove with dimensions |
| Stake weld | Use comparable spot or seam weld with process info |
| Toe grinding symbol | No symbol (specified in notes) |
CWI Exam Tip: The CWI exam tests the ability to read AWS A2.4 symbols, not ISO 2553. However, knowing the differences is tested in Part A (Fundamentals). The most frequently tested concept is the arrow-side reversal: in AWS A2.4, below the line = arrow side. In ISO 2553 System A, the solid line = arrow side, regardless of position.
Common Questions
ISO 2553 vs AWS A2.4 FAQ
What is the difference between ISO 2553 and AWS A2.4?
ISO 2553 and AWS A2.4 are both welding symbol standards, but they differ in three critical ways. First, arrow-side indication: in AWS A2.4, information below the reference line is on the arrow side; ISO 2553 System A uses a dashed identification line where information on the solid reference line is always arrow-side regardless of position. Second, fillet weld sizing: AWS A2.4 always specifies the leg dimension, while ISO 2553 uses modifiers a (throat dimension) and z (leg dimension) to distinguish measurement method. Third, intermittent welds: ISO 2553 specifies number of elements, length, and distance between elements, while AWS A2.4 uses length-pitch notation. ISO 2553 provides two systems: System A (dual reference line with dashed identification line) and System B (single reference line similar to AWS A2.4). Per AWS A2.4:2020 Annex F, System B functions similarly to the AWS convention.
What is the ISO 2553 dashed line?
In ISO 2553 System A, the welding symbol has two horizontal lines: a continuous solid reference line and a dashed identification line. The dashed line indicates the other side of the joint. Information placed on the solid reference line always applies to the arrow side, and information on the dashed identification line always applies to the other side. The dashed line may be drawn above or below the solid line without changing the meaning. This is the most significant difference from AWS A2.4, where arrow-side information goes below a single reference line and other-side information goes above it. System A eliminates the above/below ambiguity by using line type (solid vs dashed) rather than position to convey side significance.
What do a and z mean on an ISO 2553 fillet weld symbol?
In ISO 2553, the letters a and z are fillet weld dimension modifiers placed before the size value. The modifier a indicates the fillet weld throat dimension, the shortest distance from the root to the weld face. The modifier z indicates the leg dimension, the distance along each fusion face. For example, a4 means a 4 mm throat dimension, while z4 means a 4 mm leg dimension. To convert between the two for equal-leg 45-degree fillet welds: z equals a multiplied by the square root of 2 (approximately 1.414). AWS A2.4 always uses the leg dimension without any modifier letter. This difference is critical when reading drawings that reference ISO 2553, because omitting the modifier conversion can result in an undersized weld if a throat value is mistakenly treated as a leg value.
Is ISO 2553 System B the same as AWS A2.4?
ISO 2553 System B is similar to AWS A2.4 but not identical. Both use a single reference line where the symbol below the line indicates the arrow-side weld and the symbol above indicates the other side. However, differences remain in fillet weld dimensioning (ISO uses a/z modifiers, AWS uses leg dimension only), intermittent weld notation (ISO uses element count and distance, AWS uses length-pitch), and several weld types that exist in ISO 2553 but have no direct AWS A2.4 equivalent, such as the steep-flanked V-groove and steep-flanked bevel-groove. Per AWS A2.4:2020 Annex F, the user should refer to the primary ISO document for complete requirements.
Which standard should I use, ISO 2553 or AWS A2.4?
The governing code or contract specification determines which welding symbol standard to use. In the United States, AWS A2.4 is the default for structural steel (D1.1), sheet metal (D1.3), aluminum (D1.2), and most other AWS-governed codes. Internationally, ISO 2553 is more common, particularly in the EU, Middle East, and Asia-Pacific. When working on a project that crosses jurisdictions, the contract documents should specify which symbol standard applies. If drawings reference both standards, AWS A2.4:2020 Annex F provides conversion guidance. Never mix symbol conventions on the same drawing without clear notation, as the arrow-side reversal between System A and AWS A2.4 can lead to welds being placed on the wrong side of the joint.
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