How to read a
welding symbol
A welding symbol is a compact instruction set that tells a welder exactly what type of weld to make, where to put it, and how big it should be. This guide walks through every part of the symbol, step by step, covering both ISO 2553 and AWS A2.4 conventions.
Once you understand the five building blocks, you can decode any welding symbol on a drawing.
1. The basic structure
Every welding symbol is assembled from five core components. Not every symbol uses all five, but the reference line and arrow are always present.
Reference line
The horizontal backbone of the symbol. Everything is positioned relative to this line.
Arrow line
Points to the joint on the drawing. Connects the symbol to the physical weld location.
Elementary symbol
The weld type: fillet, V-butt, U-butt, spot, seam, and so on. Placed on or against the reference line.
Dimensions
Cross-sectional size to the left of the symbol, length to the right. Specifies weld geometry.
Tail
Optional forked end for process numbers, quality levels, or filler material specifications.
2. The reference line
The reference line is the horizontal line at the heart of every welding symbol. It is the datum from which all other information is positioned. The two major standards treat it differently.
Dual reference line
ISO 2553 System A uses two parallel lines: a solid line and a dashed line. The dashed line indicates the "other side" of the joint. Either line can be on top, depending on which side is the arrow side.
Single reference line
AWS A2.4 and ISO System B both use a single solid line. The position of the weld symbol relative to this line (above or below) determines which side of the joint receives the weld.
Key point: ISO 2553 recognises both System A (dual line) and System B (single line). System B is functionally very similar to AWS A2.4. Many drawings specify which system is in use in the title block.
3. Arrow side vs other side
This is the single most important concept in reading welding symbols and the most common source of misinterpretation. The arrow points to a joint. The arrow side is the face of the joint that the arrow touches. The other side is the opposite face.
The two standards use different visual conventions to tell you which side gets the weld, but the result is the same physical weld.
Symbol placed on the solid line = weld on arrow side.
Symbol placed on the dashed line = weld on other side.
Symbol placed below the line = weld on arrow side.
Symbol placed above the line = weld on other side.
| Convention | ISO System A | AWS / ISO System B |
|---|---|---|
| Arrow side indicator | Symbol on solid reference line | Symbol below reference line |
| Other side indicator | Symbol on dashed reference line | Symbol above reference line |
| Both sides | Symbol on both lines | Symbol above and below |
| Physical weld result | Identical — both systems produce the same weld | |
Common mistake: Assuming "below the line" always means arrow side. This is true for AWS and ISO System B, but in ISO System A the dashed/solid line distinction is what matters, not above/below. Always check which system the drawing uses.
For a detailed comparison of both standards, see the AWS vs ISO comparison page.
4. Reading dimensions
Dimensions are placed alongside the elementary symbol. Their position follows a consistent pattern in both standards.
Left of the symbol — cross-sectional dimension. For fillets this is the throat thickness or leg length. For grooves this is the depth of preparation or penetration.
Right of the symbol — longitudinal dimension. This is the weld length, or for intermittent welds, the length and pitch in the format l(e) where l is length and e is pitch.
Fillet weld sizing: ISO vs AWS
This is a key difference between the standards. ISO requires a prefix letter to clarify what the number represents.
| Notation | Meaning | Standard |
|---|---|---|
| a5 | 5 mm throat thickness (design throat) | ISO 2553 |
| z7 | 7 mm leg length | ISO 2553 |
| 5 | 5 (assumed leg length, no prefix) | AWS A2.4 |
For an equal-leg fillet weld, the relationship between throat and leg is: a = z × 0.707. So ISO a5 is approximately equivalent to z7 (and AWS 7).
5. Supplementary symbols
Supplementary symbols modify the elementary weld symbol to specify finishing, extent, or site conditions. They are added around the reference line and arrow junction.
Flush (flat)
Contour symbol
Weld face finished flush with the parent material surface. Placed over the weld symbol.
Convex
Contour symbol
Weld face finished with a convex (raised, rounded) profile.
Concave
Contour symbol
Weld face finished with a concave (dished, scooped) profile.
Weld-all-round
Extent symbol
Circle at the arrow/reference line junction. Weld continues around the entire joint perimeter.
Field weld
Site symbol
Flag at the junction. The weld is to be made on site (in the field), not in the workshop.
Backing / back weld
Supplementary
Half-circle opposite the weld symbol. Indicates a backing or back run weld on the reverse side of the joint.
For the full list, see all symbols filtered by the supplementary category.
6. The tail
The tail is the forked end of the reference line, opposite the arrow. It is optional and only included when additional information is needed. If no extra information is required, the tail is omitted and the reference line simply ends.
Process / Quality / Filler Welding process number (ISO 4063)
111 = manual metal arc (MMA/SMAW), 135 = MAG, 136 = FCAW, 141 = TIG, etc. AWS uses letter codes like SMAW, GMAW, GTAW in the tail instead.
Quality level (ISO 5817)
B = stringent, C = intermediate, D = moderate. Defines acceptable imperfection limits.
Filler material / consumable specification
References the specific electrode or filler wire standard and classification.
Each piece of information is separated by a forward slash. The tail can contain one, two, or all three items. If only the process is specified, the tail might simply read 135 with no separators.
7. Quick reference checklist
Follow these steps in order to decode any welding symbol you encounter on a drawing.
- 1
Identify the standard
Check the drawing title block for "ISO 2553" or "AWS A2.4". Look for a dual (dashed) reference line, which confirms ISO System A.
- 2
Follow the arrow
The arrow points to the joint. The side the arrow touches is the arrow side.
- 3
Determine arrow side vs other side
Use the convention for your standard: solid/dashed line (System A) or above/below (AWS / System B).
- 4
Read the elementary symbol
Identify the weld type from the shape on the reference line. If you are unsure, look it up in the symbol reference.
- 5
Read the dimensions
Left of the symbol = cross-section size (a, z, or bare number). Right of the symbol = weld length.
- 6
Check for supplementary symbols
Look for contour marks (flush, convex, concave), the weld-all-round circle, or the field weld flag.
- 7
Read the tail (if present)
Process number / quality level / filler material, separated by forward slashes.
- 8
Confirm the full instruction
Assemble the pieces: weld type + side + size + length + finish + process. That is your complete welding instruction.
Ready to identify specific symbols?