In short: Weld symbols on drawings are a shorthand that tells the fabricator what weld to make, where, and in what form. This guide covers the arrow, reference line, fillet and butt symbols, all-round and site welds, and how AS 1101.3 sits alongside ISO 2553 and the American system.
Why weld symbols on drawings matter
A welded joint can be described in a paragraph of notes, or in one small symbol. Weld symbols on drawings exist so the designer, drafter, fabricator and inspector all read the same instruction. ISO describes its welding symbol standard as showing, on technical drawings, how and where welds are to be made, including geometry, manufacture, quality and testing.
A misread symbol is not cosmetic. A weld on the wrong side of a joint, or a shop weld that should have been done on site, means rework. This post is about reading and drafting the symbols correctly. It is not about sizing welds, which is an engineering decision.
One naming point first. The AS 1100 series covers general technical drawing practice. Its Part 101 sets out the basic principles and is stated to be in agreement with ISO standards such as ISO 128. Weld symbols sit in a separate Australian Standard, AS 1101.3, Graphical symbols for general engineering, Part 3: Welding and non-destructive examination. A search for “AS 1100 welding” usually ends up there. Always read the current edition of both.
The parts of a weld symbol
Every system builds the symbol from the same few parts:
- Arrow line. It points at the joint to be welded.
- Reference line. A horizontal line the arrow connects to. It carries the instruction.
- Elementary (weld) symbol. A small shape on the reference line showing the weld type: a triangle for a fillet, a V for a V butt weld, and so on.
- Supplementary symbols. Marks at the junction of the arrow and reference line, such as the all-round circle and the site-weld flag. Face finish marks (flush, convex, concave) also belong here.
- Dimensions. Size to the left of the symbol, length and spacing to the right.
- Tail. An optional fork at the far end for a process number or a reference to a welding procedure.
An arrow and reference line with no weld symbol is still legitimate. ISO 2553 says that basic symbol is used when the joint is not specified, and it is often used to show tack weld locations. The detail then has to come from another document.
Arrow side and other side: the part people get wrong
Every welded joint has two sides. The side the arrow touches is the arrow side. The opposite side is the other side. How the symbol shows which side gets the weld depends on the system.
ISO 2553 recognises two approaches. System A uses a dual reference line: a continuous line plus a dashed identification line. A symbol on the continuous line means the arrow side, and a symbol on the dashed line means the other side. System B uses a single reference line. There, a symbol below the line is the arrow side, a symbol above is the other side, and a symbol on each side means both sides get a weld.
The two systems can give opposite readings of similar marks. Read a single-line drawing as system A and you may weld the wrong side.
AS 1101.3, ISO 2553 and AWS A2.4: how they relate
Weld symbols are similar from one system to the next, but they do not all show the arrow side the same way. The three you are most likely to meet are AS 1101.3 (Australia), ISO 2553 and the American AWS A2.4. Published guidance on how AS 1101.3 treats the reference line is not consistent, so check the edition of the standard the drawing names rather than relying on a summary, including this one.
| ISO 2553 system A | ISO 2553 system B | AWS A2.4 | |
|---|---|---|---|
| Reference line | Continuous plus dashed | Single | Single |
| Arrow side | On the continuous line | Below the line | Below the line |
| Other side | On the dashed line | Above the line | Above the line |
| Groove-type weld called | Butt weld | Butt weld | Groove weld |
The practical rule: never assume. Look for a dashed line beside the reference line. If there is one, it is ISO system A. If not, treat it as a single-line system. Check the general notes for the standard the drawing claims. On drawings that cross borders, such as an imported machine welded locally, state the standard in words.
Fillet weld symbol vs butt weld symbol
The fillet weld symbol
A fillet weld is the triangular weld joining two surfaces that meet at an angle, such as a T-joint or lap. Its symbol is a triangle, and its position on the reference line gives the side. Triangles on both sides call for welds on both sides. If they are offset along the line, the intermittent welds are staggered.
On ISO drawings the fillet size carries a letter. In ISO 2553, a is the throat thickness and z is the leg length. Mixing them up is a classic error, so the letter must always be there. Under AS 1101.3 or AWS, use that standard’s size notation instead of carrying ISO letters across. We give no sizes here: they come from the engineer, not the drafter.
The butt weld symbol
AS 1101.3 calls it a butt weld; the American standard calls the equivalent a groove weld. The shape shows the joint preparation: square, V, bevel, U, J and flared variations are all in the standard’s list of basic symbols. Preparation details such as root gap and included angle come from the weld procedure or the engineer’s specification, not from habit.
All-round and site welds
Two supplementary symbols sit at the junction of the arrow and the reference line.
- Weld all round. A circle means the weld continues around the whole joint, for example a tube welded to a base plate.
- Site (field) weld. A flag means the weld is made on site. No flag means the shop.
The two can be combined for an all-round weld made on site, and both mean the same in ISO 2553 and AWS A2.4. Site welds matter for planning, because access, position and working conditions differ from the workshop. Check them in a safety in design review, not after the steel arrives.
Reading a weld symbol step by step
- Find the arrow and the joint it touches.
- Check the reference line: single or dual? That decides how you read the side.
- Identify the weld shape on the line.
- Read the dimensions and any letters.
- Check the junction for the circle and flag.
- Read the tail for the process or document reference.
- If there is no weld symbol at all, find the detail in another document.
Common drafting mistakes
- Mixing conventions within one drawing set.
- Leaving the standard unstated in the general notes.
- A fillet size with no letter or notation to say what it measures.
- A flag or circle that does not match the intended build.
- No tail reference, so the fabricator cannot find the procedure.
- Changed welds that never reach the shop because the revision table was not updated.
If you also work to AS/NZS 1554, the structural welding standard, remember the symbol only says what is wanted. The welding standard and procedure control how it is done.
Getting weld details onto the drawing properly
Clear welding detail is both a drafting and an engineering task. The drafter makes the symbol correct and readable; the engineer decides what the weld has to be. For weld details drawn up from a design, or an existing assembly documented, see our mechanical drafting and 3D modelling service. The design side is covered under mechanical engineering design, and you can browse all engineering and drafting services.
For the standards themselves, see ISO’s page for ISO 2553:2019 and its news item on the update. Read the current edition rather than relying on a summary, including this one.
Frequently asked questions
What standard covers weld symbols on Australian drawings?
AS 1101.3, Graphical symbols for general engineering, Part 3: Welding and non-destructive examination, published by Standards Australia. The 2005 edition was reconfirmed in 2018. The AS 1100 series covers general drawing practice, while the welding symbols sit in AS 1101.3. Read the current edition.
What is the difference between arrow side and other side?
The arrow side is the side of the joint the arrow touches. The other side is the opposite face. On a single reference line, below the line is the arrow side and above is the other side. On an ISO 2553 system A drawing, the continuous line is the arrow side and the dashed line is the other side.
What does the circle on a weld symbol mean?
A circle at the junction of the arrow and the reference line means weld all round. The weld continues around the whole joint. It means the same in ISO 2553 and AWS A2.4.
What does the flag on a weld symbol mean?
The flag marks a site (field) weld, made during erection and not in the workshop. A weld with no flag is made in the shop. It can be combined with the all-round circle.
Is AS 1101.3 the same as ISO 2553?
Similar in purpose, but not identical. ISO 2553 allows a dual reference line (system A) or a single one (system B), and other systems differ again. Check which convention a drawing uses, and which standard its general notes name, before you read the symbols.
Where to start
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