Standards library · Updated 4 August 2026 · Free to read
Welding symbols explained: reading a drawing to AS 1101.3
A welding symbol tells you four things: what kind of weld, which side of the joint, how big, and anything special about it. AS 1101.3 is the Australian Standard that defines how those four things are drawn. Get one of them wrong and you either build a weld that fails inspection or one that costs several times what it needed to.
The one that costs real money
z6 and a6 are not the same weld. z6 is a 6 mm leg length. a6 is a 6 mm design throat, which needs roughly an 8.5 mm leg to achieve. Read one as the other and you are either 40 per cent under size, or you are depositing about twice the weld metal the drawing asked for. Section on weld size below.
The anatomy of a welding symbol
Every welding symbol is built from the same parts. Once you can name them, the rest is a lookup.
- Arrow line. Points at the joint. Which side it touches matters, and it is the single most misread element on a drawing.
- Reference line. The horizontal line the symbol sits on. Always drawn horizontal, regardless of the joint orientation.
- Identification line. The dashed line drawn above or below the reference line. This is what tells you arrow side from other side, and it is the biggest difference between the Australian and ISO system and the American one.
- Weld symbol. The small geometric shape that says what kind of weld: triangle for a fillet, V for a single V butt preparation, and so on.
- Dimensions. Size to the left of the symbol, length and pitch to the right.
- Supplementary symbols. Weld all round, field weld, surface finish, backing run.
- Tail. Where the process, the procedure number, the acceptance category or a note goes.
Arrow side and other side
The convention in AS 1101.3, which follows ISO 2553, is the identification line:
- Weld symbol on the continuous reference line: the weld is on the arrow side of the joint.
- Weld symbol on the dashed identification line: the weld is on the other side.
- Symbols on both lines: welds both sides. A double sided fillet, or a double V butt.
- For a symmetrical weld, the identification line may be omitted altogether.
The arrow side is whichever face of the joint the arrow actually touches. This is a physical question about the drawing, not a rule to memorise. Where the arrow lands is the arrow side.
Why the same drawing can mean two different things
The American system, AWS A2.4, does not use a dashed identification line. It places the symbol below the reference line for the arrow side and above it for the other side. So a symbol sitting above a plain reference line means other side under AWS and, if there is no dashed line present, may be read as arrow side under the Australian and ISO system. On an imported drawing, or a drawing produced by a detailer trained overseas, establishing which system is in use is the first thing to do and not the last.
The weld symbols you will actually meet
| Symbol | Weld | What it means in practice |
|---|---|---|
| Right angled triangle | Fillet | The workhorse. No edge preparation, weld metal laid in the corner of the joint |
| Two parallel lines | Square butt | Square edges, no bevel. Thin material, or a gap and a backing |
| V | Single V butt | Both edges bevelled from one side |
| V above and below | Double V butt | Bevelled both sides. Less weld metal and less distortion than a single V on thick material, if you can access both sides |
| Half V | Single bevel butt | One member bevelled, the other square. Common on T joints needing penetration |
| U | Single U butt | Less weld metal than a V on thick sections, more expensive to prepare |
| J | Single J butt | The one sided equivalent of a U |
| Curved line over a V | Butt with backing run | A sealing run from the far side |
| Circle | Spot or plug | Resistance spot, or a hole filled with weld metal |
| Two lines with a circle | Seam | Continuous resistance seam |
| Rectangle | Edge or flange | Weld along the abutting edges |
Weld size: the z and a convention
This is where drawings are most often misread, and the consequences are dimensional and expensive.
For a fillet weld, two dimensions can define the same weld:
- Leg length, prefix z. The distance along each fusion face from the root to the toe. What a fillet weld gauge measures directly.
- Design throat thickness, prefix a. The perpendicular distance from the root to the hypotenuse of the largest inscribed triangle. What the structural engineer actually designed to.
For an equal leg fillet the relationship is fixed by geometry: the throat is the leg divided by the square root of two, roughly 0.707 times the leg. Which means:
| Called up as | Leg length | Design throat | Relative weld metal |
|---|---|---|---|
| z6 | 6.0 mm | 4.2 mm | 1.0 |
| a6 | 8.5 mm | 6.0 mm | 2.0 |
| z8 | 8.0 mm | 5.7 mm | 1.8 |
| a8 | 11.3 mm | 8.0 mm | 3.6 |
| z10 | 10.0 mm | 7.1 mm | 2.8 |
Weld metal volume goes with the square of the leg, which is why the right hand column climbs so fast. Reading a8 as z8 puts a weld on the job at half the required throat. Reading z8 as a8 doubles the deposit, the time, the consumable and the distortion, on every joint on the drawing.
If a drawing gives a bare number with no prefix, do not guess. Ask. It is a five minute question and the alternative is a rework claim.
Length, pitch and intermittent welds
Dimensions to the right of the symbol describe how much weld and where:
- A single number: the length of the weld.
- n × l: n welds, each l long.
- n × l (e): n welds each l long, with e being the gap between them.
- Staggered intermittent: shown with the symbols offset either side of the reference line.
- No length given: the weld runs the full length of the joint.
Intermittent fillet welds are a cost saving on paper and a fatigue liability in the wrong application. Each stop and start is a discontinuity. On anything cyclically loaded, check what the design standard permits before treating an intermittent call up as an invitation.
Supplementary symbols
| Symbol | Meaning |
|---|---|
| Circle at the junction of arrow and reference line | Weld all round |
| Flag at the junction | Field weld, made on site rather than in the shop |
| Straight line across the face of the symbol | Flush finish, dress the weld flat |
| Convex arc across the face | Convex finish |
| Concave arc across the face | Concave finish, toes blended |
A flush finish call up is not cosmetic. Grinding a weld flush removes the toe stress concentration, which is why it appears on fatigue critical joints, and it is a real cost line that needs to be in the estimate rather than discovered at final inspection.
What goes in the tail
The tail is where the drawing tells you which rules apply. Expect to find:
- the welding process, often as a numeric reference or an abbreviation such as GMAW or MMAW
- the welding procedure specification number
- the weld quality category, for example SP or GP under AS/NZS 1554
- the non-destructive examination required, and the extent
- a reference to a note elsewhere on the drawing
An empty tail on a structural drawing is worth a query. Weld category drives the acceptance criteria, the inspection level and the cost, and a drawing that does not state it has left the most consequential decision undefined.
AS 1101.3, ISO 2553 and AWS A2.4
| AS 1101.3 | ISO 2553 | AWS A2.4 | |
|---|---|---|---|
| Side convention | Dashed identification line | Dashed identification line | Position above or below the reference line |
| Fillet size | z | z | Leg length, no prefix |
| Fillet throat | a | a | Stated as throat where used |
| Where dimensions sit | Size left, length right | Size left, length right | Size left, length right |
| Common on | Australian and New Zealand drawings | European drawings | North American drawings, and much oil and gas work |
The practical rule: find out which system the drawing was produced to before you interpret a single symbol on it. A title block, a general note, or the origin of the detailer will usually tell you. If nothing does, that is a query, not an assumption.
Check the edition your specification names. This page explains what AS 1101.3 requires and how it sits with the standards around it. It does not reproduce the standard, and clause numbers, symbol conventions and tables move between editions. Work from the edition your contract calls up rather than from a website. If you are not sure which edition or even which standard the specification means, ask AWI before anybody strikes an arc.
Where this fits
Reading the symbol correctly is the first step. What the weld then has to achieve, and how it will be judged, comes from the fabrication standard the drawing calls up: AS/NZS 1554 for structural steelwork, AS 3992 for pressure equipment qualification.
If you are training welders or supervisors on drawing interpretation, the symbols are the easy part to teach and the z against a distinction is the one worth spending real time on, because it is the one that shows up in rework.