How to read welding symbols on a drawing
A welding symbol is a compressed instruction: which joint, which side, what shape, what size, how long, by what process. Read it in that order and it stops being cryptic.
The reading order
1. The arrow. Which joint is being specified. On an asymmetric preparation the arrow is bent and points at the member to be prepared.
2. The line and the side. ISO 2553 system A uses a continuous reference line with a dashed line alongside: symbol on the continuous line means arrow side, on the dashed line means other side. AWS A2.4 uses one line: below it means arrow side, above it means other side.
3. The shape. Triangle — fillet. V — single-V butt. Square — square butt. Half-V — single bevel. U and J — those preparations. A symbol on both sides of the line means welds on both sides.
4. The size, on the left. a followed by a number
is the design throat; z is the leg length. For butt welds the
preparation depth may appear here instead.
5. The length, on the right. A plain number is the length. An
expression such as 3×50 (100) is an intermittent weld: three
segments of 50 mm at 100 mm pitch.
6. The tail. Process number, WPS reference, quality level, NDT requirement — whatever the drawing office chose to put there.
Supplementary symbols
| Mark | Where | Meaning |
|---|---|---|
| Circle | At the elbow | Weld all round the closed contour |
| Flag | At the elbow | Field weld — made on site, not in the shop |
| Straight line over the symbol | On the symbol | Surface to be finished flush |
| Convex arc | On the symbol | Convex surface required |
| Concave arc | On the symbol | Concave surface required |
| M or MR | On the symbol | Permanent or removable backing |
A worked example
A symbol showing a triangle on the continuous line, a5 on its
left, 3×80 (200) on its right, a circle at the elbow and
135 in the tail reads as: a fillet weld on the arrow side, design
throat 5 mm, intermittent — three segments of 80 mm at 200 mm
pitch — running all round the contour, made by MAG welding with solid wire.
Assemble your own and see it drawn in the
welding symbol builder,
and check the a-to-z arithmetic with the
fillet weld
calculator.
The same drawing element means different sides in ISO and AWS. A dashed reference line means ISO; a symbol placed above or below a single line means AWS. Older drawings from the former Soviet system are different again — see GOST 2.312 and ISO 2553.
Frequently asked questions
Where do I start reading?
At the arrow: it tells you which joint. Then the reference line and where the symbol sits on it, which tells you which side. Then the symbol shape, then the dimensions, then the tail. Reading the dimensions before establishing the side is how people weld the wrong face of a plate.
The drawing has no welding symbols at all. Now what?
Ask. An unmarked joint is not "welder's choice" — it is a gap in the documentation, and on anything covered by EN 1090 it has to be resolved before welding. A general note such as "all fillets a4 unless stated" is common and counts as the specification.
What does a symbol on both sides of the line mean?
A weld on both sides of the joint — a double fillet or a double-V butt. The two halves can differ: a5 on one side and a3 on the other is perfectly normal on an asymmetrically loaded joint.
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