Weld symbol
Weld symbol reference line basics
Every welding symbol is built on one horizontal reference line with an arrow at one end and an optional tail at the other. Where information sits on that line is what gives it meaning.
Reads as: Arrow side: fillet weld, 5/16 size, 3 long, 6 pitch.
How to read it
The reference line is the spine of the whole notation. It is always drawn horizontally, regardless of how the joint sits on the part, and everything else hangs off it: the leader and arrow at one end, an optional tail at the other, the weld symbols above and below it, and the supplementary marks at the elbow where the leader meets it.
Position on that line is meaning. Below the line is arrow-side information, above the line is other-side information, left of a weld symbol is size, right of it is length and pitch, inside it is root opening, and beyond its outer face is angle and contour. None of those placements is decorative. Move a number from one side of a symbol to the other and you have changed the instruction.
The line's horizontality is worth dwelling on because it is the source of the notation's greatest strength and its most common confusion. Because the line never rotates, the same symbol reads identically wherever the joint sits on the drawing — but that also means the symbol's shape carries no information about the joint's orientation on the part. The arrow, and the drawing it points into, carry that.
One reference line describes one joint, however many weld symbols it holds. A fillet below and a fillet above is one joint welded on both faces, not two joints. A groove below and a semicircle above is one joint welded from one side and treated from the other. Keeping that in mind stops the most damaging class of misreading, where one instruction gets executed as two.
Where each dimension goes
- Below the reference line
- Arrow-side information: what happens on the face of the joint the arrow tip lands on.
- Above the reference line
- Other-side information: what happens on the opposite face of that same joint.
- Left of a weld symbol
- Size — leg length, groove depth, nugget diameter, or hole diameter depending on the weld type.
- Right of a weld symbol
- Length, and after it the pitch, for welds that do not run continuously.
- The elbow
- Where the leader meets the line. Home of the all-around circle and the field flag, which govern the whole line.
- The tail
- The far end of the line, carrying process, procedure, or notes the symbol cannot express.
Worked examples
A complete symbol read place by place
“Weld a 1/4 fillet, four inches long on eight-inch centres, by the process in the tail.”
Size sits left of the triangle, length and pitch sit right of it, the triangle sits below the line for the arrow side, and the tail names the process. Every element is located by position.
One line describing one joint twice welded
“Weld the groove from the arrow side and put a back weld on the far side.”
Two symbols on one reference line still describe a single joint. The groove below and the semicircle above are two operations on the same seam, not two separate joints.
Common mistakes
- Reading one reference line carrying two weld symbols as two separate joints.
- Expecting the symbol's shape or the line's direction to show how the joint sits on the part.
- Putting a size on the wrong side of a weld symbol, which turns it into a length.
- Attaching an all-around circle or field flag to a weld symbol instead of the elbow.
Typical joints
Related symbols
Practise reading welds, not just symbols
The drawing tells you what the weld should be. The gauge trainer is where you find out whether you can tell what it actually is.