How to Read Weld Symbols

A weld symbol packs a whole sentence into a few lines: which joint, which side, what kind of weld, how big and how long. Once you know where each piece sits, you can read most of them in a few seconds.

The catch is that there are two main ways of drawing them. One uses a single reference line (the American style, which the Australian symbol standard follows). The other is the ISO style with a second, dashed line. You will see both on Australian jobs, so this guide covers both and shows where they differ.

What are the parts of a weld symbol?

Every weld symbol is built from the same pieces:

• The arrow. It points at the joint to be welded. The side it touches is the arrow side.
• The reference line. A horizontal line joined to the arrow. All the other information hangs off it.
• The weld symbol. A small shape on the line that says what type of weld: a triangle for a fillet, a V for a butt, and so on.
• Sizes. The weld size sits to the left of the symbol. The length (and pitch, for a broken weld) sits to the right.
• Extra marks. A circle for all round and a flag for a site weld, both at the bend where the arrow meets the line.
• The tail. An optional fork at the far end for the process, a procedure number or a note.

Read it left to right like a sentence: "At this joint, on the arrow side, run a 6 mm fillet weld, 50 mm long, every 150 mm, by TIG."

How do you tell the arrow side from the other side?

This is the part that trips people up, and it is where the two styles split.

Single reference line (AWS A2.4 and the Australian standard, AS 1101.3, which is based on it). A symbol under the line means weld the arrow side. A symbol above the line means weld the other side. A symbol on both sides means weld both sides. It does not matter which way the arrow points.

Dual reference line (ISO 2553, System A). There is a solid line and a dashed line next to it. A symbol on the solid line means the arrow side. A symbol on the dashed line means the other side. The dashed line can sit above or below the solid one; what counts is which line the symbol touches. When a weld is the same on both sides, the dashed line is left off.

ISO 2553 also has a System B, which uses one line like the American style. A drawing should use one system all the way through, never a mix.

What do the numbers on a fillet weld mean?

The fillet weld is the one you will see most on gate and fence work: frame corners, T-joints, hinges and footplates. Its symbol is a right-angle triangle, always drawn with the upright leg on the left.

• Left of the triangle: size. On single-line drawings a plain number is the leg length. "6" means a 6 mm leg.
• ISO adds a letter. "z6" is a 6 mm leg. "a4" is a 4 mm throat, which works out to a leg of about 5.7 mm on an equal fillet. Mixing up a and z gives you a weld that is too big or too small.
• Right of the triangle: length. No number means weld the full length of the joint.

How do you read an intermittent weld?

An intermittent weld, often called a stitch weld, is a row of short welds with gaps between them. It saves time, heat and distortion on long joints like battens on a frame. The two styles write it differently, and this is where mistakes get made.

• Single-line style: length-pitch. "50-150" means 50 mm welds at 150 mm centres. The pitch is measured centre to centre, so the gap is 100 mm.
• ISO style: n x l (e). n is the number of welds, l is the length of each weld and e, in brackets, is the space between welds. The same weld as above, with 8 runs, reads "8 x 50 (100)".

Same weld, different second number. Read a 150 as a gap when it was a pitch and you put down a quarter less weld than the drawing asks for.

When there are welds on both sides, they can be chained (opposite each other) or staggered (offset, so a weld on one side sits across from a gap on the other). On single-line drawings a staggered weld has its two triangles drawn offset along the line.

What do butt, plug and other weld symbols look like?

The shape on the line tells you the type of weld. A butt weld is called a groove weld on American drawings. These are the ones a gate or fence shop meets most.

A short straight line drawn over the symbol means the weld face is finished flush. On American drawings a letter can follow it to say how, such as G for grind. See butt weld and plug weld in the glossary for more.

What do the circle, the flag and the tail mean?

• Circle at the bend: weld all round. Run the weld right around the joint. You see it on posts welded to footplates and on tube going through plate.
• Flag at the bend: site weld. This weld is done on site, not in the shop. Plan for it: the part ships with that joint open, and someone takes a welder to the job.
• Tail: extra information. The process, a procedure number or a note. ISO drawings use process numbers, such as 131 for MIG and 141 for TIG. American drawings use letters, such as GMAW for MIG and GTAW for TIG.

What are the main differences between ISO and AWS weld symbols?

Before you read a single size, find out which system the drawing uses. It is often in the title block or the general notes. A dashed line next to the reference line is a sure sign of ISO System A.

If you draw your own frames, keep it simple. Most gate and fence drawings get by with one general note, such as "all joints welded all round unless noted", plus a symbol where a joint is different. The how to draw a fabrication drawing guide covers weld notes, and how to weld an aluminium gate covers the welding itself.

Common questions