Welding symbols explained: how to read a weld callout
A buyer's guide to reading welding symbols on a drawing: the reference line, arrow side vs other side, the fillet and groove symbols, size, length and pitch, and AWS vs ISO.
A welding symbol tells the shop what weld to make, where, and how big, from one callout. The reference line is the spine: a symbol below it is the arrow side, above it the other side. The shape is the weld type (a triangle is a fillet), the number to its left is the size, and numbers to the right are length and pitch. Get those four right and you get the joint you actually specified, not the one the welder guessed at.
The anatomy of a welding symbol
Every callout is built on a horizontal reference line with an arrow pointing at the joint. The weld symbol, its size and any supplementary symbols hang off that line.
| Element | What it tells the shop |
|---|---|
| Reference line | The spine everything attaches to |
| Arrow | Points at the joint to be welded |
| Arrow side | Symbol placed below the reference line (AWS) |
| Other side | Symbol placed above the reference line (AWS) |
| Tail | Holds the process or spec (for example GMAW, or a WPS number) |
| Dimensions | Size to the left of the symbol, length and pitch to the right |
Arrow side vs other side (the part people get wrong)
In the AWS A2.4 convention used across North America, a symbol below the reference line means weld the arrow side of the joint, and above the line means the other side. Symbols on both sides mean weld both. This is purely positional, it has nothing to do with up or down on the part. Reading it backwards puts the weld on the wrong face, so confirm it before the part is fixtured.
The common weld symbols
| Weld | Symbol | Typical use |
|---|---|---|
| Fillet | Triangle | Lap, T and corner joints, the workhorse weld |
| Square / V / bevel / U / J groove | Matching groove shape | Butt joints, partial or full penetration |
| Plug or slot | Rectangle | Joining overlapping plate through a hole |
| Spot or seam | Circle | Sheet metal resistance welds |
| Backing or back | Curved cap | Reinforcement or a backing pass |
Reading size, length and pitch
For a fillet, the number to the left of the triangle is the leg size. Numbers to the right are length, and for an intermittent weld, length-pitch: 50-150 means 50 mm welds on 150 mm centres. A circle at the bend of the arrow means weld all around; a filled flag means a field weld (done on site, not in the shop). A contour and finish symbol on top says the weld face must be ground flush or convex.
AWS vs ISO symbols
Canadian drawings mostly use AWS A2.4. ISO 2553 looks similar but adds a dashed identification line and uses letter prefixes for size (a for throat, z for leg), and the arrow/other-side rule is read differently. If your drawing came from a European OEM, confirm which standard it follows before quoting. Structural welding in Canada is done to CSA standards by CWB-certified shops regardless of the symbol style.
What to put on your drawing for a clean quote
- The weld type and size (for example a 6 mm fillet).
- Which side: arrow, other, or both.
- Length and spacing, or the weld-all-around symbol.
- The process or WPS if it matters, and any inspection (visual, or NDE such as UT or RT).
- CWB or ASME requirements if the weldment is structural or pressure-containing.
Send the drawing and we will match it to welding and fabrication shops that read the symbols the same way you drew them. Request quotes and we confirm certifications before we route.
FAQ
Which side of the reference line is the arrow side?
Below the line. In AWS A2.4, a symbol below the reference line means weld the arrow side of the joint, above means the other side, and both means weld both sides.
What does the number before the fillet symbol mean?
It is the leg size of the fillet weld. A larger number is a bigger, stronger (and more expensive) weld, so do not oversize it out of habit.
What does 50-150 mean on a weld symbol?
It is an intermittent fillet: 50 mm of weld on 150 mm centres. Intermittent welds save weld metal and distortion where a continuous weld is not needed.
What is the circle at the bend of the arrow?
Weld all around. It tells the welder to carry the weld continuously around the entire joint.
Do Canadian shops use AWS or ISO symbols?
Mostly AWS A2.4. ISO 2553 shows up on drawings from European OEMs. They convey the same information but read differently, so flag which one your drawing uses.
Last reviewed: October 2026. Related reading: types of welding and MIG vs TIG welding.
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