A welding symbol looks like a coded scribble on a drawing, and it is: a horizontal reference line with an arrow at one end and information packed above and below it. Once you know that above and below mean opposite sides of the joint, most of it decodes itself.
The Anatomy of a Welding Symbol
The pieces, in the order you read them:
| Element | Where | Meaning |
|---|---|---|
| Arrow | Points to the joint | Identifies which joint is being welded |
| Reference line | Horizontal | Everything hangs off this |
| Below the line | Arrow side | Weld goes on the side the arrow touches |
| Above the line | Other side | Weld goes on the far side |
| Weld symbol | On the line | The shape of weld: fillet, groove, plug |
| Size | Left of the symbol | Leg length for a fillet, depth for a groove |
| Length and pitch | Right of the symbol | Weld length, and spacing if intermittent |
| Circle at the bend | At the arrow elbow | Weld all around the joint |
| Flag at the bend | At the arrow elbow | Weld in the field, not in the shop |
| Tail | Far end | Process, procedure or a note reference |
Both sides can be welded at once, in which case the symbol appears above and below the line together. A fillet symbol on both sides with a size on each is the most common detail in structural steel, and it is what joins most of the shapes in our guide to steel beam types.
The Symbols You Will Actually See
| Weld type | Symbol shape | Used for |
|---|---|---|
| Fillet | Right triangle | The workhorse: lapped plates, stiffeners, gussets, most structural connections |
| Square groove | Two parallel lines | Thin plates butted together with a small gap |
| V groove | V shape | Full-strength butt joints in thicker material |
| Bevel groove | Half a V | One member prepared, the other left square |
| Plug or slot | Rectangle | Welding through a hole in the upper plate |
The vertical leg of a fillet or bevel symbol is always drawn on the left, whichever side of the line it sits on. That is a drafting convention, not information about the joint, and it trips people up regularly.
Reading a Fillet Weld Callout
Take a common one: a triangle below the reference line, with 1/4 to its left and 3-6 to its right.
That reads as a fillet weld on the arrow side, with a 1/4 in leg, 3 in long, repeating every 6 in along the joint. The two numbers on the right are length and pitch, and the pitch is measured center to center, not as a gap.
Three details that change the meaning entirely:
No length given means weld the full length of the joint. Leaving the length off is not an omission; it is an instruction.
A circle at the elbow means continue the weld all the way around the member, including around corners.
A flag at the elbow means the weld happens at the site rather than in the shop, which affects position, access, inspection and cost.
Why the Size Is Not Arbitrary
A fillet weld’s strength is proportional to its throat, and the throat is proportional to the leg size, so doubling the leg roughly doubles the capacity per inch of weld. That makes oversizing look tempting.
It is a bad habit for three reasons. The volume of deposited metal goes up with the square of the leg, so a 1/2 in fillet costs about four times as much to deposit as a 1/4 in one. More heat means more distortion, and a member that pulls out of line during welding has to be straightened. And on thin material an oversized weld can do more harm than good.
There are also minimum sizes tied to the thickness of the parts being joined, because a small weld on thick plate cools too quickly and can crack. Both the minimum and the maximum come from the code, not from judgement.
Where the Rules Come From
Welding symbols and the welding itself are governed separately but by the same body. The American Welding Society has authored more than 350 standards for welding practices and procedures, including D1.1, the Structural Welding Code for steel, which it describes as one of the most-referenced codes in the world (checked September 2026). D1.1 sets the procedures, the welder qualifications, the acceptance criteria and the inspection.
On bolted and welded connections in the same joint, the bolting side is covered by the Research Council on Structural Connections and its Specification for Structural Joints Using High-Strength Bolts (checked September 2026). Plenty of real connections use both, with the shop welds done first and the field connection bolted.
Those connections are usually joining members from the standard families, and the weld size follows from the member being connected. Our wide flange beam sizes chart gives the flange and web thicknesses the weld has to suit, and connection plates at a truss node are where welded and bolted details most often meet.
Frequently Asked Questions
What does the flag on a welding symbol mean?
Field weld: the weld is made at the site rather than in the fabrication shop. It matters because field welding usually happens in a harder position, with more limited access and different inspection arrangements.
What does a circle at the bend of the arrow mean?
Weld all around. The weld continues around the full perimeter of the joint rather than stopping at the end of one face.
Which side of the line is the arrow side?
Below. Information below the reference line applies to the side the arrow touches; information above applies to the opposite side. This is the reverse of what most people guess.
What do the two numbers to the right of the symbol mean?
Length and pitch, as in 3-6: weld 3 in long, repeated every 6 in center to center. One number alone is a length. No number means weld the full length of the joint.
Can a welder decide to make the weld bigger?
No. The size is specified for a reason, and both minimum and maximum sizes are set by the code. An oversized weld adds heat, distortion and cost, and on thin material it can reduce the strength of the connection rather than improve it.
The Short Version
Arrow points at the joint. Below the line is the arrow side, above is the far side. Size sits left of the symbol, length and pitch to the right, a circle means all around and a flag means field weld. The shape tells you fillet or groove, and the numbers are specified by the engineer rather than chosen at the torch.
Reference information only, not engineering advice. Disclaimer.