Weld symbol
Fillet weld symbol
The right triangle on the reference line calls for a fillet weld. The number to its left is the leg size, and the side of the line it sits on tells you which face of the joint gets welded.
Reads as: Arrow side: fillet weld, 1/4 size.
How to read it
Start at the arrow. The arrow touches the joint the symbol is talking about, and the side of the joint the arrow tip lands on is the arrow side. A triangle drawn below the reference line means weld that arrow side; a triangle above the line means weld the opposite face of the same joint. Nothing about the arrow's direction, angle, or which end of the line it leaves from changes that reading.
The triangle itself is always drawn with its vertical leg on the left, no matter which way the joint faces on the drawing. That is a drafting convention, not a picture of the weld's orientation, so do not try to infer from the triangle whether the fillet leans left or right on the part. The joint sketch beside the symbol is what tells you the geometry; the triangle only tells you the weld type.
The number to the left of the triangle is the leg size. A single number means both legs of the fillet are that dimension, so a 1/4 fillet has a quarter-inch leg along the upright member and a quarter-inch leg along the base. If no number appears at all, the size is being set somewhere else — a general note, a weld schedule, or the procedure the tail points to — and you should go find it rather than guess.
A number to the right of the triangle is a length. With no length shown, the weld runs the full length of the joint the arrow points at, stopping where the joint stops. That default trips people up more than any other part of the notation, because a bare triangle with a size looks incomplete when in fact it is a complete instruction: weld this side, this size, all the way.
Where each dimension goes
- Size (left of the triangle)
- Leg length of the fillet, measured on the two faces that form the joint. One number means both legs are equal.
- Length (right of the triangle)
- How long the weld runs. Leave it off and the weld runs the full available length of the joint.
- Side of the reference line
- Below the line is the arrow side, above the line is the far side of the same joint.
- Contour and finish (beyond the triangle)
- An optional bar or arc on the outer face calls out flat, convex, or concave, with a letter for how that face is produced.
Worked examples
Fillet welds on both faces of a tee
“Weld a 5/16 fillet on both sides of the tee, full length.”
Two triangles, one above and one below the reference line, each carrying its own 5/16 size. Same joint, both faces. Neither carries a length, so each weld runs the whole joint.
A single fillet with a stated length
“Put a 1/4 fillet six inches long on the arrow side only.”
The 1/4 to the left of the triangle is the leg size and the 6 to the right is the run length. Only one triangle appears, and it sits below the line, so the far face of the joint stays bare.
Common mistakes
- Reading the triangle's vertical leg as a picture of which way the weld faces on the part. It is drawn on the left every time.
- Assuming a symbol with no length means the size is missing. No length means full length of the joint.
- Welding only the arrow side when a triangle also appears above the reference line.
- Treating the leg size as a throat dimension. Leg and throat are different measurements and the number left of the triangle is the leg.
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.