Guide
How to Read Welding Symbols
A welding symbol is read from the reference line outward: the arrow says which joint, the side of the reference line says which face, the shape says which weld, and the numbers around it say how big and how many.
Every welding symbol on a drawing is built from the same small set of parts. Once you can name them, any symbol becomes readable — including the ones you have never seen before, because the rules for placement are the same for all of them.
Start with the reference line
The reference line is the horizontal line that everything attaches to. It is always drawn horizontal, regardless of how the joint sits in space. The arrow leads from one end of it down to the joint; the optional tail forks off the other end.
The single most important rule in the whole system is which side of the reference line a symbol sits on. A weld symbol below the reference line calls for a weld on the arrow side — the side of the joint the arrow actually touches. A weld symbol above the reference line calls for a weld on the other side. Symbols above and below mean weld both sides.
Below the line means arrow side. This is backwards from what most people guess on their first day, and it is the error that shows up most often on shop floors.
Read the shape to find the weld type
The shape drawn on the reference line says which kind of weld is required. A right triangle is a fillet. A V is a V-groove. Half a V, with one vertical leg, is a bevel-groove. A U and a half-U are the U-groove and J-groove. A rectangle is a plug or slot weld, a circle is a spot weld, a circle with a line through it is a seam weld, a semicircle is a back or backing weld, and two arcs side by side are a surfacing weld.
Look for a broken leader
Bevel-groove and J-groove welds prepare only one of the two members. When it is not obvious which one, the leader line is drawn with a deliberate kink or break, and it points at the member that gets cut. If you see a bent leader, that is what it is telling you.
Read the numbers
Dimensions always appear in the same places relative to the weld symbol, which is what makes them readable without a legend.
| Position | What it means |
|---|---|
| Left of the symbol | Size — fillet leg length, groove depth, hole diameter, nugget size |
| Right of the symbol | Length, and pitch if the weld is intermittent |
| Inside a groove symbol | Root opening |
| Outside a groove symbol | Included groove angle |
| In parentheses | Number of welds, or the groove weld size |
| In the tail | Process, specification or procedure reference |
Two numbers to the right separated by a dash, such as 3-8, describe an intermittent weld: 3 inch long segments on 8 inch centres, measured start of one weld to start of the next — not the gap between them.
Check the supplementary symbols
Small symbols at the junction of the arrow and the reference line modify the instruction rather than describing the weld itself.
- A circle at the junction means weld all around the joint.
- A flag at the junction means the weld is made in the field rather than in the shop.
- A straight line, convex arc or concave arc across the face of the weld symbol specifies the finished contour.
- A letter with the contour symbol — C, G, M, R or H — says how that contour is produced: chipping, grinding, machining, rolling or hammering.
- A filled semicircle opposite the groove symbol calls for melt-through with visible root reinforcement.
Work through one symbol
Take a fillet weld symbol below the reference line, with 1/4 to its left, 2-6 to its right, a circle at the arrow junction and GMAW in the tail. Read it in order: a fillet weld, on the arrow side of the joint, with 1/4 inch legs, in 2 inch segments on 6 inch centres, continued all the way around the joint, made by gas metal arc welding.
What the symbol does not tell you
Welding symbols specify the weld, not the procedure. Electrode classification, amperage, preheat, interpass temperature and inspection requirements come from the welding procedure specification, not from the drawing. The tail can reference that procedure, but it cannot contain it.
Frequently asked questions
Does above or below the reference line mean the arrow side?
Below. A weld symbol drawn below the reference line calls for a weld on the side of the joint the arrow touches. Above the line means the far side.
What does the tail of a welding symbol mean?
It carries a reference the symbol itself cannot express — the welding process, a specification number, or a procedure. If there is nothing to reference, the tail is omitted.
What does a circle at the bend of the arrow mean?
Weld all around: the weld continues completely around the joint rather than stopping at the point the arrow indicates.
What does a flag on a welding symbol mean?
A field weld — the weld is made on site during erection rather than in the fabrication shop.
What is the difference between a weld symbol and a welding symbol?
The weld symbol is just the shape that names the weld type, such as the triangle for a fillet. The welding symbol is the whole assembly: reference line, arrow, weld symbol, dimensions, supplementary symbols and tail.