How to read welding symbols
Decode AWS A2.4 welding symbols on structural and miscellaneous metal drawings to verify weld quality and type.
Welding symbols tell fabricators exactly what type of weld is required, where it goes, and how large it should be. These symbols follow AWS (American Welding Society) standard A2.4, and they're essential for structural steel, miscellaneous metal, and pressure vessel work. Understanding the symbol prevents weld defects, rework, and field failures. Every symbol has three main parts: the reference line, the arrow line, and the tail, each carries specific information.
The Three Parts of a Welding Symbol
Every AWS welding symbol has a standard structure:
Common Weld Types and Symbols
The weld type symbol is placed on the reference line. Here are the most common:
Reading Fillet Weld Sizes
Fillet welds are dimensioned by leg size (the two equal sides of the triangle). Size goes to the left of the symbol on the reference line:
1/4" ∟, 1/4" leg size fillet weld (both sides 1/4")
3/8" ∟, 3/8" leg size (common for structural connections)
1/2" ∟, 1/2" leg size (heavier connection, higher cost)
1/4" - 3/8" ∟, Variable size (increases from 1/4" to 3/8" along the weld)
Larger fillet sizes mean stronger welds but also higher fabrication cost. Designers balance strength with economy.
Groove Weld Symbols and Prep Details
Groove welds require surface preparation (groove angle, depth, root opening). These details are placed around the groove symbol:
The depth of groove is shown inside or above the groove symbol. Root opening (gap between parts before welding) is shown below the groove symbol with a dimension.
Weld Location: Arrow Side vs. Other Side
The position of the symbol above or below the reference line indicates where the weld goes relative to the arrow:
Fabricators can only see one side of a joint clearly when setting up the weld. Misreading arrow side vs. other side leads to welds placed on the wrong surface, creating structural weaknesses that may require complete rework.
Supplementary Symbols and Finish
Small symbols added around the weld symbol convey important information:
Reading a Complete Example
Here's how to read a real welding symbol step-by-step:
Weld Type: Fillet welds (both sides)
Arrow Side: 3/8" fillet weld
Other Side: 1/4" fillet weld
Specification: AWS D1.1 (standard for structural steel welds)
What to Tell Fabricators: Weld both sides of this joint, 3/8" on the near side, 1/4" on the far side, per structural steel code.
Common Errors in Reading Welding Symbols
Watch for these frequent mistakes on job sites:
- Confusing arrow side with other side, resulting in welds placed on the wrong surface.
- Misreading fillet size (1/4" vs. 1/2"), cheaper or weaker than designed.
- Ignoring back weld symbols, front weld alone may not penetrate fully.
- Missing finish or contour requirements, unground welds when grinding is required.
- Overlooking the specification in the tail, wrong welding process or rod type used.
Verifying Welds on Site
Before signing off on welded connections, inspect against the symbol:
- Check both sides, are welds present where the symbol shows them?
- Measure fillet size, does the weld leg size match the symbol dimension?
- Verify finish, are welds ground if the symbol specifies finish requirements?
- Confirm location, are welds on the correct side of each joint?
- Check for defects, spatter, undercut, or incomplete fusion requires repair.
Most structural steel fabrication drawings specify AWS D1.1 inspection requirements. Reference that standard if quality questions arise during installation.
Keep exploring
Related guides for structural steel and welded connections.
Welded vs. bolted connections and when each is used.
Understand structural plans and elevations.
Common W-shapes, angles, and plates.
Common installation mistakes and how to catch them.