Why Welding Symbols Exist
A welding symbol tells the fabricator exactly what type of weld to make, where to make it, how big, and to what standard — all in a compact graphic that fits on a drawing. If you’re ordering fabricated parts, you need to understand these.
The Reference Line
Every welding symbol is built on a horizontal reference line with an arrow pointing to the joint. Information below the line = arrow side (the side the arrow points to). Above the line = other side.
Basic Weld Types
| Symbol | Name | Description |
|---|---|---|
| △ (triangle) | Fillet Weld | Triangular cross-section joining two surfaces at ~90°. The most common weld type. |
| V shape | V-Groove | Beveled joint, full or partial penetration. Structural. |
| Square | Square Groove | No bevel — just butt the pieces together. Thin material only. |
| U shape | U-Groove | J-shaped bevel on both sides. Less weld metal than V on thick plate. |
| ⌒ (arc) | Plug/Slot Weld | Weld through a hole in one piece to join to the piece behind it. |
Fillet Weld Sizing
The number next to the fillet symbol is the leg size. A 1/4″ fillet weld has two 1/4″ legs forming a triangle. The throat (effective weld area) is 0.707 × leg size.
Rule of thumb: Fillet weld leg size should not exceed the thickness of the thinner member. A 1/4″ fillet on 3/16″ plate is overwelded — you’re just adding cost and distortion.
Common Supplementary Symbols
| Symbol | Meaning |
|---|---|
| Circle at junction | Weld all around (continuous around the joint) |
| Flag at junction | Field weld (done on site, not in the shop) |
| Tail with note | Special instructions (process, spec, electrode) |
| M between arrows | Melt-through (full penetration from one side) |
Groove Weld Details
Groove welds show additional info:
- Depth of bevel (in parentheses)
- Root opening — gap between parts before welding
- Groove angle — angle of the V or bevel
- Effective throat — depth of weld penetration required
Common Mistakes on Drawings
- No weld size specified: The shop has to guess. Bad guesses cost money.
- Overwelded joints: Bigger welds ≠ stronger. They cause distortion and waste time.
- Weld symbol on wrong side: Arrow side vs other side matters. The weld goes on the wrong face.
- “Weld all around” on everything: Most joints only need intermittent welds. Continuous welding where it’s not needed causes warping.
Standard Reference
Welding symbols follow AWS A2.4 (American Welding Society). If you’re specifying welds on a drawing, this is the definitive reference.
Need welded assemblies? Send us your drawings — we’ll review the weld symbols and make sure everything is clear before fabrication starts.