Welding symbols are the language between the engineer and the welder. Every structural blueprint, fabrication drawing, and weld procedure uses AWS A2.4 standard symbols to specify exactly what weld goes where. If you can’t read them, you’re guessing. Guessing on structural steel gets people killed.
This chart covers the symbols you’ll see on 95% of drawings: basic weld types, supplementary symbols, and tail references.
The Reference Line: Anatomy of a Welding Symbol
Every welding symbol is built on the same skeleton:
- Reference line (horizontal line). All symbols attach here.
- Arrow connects the reference line to the joint being welded. It points to the arrow side of the joint.
- Basic weld symbol sits on the reference line. Below = arrow side. Above = other side.
- Dimensions appear next to the weld symbol. Size on the left, length on the right.
- Supplementary symbols (circles, flags) attach at the junction of the arrow and reference line.
- Tail (optional). Contains process designations, electrode specs, or reference notes. No tail means no special instructions beyond the symbol itself.
The critical concept: arrow side vs. other side. The arrow touches one side of the joint. That’s the arrow side. Symbols below the reference line apply to the arrow side. Symbols above apply to the other side. If you see identical symbols on both sides, weld both sides of the joint.
Basic Weld Symbols
| Weld Type | Symbol Shape | Description | Common Uses |
|---|---|---|---|
| Fillet | Right triangle | Triangular cross-section weld joining two surfaces at right angles | T-joints, lap joints, corner joints |
| Square Groove | Two vertical lines | Square-edge butt joint with no bevel preparation | Thin material butt joints up to 3/16" |
| V-Groove | V shape | Both plates beveled to form a V opening | Butt joints on plate 1/4" and thicker |
| Bevel Groove | Half V (right angle + slope) | One plate beveled, other left square | T-joints, butt joints where one side is inaccessible |
| U-Groove | U shape | J-shaped preparation on both plates forming a U | Thick plate, reduces filler metal volume vs V-groove |
| J-Groove | J shape | J-shaped preparation on one plate only | T-joints on thick plate, corner joints |
| Plug / Slot | Rectangle with lines | Weld through a hole or slot in one member | Joining overlapping plates without edge access |
| Spot | Circle | Resistance or arc spot weld | Sheet metal lap joints, automotive panels |
| Seam | Circle with lines extending from sides | Continuous resistance or arc seam weld | Tanks, containers, sheet metal seams |
| Back / Backing | Semicircle | Weld applied to back side of a joint | Sealing root side of groove welds |
| Surfacing | Horizontal semicircle (flat side down) | Layer of weld metal deposited on a surface, not a joint | Hardfacing, wear-resistant overlays, buildup |
Supplementary Symbols
| Symbol Name | Symbol Shape | Meaning | Placement |
|---|---|---|---|
| Weld All Around | Circle at junction | Weld extends completely around the joint | At the junction of reference line and arrow |
| Field Weld | Flag (filled triangle) at junction | Weld to be made at the job site, not in the shop | At the junction of reference line and arrow |
| Melt-Through | Filled semicircle | Complete joint penetration with visible root reinforcement | Opposite side from groove weld symbol |
| Backing Bar | Rectangle | Backing material placed at root of joint | Opposite side from groove weld symbol |
| Spacer | Rectangle inside groove symbol | Spacer strip used in root of double groove welds | Between groove symbols on reference line |
| Flush Contour | Straight line | Weld face finished flush with base metal | Above or below weld symbol |
| Convex Contour | Arc curving outward | Weld face finished with convex (crowned) profile | Above or below weld symbol |
| Concave Contour | Arc curving inward | Weld face finished with concave (dished) profile | Above or below weld symbol |
Tail References and Process Abbreviations
When the tail of the welding symbol contains text, it specifies the welding process, electrode, or a reference document. Here are the abbreviations you’ll see most often.
| Abbreviation | Meaning |
|---|---|
| SMAW | Shielded Metal Arc Welding (Stick) |
| GMAW | Gas Metal Arc Welding (MIG) |
| GTAW | Gas Tungsten Arc Welding (TIG) |
| FCAW | Flux-Cored Arc Welding |
| SAW | Submerged Arc Welding |
| RSW | Resistance Spot Welding |
| RSEW | Resistance Seam Welding |
| E7018 | Electrode specification (example) |
| WPS-001 | Welding Procedure Specification number (example) |
If the tail is absent (no V-shaped fork at the end of the reference line), the welding process is either obvious from context or specified elsewhere on the drawing, typically in the general notes or title block.
Reading Dimensions on a Welding Symbol
Dimensions tell you weld size, length, and spacing. They follow a consistent pattern on the reference line.
Fillet welds: The number to the left of the triangle symbol is the leg size. A “1/4” to the left means a 1/4" fillet. The number to the right is the weld length. No length means continuous.
Intermittent fillets: Shown as length-pitch to the right of the symbol. “4-8” means 4" long welds on 8" center-to-center spacing. Staggered intermittent welds on opposite sides use a Z-pattern arrangement on the drawing.
Groove welds: The number to the left indicates groove depth or weld size. The number inside the V or bevel shape is the root opening. The angle of the groove is written outside the symbol.
Example: Reading a complete symbol. A reference line with a fillet triangle below, “5/16” to the left, “6-12” to the right, a circle at the junction, and “GMAW” in the tail. Translation: MIG weld a 5/16" fillet, 6" long on 12" centers, all the way around the joint, on the arrow side.
How Contour and Finish Symbols Work
A contour symbol tells the welder what the finished weld profile should look like. A finish symbol (letter) tells them how to get there.
- Flush (straight line): The weld face must be level with the base metal surface. This usually requires grinding.
- Convex (outward arc): The weld bead crowns above the base metal. This is the natural shape of most fillet welds without finishing.
- Concave (inward arc): The weld face dishes inward. Common on root passes and some fillet welds where a smooth transition reduces stress concentration.
Finish method letters appear next to the contour symbol: G = grinding, M = machining, C = chipping, H = hammering (peening), R = rolling. A flush contour with “G” means “grind this weld flush.”
Common Mistakes When Reading Welding Symbols
Confusing arrow side and other side. New welders frequently put the weld on the wrong side of the joint. Remember: below the line = arrow side. Above the line = other side.
Ignoring the tail. The tail may specify a particular electrode, WPS number, or preheat requirement. Skipping it can mean using the wrong process or missing a critical pre-heat.
Misreading intermittent spacing. The pitch dimension (second number in a length-pitch pair) is center-to-center, not the gap between welds. A “3-6” callout means 3" welds with 3" gaps, not 3" welds with 6" gaps.
Forgetting CJP vs. PJP. A groove weld symbol with no size dimension and a melt-through symbol on the opposite side means complete joint penetration (CJP). If there’s a specific depth dimension, it may be a partial joint penetration (PJP) weld. The difference matters for structural loading calculations.