AWS A2.4 is the standard that defines welding symbols used on engineering drawings. Every welded joint on a blueprint gets a symbol that tells the welder exactly what type of weld, what size, which side, and any special requirements. This reference covers all the symbols you’ll encounter on structural, pressure vessel, and fabrication drawings.
Misreading a welding symbol means you’ll either weld the wrong joint or weld the right joint incorrectly. Both cost rework time. Learn the system once and you can read any welding print in any shop.
The Reference Line System
Every welding symbol is built on the reference line. It’s a horizontal line with an arrow at one end and an optional tail at the other.
Arrow: Points to the joint being welded. The side the arrow touches is the “arrow side.” The opposite side is the “other side.”
Reference line: Carries all the weld information. Symbols below the line = arrow side. Symbols above the line = other side.
Tail: Optional. Contains process references, WPS numbers, or spec callouts. No tail means no special instructions beyond what the symbol shows.
Groove Weld Symbols
Groove weld symbols show the cross-section shape of the groove preparation. They sit on the reference line (below for arrow side, above for other side).
| Symbol Name | Description | Typical Use |
|---|---|---|
| Square groove | Two parallel vertical lines | Thin material (under 3/16"), no bevel |
| Single-V groove | V shape opening upward | Most common groove prep, 3/16" to 3/4" plate |
| Single-bevel groove | One vertical line, one angled line | Tee joints with CJP, one plate beveled |
| Single-U groove | U shape (curved bottom) | Heavy plate, less filler than V-groove |
| Single-J groove | J shape (one curved, one straight) | Tee joints on heavy plate |
| Double-V groove | V symbols both above and below reference line | Thick plate, welded from both sides |
| Double-bevel groove | Bevel symbols both above and below | Tee joints welded from both sides |
| Double-U groove | U symbols both above and below | Very heavy plate, minimum filler volume |
| Double-J groove | J symbols both above and below | Heavy tee joints, both sides accessible |
| Flare-V groove | Curved V shape | Round bar to plate, tube joints |
| Flare-bevel groove | One curved, one straight line | Round bar to flat, channel sections |
Groove Weld Dimensions
Numbers associated with groove symbols are placed in specific positions:
- Left of symbol: Depth of groove preparation (and effective throat if different, shown in parentheses)
- Inside the symbol: Root opening
- Above/below the groove angle: Groove angle in degrees
Example: A V-groove symbol with “3/8 (1/2)” to the left, “1/8” inside, and “60” at the angle means: 3/8 inch depth of groove, 1/2 inch effective throat (indicating partial penetration plus weld depth), 1/8 inch root opening, and 60-degree groove angle.
CJP vs. PJP Groove Welds
- CJP (Complete Joint Penetration): The weld extends through the full joint thickness. The symbol shows “CJP” in the tail, or no depth dimension is given (implying full penetration).
- PJP (Partial Joint Penetration): The weld doesn’t go through the full thickness. The depth of groove and effective throat are specified.
Fillet Weld Symbols
The fillet weld symbol is a right triangle sitting on the reference line. The vertical leg is always on the left side.
Fillet Weld Dimensions
- Left of symbol: Leg size (e.g., “1/4” means a 1/4 inch fillet)
- Right of symbol: Length and pitch for intermittent fillets (e.g., “3-6” means 3-inch-long welds at 6-inch center-to-center spacing)
- Both sides of reference line: Weld on both sides of the joint
Unequal-Leg Fillets
When the two legs are different sizes, both dimensions appear to the left: “3/8 x 1/4” means the long leg is 3/8 inch and the short leg is 1/4 inch.
Intermittent Fillet Welds
“2-5” to the right of the fillet symbol means 2-inch welds at 5-inch center-to-center spacing. If both sides are intermittent, they can be chain intermittent (aligned on both sides) or staggered intermittent (offset). A zigzag line between the two fillet symbols indicates staggered.
Supplementary Symbols
| Symbol | Location | Meaning |
|---|---|---|
| Circle | Arrow/reference line junction | Weld all around the joint perimeter |
| Flag (triangle) | Arrow/reference line junction | Field weld (weld on-site, not in the shop) |
| Filled circle (melt-through) | Opposite side of reference line from weld symbol | Complete penetration required from one side |
| Backing bar rectangle | Opposite side of reference line from weld symbol | Steel backing bar used behind the joint |
| Spacer rectangle | Between reference lines | Spacer strip between joint members |
| Consumable insert (square) | Opposite side from weld symbol | Consumable insert used at root |
Contour and Finish Symbols
Contour symbols specify the finished weld profile. They appear above or below the weld symbol (same side as the weld symbol itself).
| Contour Symbol | Meaning | Finish Letter | Finish Method |
|---|---|---|---|
| Straight line (flush) | Weld face must be flush with base metal | G | Grinding |
| Convex curve | Weld face convex (crowned) | C | Chipping |
| Concave curve | Weld face concave (dished) | M | Machining |
| R | Rolling | ||
| H | Hammering (peening) | ||
| U | Unspecified |
A flush contour with “G” means grind the weld flush with the base metal surface.
Other Weld Type Symbols
| Symbol Name | Description | Typical Use |
|---|---|---|
| Plug/slot weld | Rectangle on reference line | Filling holes or slots to join overlapping pieces |
| Spot weld | Circle on reference line | Resistance spot welding or plug welds in sheet metal |
| Seam weld | Circle with lines extending from sides | Resistance seam welding on lapped sheets |
| Stud weld | Circle with X inside | Shear studs, threaded studs welded to plate |
| Surfacing (buildup) | Semicircle below reference line | Weld overlay, hardfacing, cladding |
| Edge weld | Square with side flange | Edge flanges on thin sheets |
Multiple Reference Lines
When a joint needs more than one operation, multiple reference lines stack parallel:
- First reference line (closest to arrow): First operation
- Second reference line: Second operation
- Third reference line: Third operation
Example: A groove weld with a back gouge and back weld uses two reference lines. The first shows the groove weld from the arrow side. The second shows the back gouge and back weld on the other side.
Reading a Complete Welding Symbol: Step by Step
- Find the arrow. It points to the joint.
- Identify arrow side vs. other side. Arrow side is the surface the arrow touches.
- Read the weld symbol below the reference line (arrow side) and above (other side).
- Read the dimensions: size to the left, length/pitch to the right, angle and root opening inside.
- Check for supplementary symbols at the arrow/reference line junction (weld-all-around, field weld).
- Check the tail for process, WPS, or specification references.
- Check for contour/finish symbols on the weld face.
Non-Destructive Testing Symbols
NDT requirements are sometimes included on the welding symbol or nearby on the drawing:
| NDT Method | Symbol Abbreviation | Typical Placement |
|---|---|---|
| Visual Testing | VT | In the tail or on a separate symbol |
| Radiographic Testing | RT | In the tail or as a note |
| Ultrasonic Testing | UT | In the tail or as a note |
| Magnetic Particle Testing | MT | In the tail or as a note |
| Liquid Penetrant Testing | PT | In the tail or as a note |
When NDT appears in the tail of a welding symbol, it tells the shop that the weld requires that specific inspection method. When it appears as a separate note, it usually specifies the percentage of welds to inspect (e.g., “RT 10% of CJP welds”).
Practice: Reading Common Welding Symbols
Here are the most frequently encountered symbols on structural drawings:
1/4 fillet weld, arrow side only: Fillet triangle below the reference line with “1/4” to the left. The arrow points to the joint, and the weld goes on the arrow side only.
1/4 fillet both sides: Fillet triangles both above and below the reference line, both with “1/4.” Same leg size on both sides of the joint.
CJP single-V groove: V-groove symbol below the reference line with “CJP” in the tail. No depth dimension (full penetration implied).
PJP single-bevel, 3/8 effective throat: Bevel symbol with “1/2 (3/8)” to the left. The 1/2 is groove depth, 3/8 is the effective throat. The weld only penetrates 3/8 inch into the joint.
Common Symbol Reading Mistakes
Confusing arrow side and other side. Remember: below the line = arrow side, above = other side. This is the single most common error in blueprint reading.
Ignoring the depth vs. effective throat on PJP groove welds. The number in parentheses is the effective throat, which is what determines strength. The other number is the depth of groove preparation.
Misreading intermittent fillet dimensions. “3-6” means 3-inch welds at 6-inch spacing (center to center), not 3-inch welds with 6-inch gaps between them. The pitch is always center to center.
Forgetting the tail. If the tail says “A2” that’s a WPS reference. If it says “GTAW,” that specifies the process. If it says “AWS D1.1,” that’s the governing code. Ignoring the tail can mean using the wrong process or procedure.
Missing the weld-all-around symbol. A circle at the junction of arrow and reference line means the weld goes around the entire joint. Missing it means you’ll weld one side only and leave the rest undone.
For information on reading the WPS that welding symbols reference, see the how to read a WPS guide. Return to the weld symbols category page or the welding techniques pillar for more resources.