Weld Symbols: The Language Between Designer and Welder
A weld symbol on an engineering drawing is a precise instruction to the welder and to the inspector. When it is correct, it communicates exactly the type, size, location, and finish of the weld without ambiguity. When it is wrong — or when the reader misinterprets it — the weld is applied incorrectly, may not meet structural requirements, and may need costly rework or cause field failure. Understanding weld symbols is not optional for mechanical designers who work on fabricated structures.
This guide covers the reference line structure, the arrow-side and other-side convention, the most common weld types, weld size and length notation, and the key differences between AWS A2.4 (US standard) and ISO 2553 (international standard).
The Reference Line Structure
Every complete weld symbol is built on a reference line — a horizontal line connected to the joint by an arrow. The reference line is the backbone of the symbol, and all other elements are positioned relative to it:
- Below the reference line: Information about the weld on the arrow side (the side of the joint that the arrow points to)
- Above the reference line: Information about the weld on the other side (the side of the joint opposite the arrow)
- Tail (fork at the end of the reference line opposite the arrow): Used for supplementary information such as welding process specification, weld procedure number, or filler material
- Weld-all-around symbol: A circle at the junction of the reference line and the arrow indicates a continuous weld around the entire perimeter of the joint
- Field weld symbol: A filled flag at the reference line junction indicates the weld is to be made in the field (after fabrication), not in the shop
Arrow Side vs Other Side: A Critical Convention
Confusing arrow side and other side is one of the most common and consequential errors when reading weld symbols. The arrow always points to a specific surface of the joint. When a weld symbol element appears below the reference line, the weld is on the same side as the arrow. When it appears above, the weld is on the opposite side.
For a T-joint with a fillet weld on both sides, the fillet weld symbol appears both above and below the reference line, with each side’s dimensions specified independently. A fillet weld on one side only has a symbol on one side of the reference line, and the other side is blank.
Common Weld Types and Their Symbols
| Weld Type | Symbol Shape | Typical Application |
|---|---|---|
| Fillet weld | Right triangle | T-joints, lap joints, corner joints |
| Square groove | Two vertical lines | Thin plate butt joints, edge welds |
| V-groove | V shape | Butt joints requiring full penetration |
| Single bevel groove | Angled line on one side | T-joints requiring penetration from one side |
| U-groove | U shape | Thicker butt joints, reduced included angle |
| J-groove | J shape | T-joints in thick material |
| Plug weld | Circle | Lap joints, attachment through a hole |
| Slot weld | Rectangle | Lap joints, elongated plug weld |
| Seam weld | Two semicircles | Resistance seam, sheet metal |
Fillet Weld Notation: Size and Length
For fillet welds, the primary dimension is the leg size — the dimension from the root to the toe of the fillet in the throat plane. For a symmetrical fillet, one number is given (e.g., 6 for a 6 mm fillet). For an asymmetric fillet, two dimensions are given separated by a multiplication sign.
When a fillet weld does not run the full length of the joint, the length is specified after the weld symbol and the pitch (center-to-center spacing of intermittent welds) follows after a hyphen:
- 6-50-150 means a 6 mm fillet, 50 mm long, at 150 mm pitch (center-to-center)
- A fillet with no length specified runs the full length of the joint
The effective throat of a fillet weld (for structural calculation) is 0.707 × leg size for a symmetric 45° fillet. The throat is the minimum dimension through the weld cross-section and is what carries the load in a shear-loaded fillet.
AWS A2.4 vs ISO 2553: Key Differences
Both standards define welding symbol systems, but there are meaningful differences that cause confusion in international projects:
- Arrow side / other side convention: AWS A2.4 uses below = arrow side, above = other side. ISO 2553 uses the same convention, but historical ISO practice had it reversed in earlier editions — verify which version is being used if working with older drawings.
- Fillet symbol orientation: AWS uses a right-triangle fillet symbol. ISO 2553 uses a filled triangle, sometimes with different orientation conventions.
- Weld size placement: AWS places the weld size to the left of the weld symbol; ISO places it in a similar position but with potential for different notation for throat vs. leg size (ISO distinguishes throat dimension (a) and leg size (z) explicitly).
- Supplementary symbols: Flush contour, convex contour, and concave contour symbols are standardized in both but with slightly different graphic representations.
When working on international projects, explicitly state on the drawing which standard governs: Welding symbols per AWS A2.4 or Welding symbols per ISO 2553. This eliminates ambiguity that can otherwise only be resolved by guessing which convention was intended.
Common Errors in Weld Symbol Specification
- Placing the fillet symbol on the wrong side of the reference line (arrow side vs other side confusion)
- Specifying a fillet size that requires multiple passes but not indicating this in the welding procedure reference
- Omitting the weld-all-around symbol when a continuous perimeter weld is intended
- Not specifying groove angle, root opening, and root face for groove welds — leaving the prep to the welder’s interpretation
- Using the same weld symbol for two different weld types because the symbols look similar (V-groove vs single bevel)
FAQ
Q: How do I specify a full penetration weld on a drawing?
A: Use a groove weld symbol (V, J, U, bevel, etc.) with the letters CJP (Complete Joint Penetration) in the tail of the reference line. CJP indicates that the weld must achieve fusion through the full thickness of the joint, regardless of the specific groove geometry used to achieve it. This delegates the joint prep choice to the fabricator within the applicable welding code. If a specific groove preparation is required, dimension it on the drawing: root opening, included angle, and root face dimension.
Q: When should I use intermittent fillet welds instead of a continuous fillet?
A: Intermittent welds are appropriate when: the joint does not require continuous strength along its length (load transfer is localized), when continuous welding would cause excessive heat distortion in thin plate, or when cost reduction is needed for lightly loaded attachments. Continuous welds are required when: the joint must be watertight or airtight, when the full length of the joint carries shear load, or when the structural code requires continuity. Never use intermittent welds in corrosive environments without sealing the gaps — moisture trapped between intermittent welds accelerates corrosion.
Q: What does the weld symbol contour modifier mean and when should I use it?
A: Contour symbols indicate the required finishing of the weld face. A flat line through the weld symbol means the weld must be finished flush (by grinding or machining). A convex arc means convex profile is acceptable (as-welded is typically convex). A concave arc means a concave profile is required, often for root passes in groove welds. Specifying flush finishing is only necessary when the weld profile would otherwise interfere with fit-up, appearance, NDT access, or stress concentration. Unnecessary finishing specifications add cost; specify them only when the function requires it.
Reference
Weld joint design has its own rules of thumb that are easy to get wrong without a dedicated reference.
Design of Welded Structures (Blodgett)
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