Welding Symbols — Complete AWS A2.4 Guide for Beginners and Professionals

Welding symbols are the universal language of fabrication drawings. Defined by AWS A2.4 (Standard Symbols for Welding, Brazing, and Nondestructive Examination) in North America and ISO 2553 internationally, welding symbols convey in a compact graphic notation exactly what type of weld is required, where it goes, how large it must be, and what supplementary operations are needed — without a word of written description. Every engineer, drafter, welder, and welding inspector who works with fabrication drawings must be able to read welding symbols fluently.

A misread welding symbol can mean the wrong weld type, the wrong size, weld on the wrong side of the joint, or missed PWHT requirements. In pressure vessel and structural fabrication, such errors translate directly into failed inspection, costly repairs, and schedule overruns. This guide covers every element of the welding symbol system in the depth needed for real fabrication practice — from the basic arrow-side / other-side convention through to complete joint penetration groove weld notation, intermittent fillet weld designation, and process references in the tail.

You will also find original SVG diagrams for every major concept, a complete weld symbol library, worked reading examples step by step, and a comparison of the AWS A2.4 and ISO 2553 systems. For related drawing interpretation topics, see the guide to welding joint types and the article on welding positions and their designations.

Why Welding Symbols Matter

In fabrication practice, weld joints are designed by engineers, detailed by drafters or CAD operators, manufactured by welders, and inspected by quality inspectors — typically in different locations and often at different times. Welding symbols are the mechanism that transfers the design intent precisely through all of these handoffs without ambiguity.

The specific benefits of a correctly used welding symbol system are:

• Clarity and consistency — any qualified reader anywhere in the world (using the same standard) interprets the symbol identically, eliminating verbal misunderstandings
• Precision — a welding symbol specifies type, size, length, pitch, position, process, and finish in a single compact notation that would otherwise require a lengthy written note
• Cost reduction — incorrect welds (wrong size, wrong type, wrong side) are caught at the drawing stage rather than after fabrication, avoiding expensive repairs
• Document control — welding symbols on drawings create a permanent, auditable record of the design intent, which is essential for code compliance (ASME, AWS D1.1, API 1104)

Weld Symbol vs Welding Symbol — Key Distinction

AWS A2.4 makes a precise and important distinction between two closely related terms that are frequently confused:

Complete Components of a Welding Symbol

AWS A2.4 defines up to eight distinct elements that may appear in a complete welding symbol. Not every welding symbol uses all eight — many simple symbols use only three or four. The table below summarises every element, its location relative to the reference line, and when it is required.

The Reference Line

The horizontal reference line is the foundation of every welding symbol. All other elements are positioned relative to it. The reference line always appears as a straight, horizontal line on the drawing, regardless of the orientation of the joint itself. Key rules for the reference line:

• The reference line is always drawn horizontally — never at an angle — even if the joint is inclined
• The arrow connects to one end (either end — the standard permits both)
• The tail, when used, occupies the opposite end from the arrow
• Below the reference line = arrow side of the joint
• Above the reference line = other side (the side opposite the arrow)
• When weld symbols appear both above and below, welding is required on both sides

The Arrow and Arrow-Side Convention

The arrow connects to the reference line and points to the joint to be welded. In most cases the arrow is a simple straight line with an arrowhead at the joint end. AWS A2.4 makes one important special case: the break arrow.

Break Arrow (Arrow with Bend)

For certain groove welds — particularly bevel and J-groove welds — one member of the joint must be prepared (beveled or formed) while the other remains square-cut. In these cases, the standard requires a break (a bend or kink) in the arrow line. The break in the arrow is always directed toward the member that must be prepared. This removes any ambiguity about which piece the groove must be cut into.

Arrow-Side and Other-Side Convention Summary

Basic Weld Symbol Library

The weld symbol identifies the type of weld required. AWS A2.4 defines symbols for arc and gas welds, resistance welds, and brazing. The most commonly encountered symbols in structural and pressure vessel fabrication are shown below.

Fig. 3 — AWS welding symbol chart (from original article): complete symbol anatomy per AWS A2.4 showing all elements in their standard positions.

Fig. 4 — AWS basic weld symbol library (AWS A2.4): arc and gas weld symbols, resistance weld symbols, and brazing symbols as standardised graphical elements.

Arc and Gas Weld Symbols — Reference Table

Supplementary Welding Symbols

Supplementary symbols add specific requirements that cannot be conveyed by the basic weld symbol alone. AWS A2.4 defines five key supplementary symbols in common use:

Fig. 5 — AWS supplementary welding symbols per A2.4: all-around, field weld, melt-through, backing, spacer, and contour symbols used in conjunction with basic weld symbols.

The Tail — Process and Specification References

The tail is a forked extension at the end of the reference line opposite the arrow. When no additional information is required, the tail is omitted entirely. When present, it carries information in a standardised shorthand notation. AWS A2.4 permits the following information in the tail:

• Welding process designation — AWS letter abbreviations such as GTAW (TIG), SMAW, GMAW, FCAW, SAW, PAW, EBW
• WPS reference — the specific welding procedure specification number, e.g., “WPS-014”
• Filler metal classification — e.g., “ER308L” or “E7018”
• Special inspection requirements — e.g., “RT” or “PT” to indicate that specific NDT is required at that joint
• Peening, backgouging, or other special operations
• Strength requirements — for certain codes that permit specifying minimum weld strength in the symbol

Fig. 6 — Tail example 1: the tail references the welding process (SMAW) and the applicable WPS number, removing any ambiguity about which procedure governs this joint.

Fig. 7 — Tail example 2: the tail references the filler metal classification and specifies that radiographic testing (RT) is required at this joint after welding.

Weld Dimensions and Their Placement

Weld dimensions are always placed on the same side of the reference line as the weld symbol. Their precise location (left or right of the symbol) is standardised and must be observed correctly:

Fillet Weld Dimensions

For intermittent fillet welds (chain or staggered), the designation “6-50-150” means: 6 mm leg size, 50 mm weld length, 150 mm pitch (centre-to-centre spacing). Chain intermittent welds use identical symbols above and below the reference line. Staggered intermittent welds use symbols that are offset (staggered) above and below.

Groove Weld Dimensions

Weld Contour and Finish Symbols

The contour symbol specifies the required profile of the weld face after welding. The finish symbol specifies the method by which that contour is achieved.

Finish symbols appear adjacent to the contour symbol and specify the method used to achieve the required profile:

Reading Welding Symbols — Worked Examples

The following worked examples take you through the step-by-step process of reading complete welding symbols of increasing complexity. This is the skill that separates a drawing interpreter from someone who has only memorised symbol shapes.

Example 1 — Simple Arrow-Side Fillet Weld

Example 2 — Double Fillet Weld with Length and Pitch

Example 3 — Single-V Groove with Root Opening and CJP

Example 4 — Partial Penetration Groove with GTAW in Tail

AWS A2.4 vs ISO 2553 — Key Differences

Engineers working internationally encounter both AWS A2.4 (North American) and ISO 2553 (international) systems on the same project. The two standards use the same concept — a reference line — but have important differences in convention that must be understood to avoid misreading.

Recommended Reference Books on Welding Symbols and Drawing Interpretation

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