Understanding Weld Symbols
on Engineering Drawings

Weld symbols on engineering drawings are a compact and precise language for communicating exactly what joint is required, where it is, which side it goes on, how large it should be, and what the finished surface should look like. They are also frequently misread, inconsistently applied, and sometimes ignored entirely in favour of notes and sketches that introduce ambiguity. An engineer or fabricator who can read weld symbols fluently can extract all of this information from a drawing in seconds. One who cannot will either guess or ask — both of which cost time.

This article covers the BS EN ISO 2553:2019 weld symbol system — the current European and UK standard — explaining each component of the symbol, the common weld types and their representations, key dimensions, supplementary symbols, and the mistakes that most commonly cause misreads.

The Standard — BS EN ISO 2553

BS EN ISO 2553:2019 is the current standard for symbolic representation of welds on drawings, replacing the earlier BS 499-2 and harmonising the European and ISO systems. Drawings produced to the older BS 499-2 remain in circulation — the fundamental symbol logic is similar but some details differ, particularly in the representation of butt weld preparations.

The American AWS A2.4 system, used on US-origin drawings and some oil and gas projects, is similar in structure but differs in a number of important details — particularly in where dimensions are placed relative to the reference line and in some symbol shapes. If you are reading an American drawing, check whether it references AWS A2.4 and treat the symbols accordingly.

Anatomy of a Weld Symbol

Every weld symbol has the same basic structure, built around a horizontal reference line with an arrow pointing to the joint location. Understanding the geometry of this structure is the foundation for reading any weld symbol.

The Reference Line and Arrow

The reference line is a horizontal line. The arrow line connects the reference line to the joint being specified — the arrow points directly at the weld location on the drawing. The end of the reference line opposite the arrow may carry a tail, used for supplementary information such as welding process references or standards.

Arrow Side and Other Side

This is the most important concept in reading weld symbols — and the most frequently confused. The reference line has two sides:

• Arrow side: The side of the reference line closest to the arrow. Weld symbols placed below the reference line (in the ISO/EN system) specify a weld on the arrow side — the side of the joint that the arrow is pointing to.
• Other side: The side of the reference line away from the arrow. Weld symbols placed above the reference line specify a weld on the other side of the joint from the arrow.

This convention means that a single weld symbol can specify welds on both sides of a joint simultaneously — with different weld types or sizes on each side if required.

The Basic Weld Symbol

The basic weld symbol is a graphical representation of the weld joint cross-section placed on the reference line. The most common basic symbols:

Dimensions

Dimensions are written alongside the weld symbol. Their position relative to the symbol depends on the weld type:

Fillet welds: The leg length (side length of the fillet triangle) is written to the left of the symbol, preceded by the letter z (leg length) or a (throat thickness). z6 means a 6mm leg length fillet weld; a4 means a 4mm throat thickness fillet weld. The throat thickness a is related to leg length z by the formula a = z × cos 45° ≈ 0.7z for an equal-leg fillet.

If a length and pitch are specified (for intermittent welds), they appear to the right of the symbol in the format length × pitch (e.g. 50×150 means 50mm long welds at 150mm pitch).

Butt welds: The included angle of the V preparation, the root gap, and the root face depth may all be specified. These are written in the tail or as annotations to the symbol. For a full penetration single V butt weld with no specific preparation geometry, the symbol alone with a flush or convex contour supplementary symbol is often sufficient.

The Tail

The tail is a fork-shaped extension at the end of the reference line opposite the arrow. It is used to communicate supplementary information that cannot be shown symbolically — typically the welding process reference (ISO 4063 process number), the applicable welding standard, or a reference to a Welding Procedure Specification (WPS). A tail containing "111" refers to manual metal arc (MMA/SMAW) welding per ISO 4063; "141" is TIG (GTAW); "135" is MIG/MAG (GMAW). If no tail is shown, the welding process is not specified on the drawing.

Supplementary Symbols

Supplementary symbols are added to the basic weld symbol to convey additional requirements about the weld contour, extent, or location.

Weld All Round

A small circle at the junction of the arrow line and the reference line means the weld is required all around the joint — typically on a tube or column base plate where the weld runs continuously around the full perimeter. Without this symbol, a weld shown at one location applies only to that visible joint line.

Field Weld (Site Weld)

A filled flag at the junction of the arrow and reference line indicates that the weld is to be made in the field (on site) rather than in the workshop. This distinction matters for inspection, quality plan, and logistics purposes — site welds typically attract more rigorous pre-weld inspection requirements and may need to be performed by welders with specific site WPS qualification.

Contour Symbols

Placed above or below the basic weld symbol, contour symbols specify the required shape of the weld face after completion:

• Flush (flat): A straight horizontal line above the symbol. The weld face must be flush with the parent material — typically requiring dressing or grinding.
• Convex: A curved line bowing outward above the symbol. The weld face is to be left convex as welded — not ground flush.
• Concave: A curved line bowing inward. Used where a concave fillet profile is required, typically for aesthetic or fatigue reasons.

Backing Strip or Backing Weld

A rectangle below a butt weld symbol indicates a backing strip — a permanent or temporary backing bar used to support the root pass. If the backing strip symbol has a small circle at the ends of the rectangle, it is a temporary backing that is to be removed after welding.

Fillet Welds in Detail

The fillet weld is the most common weld type in fabrication and deserves detailed coverage. A fillet weld joins two surfaces that meet at an angle — most commonly a right angle — by filling the root of the joint with weld metal. The resulting cross-section is approximately triangular.

Leg Length vs Throat Thickness

Two dimensions characterise a fillet weld size:

• Leg length (z): The distance from the weld toe to the weld root along the fusion face. For an equal-leg fillet this is the same on both legs.
• Throat thickness (a): The perpendicular distance from the weld root to the weld face. For an equal-leg right-angle fillet: a = 0.707z. The throat is the critical dimension for stress calculation — design codes base fillet weld capacity on throat area.

BS EN ISO 2553 permits both conventions — specifying either z (leg length) or a (throat) — and the letter prefix on the drawing indicates which is being used. z6 and a4.2 describe essentially the same weld. When reading a drawing, check the prefix carefully — specifying the throat where the leg is intended (or vice versa) produces either an under-sized or over-sized weld.

Intermittent Fillet Welds

Where continuous fillet welds are not required — for reasons of heat input control, distortion management, or cost — intermittent welds are specified. The format is: symbol z(leg) — (weld length) × (pitch). For example, z6 — 50×150 means 6mm leg fillet welds, 50mm long, at 150mm centre-to-centre pitch.

Staggered intermittent welds — welds on alternating sides of the joint, staggered in position — are indicated by placing the symbols on opposite sides of the reference line with the Z-pattern notation. This arrangement distributes heat more evenly and reduces distortion on thin material.

Deep Penetration Fillet Welds

A standard fillet weld fuses into the parent material to the depth of the throat. A deep penetration fillet, indicated by a vertical line added to the left of the fillet triangle symbol, penetrates significantly beyond the standard throat depth. This requires a specific welding procedure and is typically specified where the joint must carry higher loads without increasing the visible weld size.

Butt Welds in Detail

Butt welds join two parts in the same plane. The joint preparation — the shape cut or machined into the edges before welding — determines how deeply the weld penetrates, how much filler metal is required, and how accessible the root is for fusion.

Single V Butt

The most common butt preparation for material above approximately 6–8mm thick. Both edges are bevelled to produce a V-groove which is then filled with weld passes. Key parameters: included angle (typically 60° for manual processes), root face (the unbevelled land at the root, typically 1–3mm), and root gap (the space between the parts at the root, typically 2–3mm for manual TIG root, 0–1mm for orbital TIG).

Double V Butt (X Weld)

Bevelling from both sides produces an X preparation. Used on thicker material (typically above 20–25mm) where welding from one side only would require excessive filler metal volume and produce unacceptable distortion. The double V distributes the weld volume between both sides and significantly reduces angular distortion. Requires access for welding from both sides — not applicable where one side is inaccessible.

Single and Double U Butt

The U preparation has a curved root profile machined or gouged rather than a straight bevel. This reduces the included angle at the root and produces a narrower, deeper groove — reducing total weld volume compared to a V for the same penetration depth. U preparations are more expensive to prepare (requiring machining rather than simple flame or plasma cutting) but are standard on thick-section pressure vessel welds where minimising heat input and distortion is critical.

Full Penetration vs Partial Penetration

A full penetration butt weld — indicated by the symbol with no additional notation, or explicitly noted as PJP (partial joint penetration) when partial is intended — is assumed to penetrate the full thickness of the thinner part. Where only partial penetration is required, the depth of penetration (s dimension) is written to the left of the symbol inside parentheses: s(8) means 8mm penetration depth.

Misreading a partial penetration as full penetration, or vice versa, has significant structural consequences. Where the joint is load-carrying, the penetration requirement must be unambiguous on the drawing.

Reading a Complete Weld Symbol — Worked Examples

Applying the above, some representative complete weld symbols and their meaning:

Common Misreads and Mistakes

1. Confusing arrow side and other side. The most frequent error. A fillet symbol below the line goes on the arrow side. Above the line means the other side. Getting this wrong produces a weld in the wrong location — which may pass visual inspection on the wrong side and fail when the joint is loaded.
2. Confusing leg length and throat notation. z6 and a6 are not the same weld. z6 gives a throat of approximately 4.2mm; a6 requires a throat of 6mm which corresponds to a leg length of approximately 8.5mm. The distinction is significant for structural adequacy.
3. Omitting the weld-all-round symbol. A fillet symbol at a tube-to-plate joint without the all-round circle is ambiguous — it might be read as a single-side weld. Always add the circle where continuous perimeter welding is intended.
4. No contour symbol where finish matters. A butt weld on a pressure vessel nozzle that must be ground flush for flush-mount instrument installation needs a flush contour symbol. Without it, the fabricator will leave the weld in the as-welded condition.
5. Assuming full penetration without specifying it. On a T-joint where full penetration is required for the design load, the weld symbol must clearly indicate this — either with a penetration depth annotation or with a note. A standard fillet symbol on a T-joint does not imply penetration through the web.
6. Mixing BS EN ISO 2553 and AWS A2.4 conventions on the same drawing. The two systems look similar enough to be confused but differ in dimension placement. If the drawing origin is mixed — some details from a US project, some from a UK drawing — this creates silent errors that may not be caught until the joint is inspected.
7. Not specifying intermittent weld pitch correctly. The pitch in intermittent weld notation is centre-to-centre, not gap. z6 — 50×150 means 50mm of weld every 150mm — the gap between welds is 100mm, not 150mm. This is a frequent point of confusion on site.
8. Relying on notes instead of symbols. "Weld as required" or "full penetration butt weld — see detail" without a proper symbol is not a specification. It transfers responsibility for joint design to the fabricator and leaves no unambiguous basis for inspection.

Practical Reading Strategy

When reading a weld symbol on a drawing for the first time, work through it systematically:

1. Locate the arrow. Identify which joint the arrow is pointing to and which side the arrow is on.
2. Check above and below the reference line. Symbol below = arrow side. Symbol above = other side. Symbol both sides = both sides of joint.
3. Read the basic symbol. Identify the weld type — fillet, V butt, U butt, plug etc.
4. Read the dimension to the left of the symbol. For fillets, note the z or a prefix. For butt welds, note any penetration depth or preparation dimension.
5. Read any dimension to the right. Length and pitch for intermittent welds.
6. Check for supplementary symbols. Circle at arrow junction (all round), flag (site weld), contour symbol above or below the basic symbol.
7. Check the tail. Welding process number, WPS reference, or other notes.

Summary

Weld symbols are a precise, compact notation system. Once the arrow-side/other-side convention is understood and the common basic symbols are familiar, the vast majority of weld symbol combinations on engineering drawings can be read correctly without reference to a chart.

The critical disciplines are: always check which side of the reference line the symbol is on; always check the dimension prefix (z vs a for fillets, s for partial penetration depth); always check for supplementary symbols that modify the basic requirement; and never substitute a written note for a proper symbol where a symbol could do the job more precisely.

If you are producing drawings with weld symbols, follow the same logic in reverse — use the symbol system to communicate precisely rather than relying on notes or sketches that introduce interpretation. An ambiguous weld specification is a non-conformance waiting to happen.