Weld Symbols: Structure, Meaning and Positioning

A shop drawing from a Far East supplier shows a weld symbol nobody in-house has seen before. Whether the weld was actually executed as required can't be judged without reading that symbol correctly, even if the finished component looks unremarkable at first glance.

In brief: Weld symbols per DIN EN ISO 2553 consist of an arrow line, a reference line, and the actual weld symbol. The reference line is made up of a continuous line and a dashed line: a symbol placed on the continuous line means the weld sits on the arrow side, a symbol on the dashed line means it sits on the opposite side. The weld size (throat thickness) sits to the left of the symbol.

What is a weld symbol?

A weld symbol is the standardized graphical shorthand used to unambiguously describe a welded joint on a technical drawing, without having to draw the weld itself. The notation is governed by DIN EN ISO 2553, which sets the symbolic representation of welds on drawings for welding and allied processes.

For buyers, the weld symbol matters because it fixes the weld type, weld size, and execution side before manufacturing even starts. Misread the weld symbol, and you might flag a correctly executed weld as a deviation, or miss an actual deviation entirely. That holds regardless of whether the design originated in-house or at the manufacturer, since the symbol remains the binding reference for both sides.

How is a weld symbol structured?

A complete weld symbol consists of three basic elements: the arrow line, the reference line, and the actual weld symbol, supplemented by optional additional data such as dimensions or the number of welds.

In our quality inspections at Far East suppliers, we regularly see additional data on surface finish or weld length missing from the drawing, even though the basic symbol itself is correctly placed. Without that data, the manufacturer has room for interpretation, especially on visible welds on housings or covers.

The weld surface finish is marked with its own supplementary symbol attached to the weld symbol, such as a flat line for a flush-dressed weld. If that supplementary symbol is missing, the weld's natural, unworked state counts as sufficient, which rarely matches the desired look on visible components.

What does the symbol's position on the reference line mean?

The reference line consists of two parallel lines, one continuous and one dashed. If the weld symbol sits on the continuous line, the weld is on the side the arrow line points to, the so-called arrow side. If it sits on the dashed line, the weld is on the opposite side of the component.

This distinction matters especially for components only accessible from one side, such as closed profiles, where the weld can only be executed from that one side. Read the side wrong, and welding may end up happening at a location the design never intended for it.

If a joint needs to be welded on both sides, two symbols appear on the reference line at once, one above and one below, on the continuous and dashed line respectively. Both sides don't necessarily need to share the same weld type or weld size either, and the two symbols make that distinction just as unambiguous.

What basic weld symbols exist?

Each basic symbol stands for a specific weld type and is placed on the reference line. The most common symbols are:

In one of our sourcing projects, a fillet weld symbol was mistaken for a single-V butt weld symbol, since the two look similar at a glance. That would have set the wrong expectation for weld size at acceptance, had the difference not been clarified with the manufacturer beforehand.

What does the weld size on a weld symbol mean?

The weld size states the throat thickness and sits to the left of the actual weld symbol on the reference line. For fillet welds, it corresponds to the height of the largest triangle that fits inside the weld's cross-section, not the visible outer width of the weld bead.

If the weld size is missing from the drawing, the throat thickness stays undefined even when the weld type is clearly identifiable from the weld symbol. For load-bearing or fatigue-loaded joints, the weld size should therefore never be left out, no matter how unambiguous the rest of the design looks. For several identical welds on the same component, it's usually enough to state the weld size once and reference the remaining welds to it, provided the drawing makes that reference clear.

Put together, this produces a fully readable symbol, for example this one for a 4-millimeter fillet weld on the arrow side:

What applies if the drawing shows no weld symbol at all?

If a weld symbol is missing entirely despite a welded joint being intended, there's no automatically applicable default value, unlike general tolerances per ISO 2768, which do apply without a special note. Weld type, weld size, and execution side have to be actively specified for every welded joint.

The same logic applies to a weld's geometric position on the component: form and position tolerances apply there as a supplement, provided the drawing doesn't state its own value.

How do you check a weld symbol during incoming goods inspection?

During incoming goods inspection, the executed weld gets checked against the weld symbol required on the drawing, not against a general visual impression. What matters most is the weld type, the execution side, and, where measurable, the weld size.

On a complete 3.1 inspection certificate, referencing the required weld symbol is part of the minimum scope you should check before releasing payment. Further guidance on the practical application of the notation is also available from the German Welding Society's technical rulework, and an international overview comes from TWI's review of weld symbols.

Frequently Asked Questions

What is a weld symbol?

A weld symbol is the standardized graphical representation of a welded joint on a technical drawing per DIN EN ISO 2553. It describes the weld type, weld size, and the side the weld is on.

How is the weld size measured?

For fillet welds, the weld size corresponds to the height of the largest triangle that fits inside the weld's cross-section. It sits to the left of the weld symbol on the drawing and has to be explicitly stated there.

What weld types exist and how do I tell the symbols apart?

The most common basic symbols are the fillet weld (triangle), the square butt weld (two parallel lines), and the single-V and V butt welds (V-shaped symbols). The exact shape determines the required weld preparation.

What does a dashed reference line mean on a weld symbol?

If the weld symbol sits on the dashed part of the reference line, the required weld is on the side opposite the arrowhead, not the side the arrow directly points to.

Conclusion: reading weld symbols correctly prevents misinterpretation

A weld symbol fixes weld type, weld size, and execution side right on the drawing, long before the first weld gets made. Read the arrow line, reference line, and weld size correctly, and you'll catch deviations reliably at goods receipt instead of relying on the visual impression of the finished weld.

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