Design & fabrication·24 January 2026·3 min read

Reading welding symbols to ISO 2553

A welding symbol is a compact instruction: it states which type of weld goes where, how large, how long and with what supplementary requirements. Read it wrongly and the weld lands on the wrong side or at the wrong size, which usually only surfaces at inspection.

This article walks through the structure of the symbol under ISO 2553 and identifies where the ISO system and the American AWS system differ.

Structure of the symbol

A welding symbol consists of an arrow, a reference line and an elementary symbol indicating the weld type: a triangle for a fillet, a V for a V groove, a half V, a U, a J, or a rectangle for a square butt. To the left of the elementary symbol sits the size, to the right the length and any pitch.

The arrow points at the joint. A symbol on the continuous reference line applies to the arrow side; below the dashed line it applies to the other side. A double sided weld therefore carries a symbol on both.

Size, length and pitch

The size for a fillet is preceded by a for throat thickness or z for leg length. For a groove weld the penetration depth is given, or nothing where full penetration is intended. See calculating throat thickness.

To the right of the symbol, an intermittent weld carries first the number of welds, then the length and then the pitch, for example 5 x 50 (100). That means five welds of 50 mm at 100 mm centres. A Z shaped symbol between them denotes a staggered intermittent weld.

Supplementary indications

The tail of the reference line may carry references to the process to ISO 4063, the quality level to ISO 5817, the welding position or the applicable WPS. A circle at the elbow means weld all round; a flag means weld on site.

A flat, convex or concave line above the elementary symbol indicates the required weld face profile: dressed flush, convex or concave. That is not a cosmetic preference: under fatigue loading that profile determines service life.

ISO versus AWS

The biggest difference lies in symbol position. In the ISO system the continuous line applies to the arrow side and the dashed line to the other side. In the AWS system there is one reference line, where below the line means arrow side and above means other side, exactly the reverse of what a European reader expects.

Drawings originating from both worlds must therefore state which system applies. Where that is missing, do not rely on habit but ask. A swapped side is an error that only becomes visible after welding.

Frequently asked questions

That the weld runs all round, across all adjoining faces of the joint. A flag in the same position means the weld is made on site rather than in the shop.

A groove weld without a size normally means full penetration. Where a size is given, that is the required penetration depth. In case of doubt it is a question for the designer, not an assumption, because the difference affects both strength and examination requirements.

Continuous is structurally stronger, but costs more weld metal and shrinkage, while in corrosion prone structures it leaves fewer crevices. In dynamically loaded structures an intermittent weld may have been chosen deliberately. Consult rather than changing it on your own initiative.

In the tail after the reference line, for example ISO 5817-B. If it is not there, it must be defined elsewhere on the drawing or in the specification. Without a defined level it cannot be judged whether a weld complies. See ISO 5817.