WELDINGResources / Welding symbolsUpdated 29 Oct 2026

[ WELDING SYMBOLS · COMPARISON ]

AWS vs ISO welding symbols

AWS A2.4 and ISO 2553 use the same weld shapes on the same reference line and arrow. They differ in three places that matter on the shop floor: which side a symbol means, how a fillet is sized, and how the process is written.

AWS: AWS A2.4: arrow side below the line, fillet sized by leg, AWS A3.0 process letters
ISO: ISO 2553 system A: arrow side on the solid line, other side on the dashed line, a or z sizes, ISO 4063 numbers

Groove welds

All groove welds

SymbolAWS A2.4ISO 2553Difference

Square groove weldAWS A2.4: Two parallel lines across the reference line, below it for an arrow side weld. ISO: The same two parallel lines, on the solid line for an arrow side weld.

V groove weldAWS A2.4: A V with its point on the reference line, below the line for an arrow side weld. ISO: The same V, on the solid line for an arrow side weld.

Bevel groove weldAWS A2.4: A vertical leg and a sloped leg meeting at the reference line. The vertical leg is always drawn on the left. ISO: The same bevel, vertical leg on the left, on the solid line for an arrow side weld.

U groove weldAWS A2.4: A U shape on a short stem from the reference line. ISO: The same U on a stem, on the solid line for an arrow side weld.

J groove weldAWS A2.4: A vertical leg with a curve, the vertical leg on the left. ISO: The same J, on the solid line for an arrow side weld.

Flare-V groove weldAWS A2.4: Two arcs facing each other, like )(. ISO: The same two arcs, on the solid line for an arrow side weld.

Flare-bevel groove weldAWS A2.4: A vertical leg and one arc, like |(. ISO: The same leg and arc, on the solid line for an arrow side weld.

Double-V groove weldSameThe same X. ISO leaves out the dashed line for symmetrical welds.

Double-bevel groove weldSameThe same K, without the dashed line because the weld is symmetrical.

V groove with broad root faceAWS A2.4: No separate symbol: a V groove with the groove depth and effective throat written to the left. ISO: A symbol of its own: a V on a short stem, like a Y.

Fillet, plug, spot and seam welds

All fillet, plug, spot and seam welds

SymbolAWS A2.4ISO 2553Difference

Fillet weldAWS A2.4: A right triangle with its vertical leg on the left, below the line for an arrow side weld. The number to the left is the leg size. ISO: The same triangle, on the solid line for an arrow side weld. The size is written with its letter: a for throat, z for leg.

Double fillet weldSameThe same pair of triangles, without the dashed line because the weld is symmetrical.

Plug or slot weldAWS A2.4: A rectangle on the reference line, below it for an arrow side weld. The arrow side member is the one with the hole. ISO: The same rectangle, on the solid line for an arrow side weld.

Spot weldSameThe same circle, centred on the line when the weld has no side.

Seam weldSameThe same symbol, centred on the line when the weld has no side.

Surfacing weldAWS A2.4: Two small arcs side by side, below the reference line. A surfacing weld is always on the arrow side. ISO: The same two arcs, on the solid line.

Back or backing weldAWS A2.4: An open half circle on the side opposite the groove symbol. A filled half circle means melt-through instead. ISO: An open half circle on the other side of the groove symbol, here on the dashed line.

Supplementary symbols

All supplementary symbols

SymbolAWS A2.4ISO 2553Difference

Weld all aroundAWS A2.4: A circle at the junction of arrow and reference line. ISO: The same circle at the junction.

Field weldAWS A2.4: A flag at the junction, pointing towards the tail. ISO: The same flag at the junction.

Backing stripAWS A2.4: A rectangle on the side opposite the groove symbol. An R inside means the backing is removed after welding. ISO: M in a rectangle for a permanent backing strip, MR for a removable one.

Flush contourAWS A2.4: A straight line beyond the weld symbol, parallel to the reference line. ISO: The same straight line.

Convex contourAWS A2.4: An arc beyond the weld symbol, bulging away from it. ISO: The same arc.

Concave contourAWS A2.4: An arc beyond the weld symbol, curving towards it. ISO: The same arc.

Parts of a welding symbol

All parts of a welding symbol

SymbolAWS A2.4ISO 2553Difference

Reference line and arrowAWS A2.4: A single reference line with the arrow at one end. Below the line is the arrow side, above is the other side. ISO: In system A a dashed identification line runs beside the solid reference line. The solid line is the arrow side, the dashed line the other side.

Other side weldAWS A2.4: A symbol above the reference line means the other side. Below means the arrow side. ISO: A symbol on the dashed line means the other side, whether the dashed line is drawn above or below.

TailAWS A2.4: AWS writes processes as letters from AWS A3.0, such as SMAW, GMAW or GTAW. ISO: ISO writes the process as its ISO 4063 number, such as 111, 135 or 141.

The differences that matter

Topic	AWS A2.4	ISO 2553 (system A)
Arrow side	Symbol below the reference line	Symbol on the solid reference line
Other side	Symbol above the reference line	Symbol on the dashed identification line
Fillet size	Leg size, no letter	a (throat) or z (leg) before the value
Intermittent welds	Length and pitch, centre to centre	Number x length (distance between welds)
Backing	Rectangle, R inside if removed	M (permanent) or MR (removable) in a rectangle
Process in the tail	AWS A3.0 letters: SMAW, GMAW, GTAW	ISO 4063 numbers: 111, 135, 141
Units	Usually inches on US drawings	Millimetres

Which one is on your drawing?

Look for a dashed line beside the reference line: if there is one, it is ISO 2553 system A. Look at the fillet sizes: a letter a or z in front means ISO. Process numbers such as 135 mean ISO; letters such as GMAW mean AWS. A single line with no letters can be AWS or ISO system B, so check the general notes.

The costly mistake is reading an ISO system A drawing with AWS habits: a symbol drawn above a solid line with the dashed line below is an arrow side weld in ISO, but would be read as the other side in AWS.

Frequently asked questions

Are AWS and ISO welding symbols the same?

The weld shapes are the same. The placement rules, the fillet size convention and the way processes are written differ.

Which side is the arrow side in ISO 2553?

In system A, the side shown by the solid reference line. A symbol on the dashed line is on the other side.

Is ISO 2553 system B the same as AWS?

It follows the AWS placement: one reference line, arrow side below. ISO size letters and process numbers can still be used.

How do I convert an ISO fillet size to an AWS leg size?

If the ISO size is z, it is already the leg. If it is a, multiply by about 1.4 for an equal-leg fillet. Always check with the design engineer.

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The symbols on this page were drawn by Deskely and are free to use. Standards are revised and every project issues its own legend sheets, which take precedence.

More referencesStandardAWS A2.4 welding symbols StandardISO 2553 welding symbols ReferenceHow to read welding symbols LegendAll welding symbols LibraryAll symbol libraries

Groove welds

Fillet, plug, spot and seam welds

Supplementary symbols

Parts of a welding symbol

The differences that matter

Which one is on your drawing?

Frequently asked questions

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