Welding symbols (ISO 2553)
A welding symbol is a compact diagram on a technical drawing that tells the welder exactly which joint to make, where to make it, and how. It is built from a small set of standardised parts that can be combined to describe almost any weld.
ISO 2553 vs AWS A2.4 at a glance
Same symbol, different rule. These are the differences that change how a weld is actually made.
| Feature | ISO 2553 | AWS A2.4 |
|---|---|---|
| Reference line | Solid line + dashed identification line | Single solid line |
| Arrow side | Symbol on the solid line | Symbol below the line |
| Other side | Symbol on the dashed line | Symbol above the line |
| Fillet size | a (throat) or z (leg), letter required | Leg length, no letter |
| Units | Millimetres | Inches (fractions) or millimetres |
| Intermittent weld | n × l (e) — count, length, pitch | length–pitch after the symbol |
| Weld all around | Circle at the junction | Circle at the junction |
| Field weld | Flag at the junction | Flag at the junction |
| Tail | Process, procedure, standard | Process, specification, reference |
Go deeper
Short, focused pages on the questions that come up most often on the shop floor.
Fillet weld: a-dimension vs z-dimension
Throat or leg? Conversion table, sizing rules and the mistakes that cost the most.
Intermittent welds and pitch
What 50 (100) means, chain vs staggered, and when you must not use them.
ISO 2553 System A vs System B
The two systems inside one standard, and how to tell which a drawing uses.
ISO 2553 vs AWS A2.4
Full comparison of the European and US welding symbol conventions.
Form and position tolerances
ISO 1101 GD&T symbols explained visually, plus ISO 2768 and 13920.
Threads and tapped holes
Metric sizes, pitch, drill diameters, minimum engagement and wall thickness.
Free welding symbol chart (PDF)
One-page reference with the elementary symbols, supplementary symbols and the ISO 2553 / AWS A2.4 differences. Print it and hang it by the bench. No email required.
Anatomy of a welding symbol
Every ISO 2553 welding symbol is built around an arrow line that points to the joint and a horizontal reference line that carries the information.
Arrow line
Points from the symbol to the joint on the drawing. The direction matters — see 'arrow side' below.
Reference line (solid)
Horizontal line that carries the main weld symbol. Anything drawn on this line refers to the arrow side.
Identification line (dashed)
Drawn above or below the reference line. Carries information for the opposite side of the joint.
Weld glyph
A small shape (V, triangle, U…) placed on the reference or identification line. Tells you the type of weld.
Dimensions
Numbers around the glyph give throat thickness, leg length, root gap, included angle, length, pitch, etc.
Tail (optional)
A forked end of the reference line used to reference the welding process, standard or filler.
Arrow side vs other side
Arrow side
Glyph drawn on the solid reference line — the weld is on the side of the joint the arrow points to.
Other side
Glyph drawn on the dashed identification line — the weld is on the opposite side of the joint.
Both sides
Glyphs drawn on both lines (mirror image) — weld on both sides of the joint.
Common weld glyphs
Each glyph corresponds to a specific weld or edge preparation.
Fillet weld
Right-angle triangle. The most common weld symbol on structural drawings.
V groove
Symmetrical V. Both edges beveled, welded from one side.
Square (I) groove
Two short parallel lines. Straight edges with a small root gap.
Single bevel
Only one edge beveled — common in T-joints.
V with root face
V groove with a horizontal bar at the bottom to indicate a root face.
U groove
Curved U shape — used for heavy plate to save filler metal.
J groove
Half U — only one edge prepared with a J profile.
Flare-V
Two opposing curves for rounded edges meeting (e.g. tubes).
Edge weld
Solid rectangle on the reference line.
Plug / slot weld
Filled rectangle representing weld metal through a hole or slot.
Supplementary symbols
Weld all around
Open circle at the intersection of arrow and reference line — weld continues around the entire joint.
Field / site weld
Filled flag at the same intersection — weld is to be made on site, not in the shop.
Flush finish
Straight bar above the glyph — weld surface to be ground flush.
Convex finish
Curved bar above the glyph — weld face is convex.
Concave finish
Inverted curve — weld face is concave.
Welding symbol FAQ
What does the circle on a welding symbol mean?
A circle at the junction of the arrow and the reference line means weld all around — the weld continues around the whole perimeter of the joint. It is the same in ISO 2553 and AWS A2.4.
What does the flag on a welding symbol mean?
A flag at the junction means field weld (site weld): the weld is made on site during erection, not in the workshop.
What is the tail of a welding symbol for?
The forked tail at the far end of the reference line carries extra information — welding process (for example 135 for MAG), procedure number, filler material or a standard reference. If there is nothing to add, the tail is left out.
Which side is the arrow side?
The arrow side is the face of the joint that the arrow tip touches. The other side is the opposite face. In ISO 2553 the arrow-side symbol sits on the solid reference line; in AWS A2.4 it sits below the line.
What does the dashed line under the reference line mean?
It is the ISO 2553 identification line. Symbols placed on it describe the other side of the joint. AWS A2.4 has no dashed line — it uses the position above or below a single line instead.
What does a4 mean?
A fillet weld with a 4 mm design throat thickness. The equivalent leg length is about 5.7 mm, normally written z6.
How do I read the numbers around a weld symbol?
The size goes before the symbol, the length and pitch after it, and the angle or root gap sits above the symbol. The process or standard goes in the tail.