ISO 2553 welding symbols: chart, how to read them, and fillet weld a / z
Published October 3, 2026 — elementary and supplementary symbols, how to read the reference line, ISO 4063 process numbers and a converter between throat a and leg z.
In short: an ISO 2553 welding symbol consists of an arrow pointing at the joint, a reference line and, in system A used in Europe, a dashed line. The elementary symbol (triangle for a fillet, V for a bevel…) placed on the continuous line means a weld on the arrow side; on the dashed line, a weld on the other side. Dimensions read to the left (thickness, e.g. a5) and to the right (length) of the symbol; the tail carries the process.
1. Elementary symbols
The elementary symbol describes the joint preparation and weld shape. It is drawn on the reference line (the grey line in the table).
For a weld on both sides, the symbol is drawn on each side of the reference line, without the dashed line: two Vs make an X, two triangles a double fillet.
2. Supplementary symbols
A backing strip is shown by M in a rectangle (permanent) or MR (removable), under the joint symbol.
3. Dimensions and process
- Left of the symbol: the section dimension. For a fillet, a (throat) or z (leg), followed by the value in mm: a5, z7. For a butt weld, the penetration depth s.
- Right of the symbol: the length. Without a length, the weld runs the whole joint. An intermittent weld is written n × l (e): n segments of length l, spaced e.
- In the tail (the fork at the end of the line): the ISO 4063 process number, and possibly the quality level or the welding procedure specification (WPS) reference.
In SOLIDWORKS, welding symbols are placed on drawings (Annotations → Weld Symbol) and the dialog follows this structure: arrow side, other side, all around, field, tail. Check that the document's drafting standard is ISO (Document Properties → Drafting Standard) to get the reading described here. Hole and thread sizing for neighbouring parts is in the metric thread chart.
4. Throat a ↔ leg z converter
For an equal-leg fillet weld between two faces at 90°, z = a × √2. Enter either value: the other is calculated.
The trap: a drawing asking for "5" without a letter is ambiguous. A weld made at z5 instead of a5 only has a 3.5 mm throat, i.e. 30 % less load-bearing section. Always write the letter.
5. FAQ
What does a5 mean on a welding drawing?
A fillet weld with a 5 mm throat thickness a: the height of the largest isosceles triangle inscribed in the weld section. For a 45° weld, the matching leg length is z = a × √2 ≈ 7.1 mm.
What is the difference between a and z for a fillet weld?
a is the throat thickness (the dimension used for strength), z the leg length (what the welder checks with a gauge). For an equal-leg weld, z = a × √2 and a = z / √2 ≈ 0.707 × z. The letter in front of the value on the drawing says which one is required.
How do you know which side to weld?
In ISO 2553 system A, the reference line is paired with a dashed line. Symbol on the continuous line: arrow side. Symbol on the dashed line: other side. Symbols on both sides with no dashed line: symmetrical weld on both sides.
What does 135 in the tail of the symbol mean?
The process number from ISO 4063: 135 is MAG welding with solid wire. The most common: 111 (stick), 131 (MIG), 135 (MAG), 136 (flux-cored), 141 (TIG).
Are ISO and AWS symbols the same?
Basic symbols are very close, but side reading differs: AWS A2.4 and ISO 2553 system B put the arrow side below the reference line, with no dashed line. On a drawing from another country, check the cited standard first.