Drawings and symbols
ISO 2553, AWS A2.4 and GOST 2.312 — how to read a welding symbol without guessing.
A welding symbol is a compressed instruction: which joint, which side, what shape, what size, how long, by what process. Read it in that order and it stops being cryptic.
The trouble starts when two systems meet. ISO 2553 uses a dashed identification line to say which side; AWS A2.4 uses the position of the symbol above or below a single line; GOST 2.312 uses the AWS convention for sides but names the joint from a catalogue instead of drawing it. The same picture means different things in each, and on international projects all three turn up.
One dimension is worth more attention than the rest: a5 and z5 are not the same weld, and reading one as the other produces a joint a third undersized.
- How to read welding symbols The arrow, the reference line, the symbol and the dimensions: reading an ISO 2553 weld symbol without guessing what it means.
- Welding symbol builder Assemble a weld symbol from the joint type and its dimensions and see exactly how it should look on the drawing.
- ISO 2553 vs AWS A2.4 The dashed identification line, which side of the joint is which, and the symbol shapes that mean different things in each system.
- GOST 2.312 vs ISO 2553 How the Soviet and European symbol systems differ, and how to read an old drawing without misreading the weld it asks for.
Frequently asked questions
How do I tell ISO from AWS on a drawing?
Look for the dashed identification line. A reference line with a dashed twin is ISO 2553 system A; a single line with symbols above and below it is AWS A2.4 — or ISO system B, which uses the same convention. The title block usually names the standard as well.
What is the difference between a5 and z5?
a5 is a design throat of 5 mm, which is a leg of about 7. z5 is a leg of 5 mm, which is a throat of 3.5. Reading one as the other produces a weld a third undersized or 40 % oversized — see the fillet weld calculator.
The drawing has no welding symbols. What do I do?
Ask. An unmarked joint is a gap in the documentation, not welder's discretion, and on anything covered by EN 1090 it has to be resolved before welding. A general note such as "all fillets a4 unless stated" counts as the specification.