Weld Symbol Decoder
Build a weld symbol and read it back in plain English — including why the same symbol means opposite faces under AWS and ISO System A.
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Read it out
The same picture, both conventions
A symbol drawn in one place means opposite faces depending on which standard the drawing was made to. This is not a subtlety — it is the difference between welding the near side and the far side.
The traps
The arrow side is the lower half of the symbol. Under AWS A2.4, a symbol placed below the reference line means the arrow side — the face the arrow is touching — and one placed above means the far side. Nearly everyone guesses the opposite the first time, because "above" feels like it should mean "the side you can see".
And the same picture means the opposite thing under ISO System A. That one is genuinely dangerous. ISO 2553 System A draws two reference lines — a continuous one and a dashed identification line, conventionally below it. Arrow-side welds go on the continuous line and far-side welds on the dashed one, so the arrow-side symbol ends up on top: the exact inverse of AWS. Nothing about the symbol itself tells you which convention applies. The tell is the dashed line: if the drawing has one, it is System A and the sides are the other way round. ISO 2553:2019 added System B, which matches AWS, precisely because of this.
- Pitch is centre to centre, not the gap. "50-150" means 50 mm of weld every 150 mm, so the gap is 100. Reading the second number as a gap puts half again as much weld on the job as the drawing asked for.
- Size goes left, length goes right. The number in front of the symbol is the leg (or, on a plug weld, the hole diameter); the number behind it is the length. Groove angle goes outside the symbol and root opening inside it — two numbers that look interchangeable and are not.
- The arrow breaks toward the prepared member. On a bevel or J-groove only one plate gets ground, and the kink in the arrow line is the only thing on the drawing that says which. It is easy to miss on a busy sheet.
- Back weld and backing weld share a symbol. They differ only in sequence — after the groove it's a back weld, before it a backing weld — so the tail or the procedure has to disambiguate them.
- Contour changes the throat in both directions. Concave reduces the effective throat below the leg size, because the throat is measured to the actual face. Convex adds metal you cannot count toward capacity at all.
- Intermittent welds are for static load only. They leave unfused root gaps that are stress raisers and moisture traps, ruling them out for fatigue, for sealed joints, and for anything exposed.
How to use
- Pick the drawing standard — this changes which side the symbol means.
- Choose the weld type and which face it goes on.
- Add size, length and any supplementary symbols.
- Read the symbol back in plain English before you cut anything.
Frequently asked questions
Does a symbol above the line mean the near side or the far side?
Under AWS A2.4 it means the FAR side, which is the opposite of what most people guess. A symbol placed below the reference line means the arrow side — the face the arrow is actually touching — and one placed above means the other side. The instinct to read above as near is strong because above feels like the side you are looking at, and it is wrong every time.
Why do AWS and ISO drawings disagree about sides?
Because ISO 2553 System A uses two reference lines: a continuous one and a dashed identification line, conventionally drawn below it. Arrow-side welds go on the continuous line and far-side welds on the dashed one, which puts the arrow-side symbol on top — the exact inverse of AWS. So the same picture means the near face under one standard and the far face under the other, and nothing in the symbol itself distinguishes them.
How do I tell which standard a drawing uses?
Look for the dashed identification line. If any weld symbol on the sheet has a dashed second reference line, the drawing is ISO 2553 System A and the sides are inverted relative to AWS. If there is no dashed line anywhere, you are reading AWS or ISO System B, which agree with each other. ISO 2553:2019 added System B for exactly this reason and requires drawings to state which system applies, though plenty of older sheets do not.
What does 50-150 mean on an intermittent weld?
Fifty millimetres of weld every hundred and fifty millimetres, measured centre to centre. So the GAP between welds is a hundred millimetres, not a hundred and fifty. Reading the second number as the gap is a common mistake and it puts half again as much weld metal on the job as the drawing asked for, which costs consumable, arc time and distortion for nothing.
Which number is the size and which is the length?
Size goes to the left of the symbol and length to the right. On a fillet the size is the leg; on a plug weld it is the hole diameter, which catches people who read every leading number as a leg length. Groove welds add two more: the included angle is written outside the symbol and the root opening inside it, and those two look interchangeable on a busy drawing but are not.
What does the kink in the arrow line mean?
On a bevel or J-groove it points at the member that gets prepared. Only one plate is bevelled in those joints, and the broken arrow is the only thing on the drawing telling the fabricator which one — so a missed kink means the wrong plate gets ground. On any other weld type the arrow is drawn straight and the break carries no meaning.
What is the difference between a back weld and a backing weld?
Only the sequence, and the symbol is identical for both. A back weld is deposited after the groove weld on the opposite side; a backing weld is deposited before it, as a foundation for the root. Since the symbol cannot distinguish them, the tail or the welding procedure has to. If neither says, that is a genuine ambiguity worth resolving before anyone starts.
Does the contour symbol affect the strength?
Yes, and in both directions. A concave contour REDUCES the effective throat below what the leg size suggests, because throat is measured to the actual weld face — so dishing a fillet that was sized for strength takes some of that strength back out. A convex contour adds no usable throat at all, since capacity is measured to the theoretical flat face, making the crown weld metal you paid for and cannot count.
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