1. The two standards you'll actually encounter
ISO 2553:2019 — Welding and allied processes — Symbolic representation on drawings — Welded joints — is the fifth edition of the international standard, maintained by ISO/TC 44/SC 7. It governs symbolic representation on drawings originating in Europe, the UK, Australia, and many Asian supply chains working to EN/ISO frameworks (EN 1090, EN 15085, ISO 3834, etc.). A revision is currently in development under the working title "Symbols for welding."
AWS A2.4:2020 — Standard Symbols for Welding, Brazing, and Nondestructive Examination — is the eighth edition of the American Welding Society's standard, first published in 1976 as a consolidation of earlier AWS welding-symbol and NDT-symbol documents. It's the reference for drawings produced in North America and for many suppliers in the Gulf region and parts of Asia who work to ANSI/AWS conventions.
Neither standard defines how to make a weld correctly — that's the job of a welding procedure specification (WPS) or a code like AWS D1.1 or EN 1090. Both standards define how to communicate what weld is required, where, and to what quality, using a compact graphical language on the drawing.
2. Anatomy of a welding symbol
Every welding symbol, in both standards, is built from the same basic skeleton:
Arrow line — points to the joint. The side it touches is the "arrow side"; the opposite side of the joint is the "other side."
Reference line — the horizontal line that carries all the coded information.
Weld symbol — a small pictogram (triangle, letter-shaped groove profile, circle, etc.) placed on the reference line to identify the type of weld.
Dimensions and letters — placed to the left and right of the weld symbol to specify size, length, pitch, and number of welds.
Tail — an optional fork at the end of the reference line used for welding process codes, specifications, acceptance criteria, or other references. If there's nothing to note, the tail is simply omitted.
Supplementary symbols — flags, circles, and contour marks added for weld-all-around, field weld, melt-through, backing, and surface finish.
The rule that trips up most non-welders: the position above or below the reference line is not decorative — it is the instruction for which side of the joint gets the weld. A symbol below the line means the arrow-side member; a symbol above the line means the other-side member; symbols on both sides mean both members are welded.
3. Weld types: the symbols you'll see on 90% of drawings
Weld type | What it looks like | Typical use |
|---|---|---|
Fillet weld | A right-triangle | T-joints, lap joints, corner joints — the most common structural weld |
Square groove | A vertical line | Thin material, simple butt joints |
V groove | A "V" shape | Thicker plate, full-penetration butt joints |
Bevel groove | Half of a "V" (one straight, one angled side) | Butt or corner joints where only one member is chamfered |
U groove | A "U" shape | Thick-section full-penetration welds where a V groove would need excessive filler |
J groove | Half of a "U" | Asymmetric version of the U groove |
Flare-V / Flare-bevel | Curved lines meeting at a point | Welding of round or formed sections (e.g., bar to plate) |
Plug / slot weld | A rectangle | Filling a round or slotted hole to join overlapping plates |
Spot / projection weld | A circle | Resistance welding of overlapping sheet metal |
Seam weld | A circle with two horizontal lines | Continuous resistance welding along a seam |
Stud weld | A circle with an "X" | Fastening studs to a base plate |
Back / backing weld | A rectangle behind a groove weld symbol | A weld pass on the reverse side of a groove weld |
Edge / surfacing weld | Specific pictograms per standard | Edge preparation or hardfacing/cladding |
Both standards use essentially the same pictograms for these basic weld types — this is the part of the two systems that is genuinely harmonized and rarely causes confusion.
4. Dimensions: where the real differences begin
This is the single most consequential difference between the two standards for a buyer, because it affects how much weld metal is actually being paid for.
Fillet weld size: leg vs. throat
AWS A2.4 dimensions a fillet weld by its leg length (Z), the distance from the root of the joint to the toe of the weld — the number that goes directly on the drawing, typically in inches or fractions of an inch.
ISO 2553 dimensions a fillet weld by its throat thickness (a) — the height of the largest right triangle that can be inscribed in the weld's cross-section — expressed in millimeters. ISO drawings can alternatively call out the leg length using the letter z, but the throat convention (a) is the default and far more common in European practice.
For an equal-leg fillet weld, the two dimensions relate by a simple trigonometric factor:
leg (z) = throat (a) × 1.414
throat (a) = leg (z) × 0.707Why this matters commercially: a drawing calling for "a6" (a 6 mm throat, ISO convention) requires roughly a 8.5 mm leg — nearly 40% more weld metal, deposition time, and heat input than a fabricator would assume if they misread the "6" as a leg dimension. On high weld-volume structural or pressure-vessel jobs, this single misreading can silently inflate (or erode) both cost and structural margin.
Other coded dimensions (ISO letter system)
ISO 2553 uses a compact set of single-letter codes placed immediately before the numeric value:
Letter | Meaning |
|---|---|
| Fillet weld throat thickness |
| Fillet weld leg length (if specified instead of throat) |
| Groove weld throat/effective depth |
| Weld length |
| Pitch (center-to-center spacing) for intermittent welds |
| Number of weld elements |
| Width of a plug, slot, or seam weld |
| Diameter of a plug or spot weld |
| Permanent / removable backing strip |
AWS A2.4 conveys most of the same information positionally (left of the symbol = size; right of the symbol = length and pitch, separated by a hyphen) rather than with letter prefixes, and expresses dimensions in inches by default rather than millimeters. A buyer comparing quotes against two drawings — one from a European supplier, one from a North American one — should never assume a bare number means the same thing on both.
5. Location: arrow side, other side, and the reference-line trap
Both standards use the arrow-side / other-side / both-sides logic described above, but ISO 2553:2019 formally recognizes two different notation systems, and telling them apart at a glance is essential:
System A (dual reference line): uses a solid line plus a parallel dashed line. Symbols on the solid line indicate the arrow-side weld; symbols on the dashed line indicate the other-side weld. This is the legacy European convention (originating in ISO 2553:1992 / EN 22553) and is still common on older European drawings and in some industries.
System B (single reference line): drops the dashed line entirely and uses the same above/below-the-line logic as AWS A2.4 — a deliberate harmonization introduced to reduce exactly the kind of confusion this article is about.
AWS A2.4 has always used a single reference line with no dashed companion line: arrow-side symbols go below the line, other-side symbols go above it, full stop.
The practical tell for a buyer: if you see a dashed line running parallel to the solid reference line anywhere on a drawing, you are looking at an ISO System A drawing, and the arrow-side/other-side logic is reversed relative to what a North American fabricator would default to reading. A supplier unfamiliar with System A can build the weld on the wrong face of the joint — a defect that is often invisible until final inspection or, worse, until the part is in service.
6. Contour and finish symbols
Once the joint and size are specified, a separate set of symbols controls the final shape of the weld face, which affects fatigue performance, fit-up with mating parts, coatings, and inspection:
Symbol | Meaning |
|---|---|
Flat/flush line above the weld symbol | Weld face must be finished flat/flush with the base material |
Convex arc | Weld face left (or built up) convex |
Concave arc | Weld face ground or dressed concave |
Letter G near the contour symbol | Finish is to be achieved by grinding |
Letter M near the contour symbol | Finish is to be achieved by machining |
Letter C (AWS) | Finish is to be achieved by chipping |
These symbols look nearly identical between ISO and AWS drawings, but they are frequently the source of scope disputes rather than technical misreads: "flush" implies an additional grinding or machining operation that a fabricator may or may not have priced into a quote if the buyer only skimmed the weld-type symbol and missed the small contour mark sitting above it.
7. Supplementary symbols every buyer should recognize
Symbol | Meaning | Cost/scope implication |
|---|---|---|
Circle at the arrow/reference-line junction | Weld-all-around | Weld must run the full perimeter of the joint, not just the arrow-side face — significantly more weld length than a single symbol might suggest |
Flag at the arrow/reference-line junction | Field weld | Weld is to be made on-site, not in the shop — implies field labor, travel, and often different (lower-productivity) welding processes |
Melt-through symbol | Full penetration is required from one side, visible as reinforcement on the reverse | Relevant to sheet metal and single-sided access joints; missing it can mean an under-penetrated, rejectable weld |
Consumable insert symbol | A specific insert ring must be used at the root | Adds material cost and typically implies orbital or precision pipe welding |
Backing/spacer symbol | A backing strip or spacer bar is required, permanent (M) or removable (MR) | Adds material and, for permanent backing, can affect corrosion or fatigue performance |
8. Where ISO and AWS conventions create costly misunderstandings for buyers
Bringing the sections above together, here are the failure points that actually show up in commercial disputes and rework:
Reading a throat dimension as a leg dimension (or vice versa). The single highest-cost misreading. A 40% swing in weld leg size changes deposition volume, cycle time, and — if under-sized — structural adequacy.
Missing a dashed reference line. Signals ISO System A; arrow-side/other-side logic is not what a North American-trained fabricator will assume by default.
Assuming units. ISO drawings are metric by default (mm); AWS drawings are typically imperial (inches, fractions). A "6" with no unit callout invites a guess.
Overlooking a small contour or finish mark. A flush/grind requirement adds a labor operation that's easy to miss next to a bold weld-type symbol.
Skipping the tail. Process codes, applicable specifications, and acceptance criteria live in the tail and are often the only place NDE (nondestructive examination) requirements are called out — silence in the tail does not necessarily mean "no requirements," it may mean "see general notes" or the referenced code.
Treating "weld-all-around" as a single, localized weld. It multiplies weld length (and time/cost) around the full joint perimeter, not just the segment the arrow points to.
Confusing a field-weld flag for a shop instruction. Field welds usually carry different cost, quality-control, and schedule implications than shop welds — a mismatch here affects logistics as much as fabrication.
None of these are exotic edge cases — they are the recurring items that appear in supplier RFQ clarifications, drawing markup redlines, and post-delivery non-conformance reports.
9. A quick pre-RFQ checklist for buyers
Before a fabrication drawing goes out for quote, or before accepting a supplier's read of your drawing, it's worth confirming:
Which standard governs the drawing (ISO 2553 or AWS A2.4) — check the title block or drawing notes, don't assume based on the supplier's country alone.
If ISO 2553: is it System A (dashed line present) or System B (single line)?
For every fillet weld: is the size a throat (
a) or a leg (z)? Is it in mm or inches?Are there contour/finish symbols (flush, convex, concave) that imply an extra grinding or machining step?
Are there weld-all-around or field-weld flags that change the true scope of work?
Does the tail reference a WPS, an acceptance standard, or an NDE requirement that needs to be priced separately?
A five-minute check against this list, before a quote is issued rather than after a part fails inspection, is usually all it takes to avoid the misunderstandings above.