In this guide
How to Read Welding Symbols on Engineering Drawings
A welding symbol on a drawing is a compact instruction set: what kind of weld, where, how big, and how long — all packed into a reference line, an arrow and a few marks. Reading them fluently is one of the most practical drawing-reading skills for mechanical students, because the symbol tells you what the welder will actually do. This guide walks through the anatomy of the symbol, the basic weld types, how dimensions attach to them, and the supplementary marks that change the meaning.
The problem it solves
Drawings can't show every weld in three dimensions with a paragraph of instructions — the sheet would be unreadable. So welding information is encoded in symbols (standardized in ISO 2553 internationally and AWS A2.4 in the US, which are very close in practice). The symbol points at the exact joint, and everything the welder needs rides on it.
The most common student mistake is treating the symbol as decoration: welding the wrong side of the joint, misreading the leg size, or missing the "all-around" circle. Each of those mistakes produces a joint that looks welded but fails inspection — or worse, fails in service.
Anatomy of a welding symbol
Every welding symbol has the same skeleton:
- Arrow line — points at the joint to be welded. It may have a bent (multi-jog) leader; the first segment from the arrowhead is the "arrow line" proper.
- Reference line — the horizontal line the arrow line meets. Everything is placed relative to this line, which is why reading position matters.
- Arrow side vs other side — a weld symbol placed below the reference line means "weld the arrow side" (the side the arrow touches); placed above means "weld the other side" (the far side of the joint). Same weld on both sides? The symbol appears on both.
- Tail — the forked end of the reference line. It carries supplementary information: the welding process (MIG, TIG, SMAW), electrode class, or a note number. No tail means no special process instruction.
Reading order that never fails: find the arrow → read what's on the reference line → note the position (above/below = other side/arrow side) → check the tail.
The basic weld symbols
Weld types divide into groove welds (the joint edges are prepared/bevelled and the weld fills the groove) and non-groove welds (no edge preparation, deposited directly).
| Symbol (shape on the line) | Weld type | What it is |
|---|---|---|
| Right-triangle on the line | Fillet | Weld in a corner/T-joint, roughly triangular cross-section. The most common weld on student fabrication |
| V across the line | Single-V groove | Both edges bevelled into a V; filled with weld metal. For butt joints needing full strength |
| Half-V (single line bevelled) | Bevel groove | One edge bevelled, the other square. Used in T-joints and corners |
| U across the line | Single-U groove | Rounded groove; less filler metal than a V on thick plate |
| J (half of U) | J groove | One edge grooved, one square — single-sided version of U |
| Square (rectangle) | Square groove | No bevelling; edges simply butted and welded. Only for thin sections |
| Circle on the line | Plug or slot | Hole (plug) or elongated hole (slot) in one plate, filled to join it to the one beneath |
| Circle between the lines | Spot / seam | Resistance spot or seam weld — symbol goes in the middle of the reference line, straddling it |
Fillet and single-V cover the large majority of what a mechanical student project needs. Learn those two cold before the rest.
How dimensions attach to the symbol
Dimensions sit at fixed positions relative to the symbol, and position carries meaning:
- Fillet size (leg size): written before the triangle, e.g. a triangle preceded by "6" means a 6 mm leg fillet. This is the dimension fabricators get wrong most often.
- Groove depth / effective throat: for groove welds, the size sits before the symbol similarly and refers to the depth of penetration or effective throat, sometimes in parentheses.
- Length of weld: written after the symbol. No length given = full length of the joint.
- Intermittent welds: "50-25" style — length of each weld segment, pitch (centre-to-centre spacing) after it. E.g. 50 mm welds at 150 mm pitch.
- Staggered intermittent: the symbol appears both above and below the line, offset — welds alternate sides of the joint.
Note: A dimension inside parentheses on a groove-weld symbol is the size of the weld or the root opening per the standard in use — check whether your drawing follows ISO or AWS conventions before you quote a value. When in doubt, ask the drawing's author rather than guessing.
Supplementary symbols that change the meaning
| Mark | Meaning |
|---|---|
| Circle at the arrow/reference junction | All-around weld — the weld continues around the whole perimeter of the joint |
| Small flag at the junction | Field weld — welded on site/installation, not in the workshop |
| Symbol with a filled/unfilled triangle on the opposite side | Melt-through / backing — the root is welded through or backed |
| Contour letter (G = grind, M = machine, C = chip) | How the weld face is finished — a "G" means grind it flush |
| Groove weld with a consumable insert note | Root pass made with a pre-placed consumable ring/strip |
The all-around circle and the field-weld flag are the two you'll see constantly on structural and piping drawings. A missing all-around circle on a handrail post drawing means a debate in the workshop; a misread one means a wasted day.
Three worked decodes
Decode 1: A fillet triangle below the reference line, "6" before it, "120" after it.
→ Arrow side, 6 mm leg fillet, 120 mm long. Weld the side the arrow touches.
Decode 2: A V symbol both above and below the line, with "G" in the tail.
→ Single-V groove welds on both sides of the joint (a double-V in effect), weld face ground flush.
Decode 3: A fillet triangle below the line with a circle at the arrow junction, no length given.
→ Arrow-side fillet, welded all the way around the joint perimeter, full length of each edge.
If you can read those three patterns instantly, you can read the majority of fabrication drawings a student will encounter.
Five mistakes that fail inspection
- Welding the wrong side — the above/below = other-side/arrow-side rule inverted in the welder's head. Always confirm side before striking an arc.
- Wrong leg size — a 6 mm fillet read as "about that big." The leg dimension is a minimum throat requirement, not a suggestion.
- Missing the all-around circle — welding one straight run when the drawing wanted the full perimeter.
- Intermittent pitch misread — welding continuous when the drawing called for staggered 50-on/150-pitch, or the reverse.
- Assuming the process — no tail doesn't mean "any process is fine"; it means the shop standard applies. If the drawing specifies SMAW in the tail, don't substitute MIG without approval.
FAQ
ISO 2553 vs AWS A2.4 — which should I learn?
Learn one system properly (ISO 2553 is common in Indian and European industry; AWS A2.4 in American-influenced shops) and know that the other exists. The symbol shapes are nearly identical; differences live in tail notation and a few dimension conventions.
Can one symbol call out two different welds on the same joint?
Yes — a symbol above and below the reference line calls different welds on each side (e.g. fillet arrow-side, V-groove other-side). Multiple reference lines stacked carry combinations of groove + fillet on the same side.
What if the drawing has no welding symbols, just "WELD ALL ROUND"?
That's a note, not a symbol — it carries no size or type. Push back: ask for the weld type and size, or cite the shop's standard practice in writing. Undocumented weld sizes are a classic viva trap.
Do I need to know the welding process to read the symbol?
No — the symbol describes geometry, not process. Process lives in the tail (or the welding procedure specification, WPS). A fillet symbol is the same whether it's MIG or stick.
Where do tolerances on welds live?
Usually in the general notes or the WPS — undercut limits, reinforcement height, acceptable porosity. The symbol itself gives nominal geometry; acceptance criteria live elsewhere.
Where to go from here
- Arc vs MIG vs TIG Welding: Which Process for Your Project — pick the process the drawing's tail calls for.
- Engineering Tolerances and Fits Explained — the companion drawing-reading skill for machined features.
- Sheet Metal Bending Basics — fabrication drawings meet bend allowances.
- FEA Basics: Stress Analysis — check whether that weld actually carries the load before you size it.