Weld Symbols and Weld Callouts: The Basics
Reading a fillet weld symbol, sizing the weld, and what to send when a drawing has no symbols at all
A weld symbol on a drawing specifies the weld type, size, location (arrow side, other side or both) and any special instructions such as all-around or field weld, in a compact form defined by ISO 2553 or AWS A2.4. Reading and specifying them correctly avoids two common problems: over-welding, which wastes material and time and increases distortion, and under-welding, which leaves a joint weaker than the design intends.
This page covers the basic symbol structure with a worked fillet weld example, and explains what to send instead of formal symbols if your drawing does not use them, since a clear written note is a workable substitute for most fabrication orders.
The basic symbol structure
A weld symbol has a reference line, an arrow pointing to the joint, and a symbol placed above or below the reference line. A symbol below the line means weld on the arrow side (the side the arrow touches); a symbol above the line means weld on the other side. Symbols on both sides mean weld both sides. A circle at the bend in the reference line means weld all around the joint; a filled flag means a field weld, done on site rather than in the shop. The most common symbol for sheet metal and structural fabrication is the fillet weld, a right-triangle shape.
| Symbol element | Meaning |
|---|---|
| Symbol below reference line | Weld on the arrow side of the joint |
| Symbol above reference line | Weld on the other side of the joint |
| Symbols both above and below | Weld both sides |
| Circle at the bend of the reference line | Weld all around the joint |
| Filled flag at the bend | Field weld (done on site, not in the shop) |
| Number to the left of the symbol | Weld size (leg length for a fillet weld, in mm) |
Worked example: fillet weld symbol
A drawing shows a right-triangle symbol below the reference line, with the number 5 to its left, and a circle at the bend of the reference line. Read left to right: the number 5 specifies a fillet weld leg length of 5 mm; the triangle below the line means the weld is on the arrow side; the circle at the bend means weld all around the joint, not just where the arrow points. If instead the triangle appeared both above and below the line, the joint would need a 5 mm fillet weld on both sides of the joint, roughly doubling the weld volume and the heat input, which matters for distortion control on thin sheet.
Sizing a fillet weld
Fillet weld leg size is usually specified as roughly 0.7 times the thinner of the two joined plate thicknesses for a full-strength joint on mild steel, though the exact figure depends on the design load and should come from a structural calculation for load-bearing joints. A 3 mm sheet joined to a 5 mm bracket typically calls for a fillet size around 2 to 3 mm, not a size matching the thicker piece; oversizing the weld on thin sheet increases heat input, distortion and burn-through risk without adding useful strength. For non-structural brackets and enclosures, a continuous fillet at the minimum practical size (around 3 mm) is standard rather than a calculated size.
| Joined thicknesses | Typical fillet size | Note |
|---|---|---|
| 1 to 2 mm sheet | 1.5 to 2 mm | Risk of burn-through above this; consider intermittent weld |
| 3 mm sheet to 3-5 mm bracket | 2 to 3 mm | Size to the thinner member, not the thicker |
| 5 to 10 mm structural plate | 4 to 6 mm, per calculation | Structural joints need engineering sign-off |
| Non-structural enclosure, general fabrication | 3 mm continuous, standard practice | No calculation needed for light-duty joints |
What to send if your drawing has no weld symbols
Many buyers do not use formal ISO 2553 or AWS symbols and that is workable: a written note on the drawing or in the RFQ specifying the joint type (fillet, butt), the weld size or 'full strength' if no size is calculated, which sides need welding, and whether the weld should be continuous or intermittent is enough for a workshop to quote and weld correctly. What causes problems is leaving welds unspecified entirely and expecting the workshop to infer intent from the 3D model alone; a bracket with no weld note might be welded on one side only when both were needed, or oversized when a light tack weld would have sufficed.
Design tips
- Size fillet welds to roughly 0.7 times the thinner joined thickness for full-strength joints, not to match the thicker member.
- Avoid full-strength continuous welds on 1 to 2 mm sheet where possible; intermittent welding reduces heat input and distortion.
- Specify 'weld all around' explicitly with the circle symbol or a written note wherever the joint is enclosed or exposed to moisture on all sides.
- State field weld locations clearly if part of the structure is welded on site rather than in the shop; this changes access and positioning requirements.
- Write a plain-language weld note (joint type, size, sides, continuous or intermittent) if you are not using formal symbols; this is a workable substitute for most fabrication orders.
- Flag any joint carrying structural load explicitly so weld size is calculated rather than assumed at a standard practice size.
Applications
| Industry | Example parts | Typical material |
|---|---|---|
| Construction and structural steelwork | Beam-to-column connections, frames | S235JR, S355J2 |
| Machinery and OEM | Welded brackets, frames, housings | S235JR, S355J2 |
| HVAC and ducting | Duct frames, support brackets | Galvanised steel |
| Electrical enclosures | Panel frames, cabinet welds | Mild steel, stainless |
| Oil, gas and energy | Structural brackets, pipe supports | S235JR, S355J2, 316L |
| Furniture and fit-out | Welded frames, table bases | Mild steel, stainless |
Pricing and lead time
Weld cost is driven by joint length and size (a larger fillet or longer continuous run takes more time and filler material), joint accessibility (welds on the inside of an assembly or in awkward positions cost more than flat, open joints), and whether inspection is required (visual inspection is standard; dye penetrant or radiographic testing on structural joints adds cost and 2 to 3 working days). Simple welded brackets ship in 3 to 6 working days; structural assemblies with calculated joints and inspection reports typically run 6 to 10 working days.
Frequently asked questions
Yes, a written note specifying joint type, weld size (or 'full strength'), which sides need welding and whether it should be continuous or intermittent is enough; we ask a clarifying question only if the note leaves genuine ambiguity.
Upload your DXF or STEP file with weld symbols or a written weld note and receive a binding quote within 12 hours.