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Welding Symbols: How to Read Weld Symbols on a Drawing
A welding symbol is an arrow, a horizontal reference line and a weld symbol: the shape of the symbol says the type of weld, its side of the line says which side of the joint, and the numbers beside it give the size and length. This guide is a welding symbols chart and a reading order for the weld callouts on a weldment drawing.
What are the parts of a welding symbol?
A welding symbol has up to eight elements: the arrow, the reference line, the weld symbol, its size, its length and pitch, the joint preparation, supplementary symbols and the tail. Only the arrow and the reference line are always there; the rest appear when the weld needs them.
| Element | Where it sits | What it tells you |
|---|---|---|
| Arrow | From the reference line to the joint | Which joint, and which side of it is the arrow side |
| Reference line | Horizontal, always | Everything else hangs off it |
| Weld symbol | Below, above or across the line | The type of weld and which side it goes on |
| Size | Left of the weld symbol | Fillet leg length, or groove depth and weld size |
| Length and pitch | Right of the weld symbol | How long each weld is and how far apart they are |
| Groove angle and root opening | Outside and inside the groove symbol | The joint preparation |
| Supplementary symbols | At the arrow corner or on the weld symbol | All around, field weld, contour, melt-through |
| Tail | The far end of the line | Process, procedure or a note |
The standard in the US is American Welding Society A2.4, which covers welding, brazing and inspection symbols. The international standard is ISO 2553. They share almost every shape and differ mainly in how the side of the joint is shown, covered further down.
What do arrow side and other side mean?
The arrow side is the side of the joint the arrow touches; the other side is the opposite side of the same joint. A weld symbol below the reference line is welded on the arrow side, above the line on the other side, and on both sides of the line means weld both.
- Below the line. Arrow side. A fillet triangle hanging under the line is a fillet on the side the arrow points to.
- Above the line. Other side. The same triangle on top of the line is a fillet on the far side of the joint.
- Both sides. A triangle above and below is a double fillet: one each side, often with its own size on each.
- Across the line. A spot or seam weld made through both pieces usually has no side, so its symbol sits centred on the line.
Which way the arrow comes off the line does not change anything: an arrow to the left and an arrow to the right read the same. For a bevel or J-groove the arrow does carry extra meaning. It points at the piece that gets the bevel, and when that matters the arrow is drawn with a break in it, aimed at that piece.
A symbol with several reference lines stacked on one arrow lists operations in order: the line nearest the arrow is done first. You will see this on a groove weld with a back gouge and a back weld, or on a weld that is inspected after it is made.
What do the basic weld symbols look like?
The weld symbol is a small sketch of the weld’s cross-section: a triangle for a fillet, a V for a V-groove, a rectangle for a plug or slot weld, a circle for a spot weld. Fillet, bevel and J symbols are always drawn with their upright leg on the left, whatever the joint looks like.
| Weld | Symbol | Used for |
|---|---|---|
| Fillet | A right triangle, upright leg on the left | Corner, T and lap joints; the most common weld on a weldment |
| Square groove | Two parallel lines | Butt joints in thin plate, no edge preparation |
| V-groove | A V | Butt joints with both edges bevelled |
| Bevel groove | Half a V, upright leg on the left | One edge bevelled, often a T joint into a plate |
| U-groove | A U | Thick butt joints; less filler than a V |
| J-groove | Half a U, upright leg on the left | Thick T and corner joints with one edge prepared |
| Flare-V and flare-bevel | Curved V or half V | Joints between rounded edges, such as bent plate or tube |
| Plug or slot | A rectangle | A hole or slot in one plate filled with weld to join the plate below |
| Spot or projection | A circle | Resistance welds in sheet metal |
| Seam | A circle with two lines through it | A continuous row of spot welds, often for tight joints |
| Back or backing | A half circle | A pass on the back of a groove weld |
| Surfacing | A row of small arcs | Weld metal built up on a surface rather than a joint |
On a typical steel weldment, fillets do most of the work. Groove welds turn up where two plates meet edge to edge or where a joint carries full load, and plug or slot welds where a plate laps over another and there is no edge to fillet.
How do you read weld sizes and lengths?
Size goes to the left of the weld symbol and length to the right. For a fillet, the left number is the leg length; the right numbers are the length of each weld and, after a dash, the pitch between their centres. A fillet with no length runs the whole joint.
- Continuous fillet. 1/4 to the left of the triangle and nothing to the right: a 1/4 in fillet the full length of the joint.
- Intermittent fillet. 1/4 on the left and 2-6 on the right: 1/4 in fillets, each 2 in long, spaced 6 in centre to centre.
- Chain and staggered. Intermittent fillets on both sides opposite each other are chain welds; when the two sides are offset, the symbols are drawn offset too, and the welds are staggered.
- Unequal legs. Two sizes, such as 1/4 x 3/8. The drawing shows which leg runs along which piece.
- Groove welds. The depth of preparation sits to the left, with the weld size in brackets after it. The groove angle is written outside the symbol and the root opening inside it.
- Plug and slot welds. The hole diameter or slot width goes to the left; the fill depth goes inside the symbol when the hole is not filled completely.
On metric drawings the sizes are in millimetres; on US drawings they are usually in fractions of an inch. A size that looks wrong for the plate thickness is worth a question before anything is welded, since a fillet much larger than the thinner plate is rarely intended.
What do the supplementary symbols mean?
Supplementary symbols add how and where: a circle at the corner of the arrow means weld all around, a flag means weld on site, and a line or arc over the weld symbol gives the finished contour, with a letter for how it is finished. They change the job, not the weld type.
| Mark | Meaning |
|---|---|
| Circle at the arrow corner | Weld all around the joint |
| Flag at the arrow corner | Field weld, made on site |
| Straight line over the weld symbol | Flush or flat contour |
| Outward-curving arc over the weld symbol | Convex contour |
| Inward-curving arc over the weld symbol | Concave contour |
| Letter above the contour mark | Finishing method, such as G for grinding, M for machining or C for chipping |
| Filled half circle opposite a groove weld | Melt-through: visible penetration on the far side |
A ground-flush weld on a face that is later machined or painted is common on frames and bases. The all-around circle is easy to miss on a crowded print, and it often doubles the weld length a shop expects, so it is the first thing to look for when quoting.
How are ISO 2553 welding symbols different?
ISO 2553 adds a dashed line beside the solid reference line. In its most common form, a symbol on the solid line is welded on the arrow side and one on the dashed line on the other side, wherever the dashed line is drawn. Without a dashed line, the weld is symmetrical or the side does not matter.
- Fillet size. An ISO fillet size carries a letter: a for the throat thickness or z for the leg length, so a5 and z7 are close to the same weld. A US fillet size is the leg length with no letter.
- Intermittent welds. Written as the number of welds times their length, with the spacing in brackets after it.
- Same shapes. The fillet triangle, the groove shapes, the all-around circle and the field flag all mean the same thing.
A drawing from Europe or Asia is likely to use ISO 2553; check for the dashed line before reading sides. The title block may also name the standard, along with the projection method; see first vs third angle projection for that symbol.
How do you read the welds on a weldment drawing?
Read the cut list and the views first, so you know which pieces meet at each joint. Then take each welding symbol in turn: find its joint from the arrow, read the side from the line, the type from the symbol, the size and length from the numbers, and finish with the supplementary marks and the tail.
- General notes. Many drawings set a default, such as “all fillets 3/16 unless noted”, and give the welding procedure there instead of in each tail.
- Joint. Follow the arrow to the line where two pieces meet. A broken arrow points at the piece that is bevelled.
- Side and type. Below the line is the arrow side, above it the other side; the shape gives the weld type.
- Size, length, pitch. Left of the symbol, then right of it.
- All around and field welds. The circle and the flag at the arrow corner.
- Machined after welding. Dimensions on faces that are machined after welding are on the finished part, not on the pieces.
Weld symbols sit on the sheet alongside the usual dimensions, tolerances and finish marks; the engineering drawing symbols guide covers those, the guide to assembly drawings explains the balloons and cut list, and the guide to reading engineering drawings covers the whole sheet in order.
Frequently asked questions
What is the difference between a weld symbol and a welding symbol?
A weld symbol is the small shape that names the type of weld, such as the triangle for a fillet weld. A welding symbol is the whole callout: the arrow, the reference line, the weld symbol, its sizes, any supplementary marks and the tail. The weld symbol is one part of the welding symbol.
Which side of the reference line is the arrow side?
On drawings to the American Welding Society standard A2.4, a weld symbol below the reference line is welded on the arrow side of the joint, and one above it is welded on the other side. A symbol on both sides means weld both sides. The arrow side is the side of the joint the arrow touches.
What does the circle on a welding symbol mean?
A circle where the arrow meets the reference line is the weld-all-around symbol: the weld goes all the way around the joint, such as a tube welded to a plate. A circle on the reference line itself, away from that corner, is a different symbol, the spot or projection weld.
What does the flag on a welding symbol mean?
The flag is the field weld symbol. It means the weld is made on site, during installation or erection, rather than in the shop. Shops quoting a weldment leave field welds out of their own work, because the pieces leave the shop unwelded at that joint.
How do you read the numbers on a fillet weld symbol?
The number to the left of the triangle is the weld size, the leg length of the fillet. Numbers to the right are the length of the weld and, after a dash, the pitch: the centre-to-centre spacing of intermittent welds. With no length given, the weld runs the full length of the joint.
What does the tail of a welding symbol mean?
The tail is the fork at the end of the reference line opposite the arrow. It holds anything the symbol cannot show: a welding process, a welding procedure number, a specification or a note. A welding symbol with nothing to add has no tail.
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