How to Read Welding Symbols on a Fabrication Drawing with AI

What is a welding symbol?

A welding symbol is a standardised notation that specifies a weld on an engineering drawing and it replaces a paragraph of instructions with one graphic that every fabricator reads the same way.

What does a welding symbol tell you?

The symbol carries everything the shop needs to make the joint.

It sets the type of weld, its size, its length and spacing, which side of the joint to weld, and any finish or process notes.

• Weld type: a fillet, a groove, a plug, or another joint, shown by the symbol on the reference line.
• Size and length: the numbers set the weld size, its length, and the spacing when it repeats.
• Which side: the position of the symbol says which side of the joint takes the weld, the arrow side or the far side.
• Process and finish: the tail and supplementary marks add the process, the contour, and any inspection note.

Where do you find welding symbols on a drawing?

Welding symbols point to the joints on a fabrication or assembly drawing.

The arrow points from the symbol to the exact joint it describes, so one drawing can carry multiple welds, each tied to its own symbol.

Which standards define welding symbols?

Two standards govern the notation.

1. In the United States it is the American Welding Society standard, AWS A2.4.
2. Internationally it is ISO 2553, used across Europe as EN ISO 2553.

The symbols overlap, but the two systems place the arrow-side weld differently. So a reader has to know which standard a drawing follows.

The anatomy of a welding symbol

Every welding symbol is built from the same parts.

Once you can name them, any symbol becomes a quick read. A clear reference on the parts is the TWI guide to reading weld symbols.

• Reference line: the horizontal line at the heart of the symbol. Everything else attaches to it.
• Arrow: the line and arrowhead that point from the reference line to the joint it marks.
• Weld symbol: the small shape on the reference line that names the type of weld, such as a triangle for a fillet.
• Dimensions: the numbers around the weld symbol that set its size, length, and spacing.
• Tail: the fork at the far end of the reference line, which holds the process or a specification note.
• Supplementary symbols: the extra marks for weld-all-around, a field weld, and the finished contour.

Arrow side and other side

The single most important rule is which side of the joint the weld goes on.

The symbol shows it by where the weld sits relative to the reference line, and this is where the two standards differ.

1. Under AWS A2.4, a weld symbol below the reference line means the arrow side, the side the arrow touches. A symbol above the line means the other side. Both above and below together means a weld on each side.
2. Under ISO 2553, the reference line appears as a solid line paired with a dashed identification line. A symbol on the solid line is the arrow side, and a symbol on the dashed line is the other side.

So the same weld can look different in the two systems, which is why the title block matters. The arrow line points to the joint, and the symbol tells you whether the arrow side or the opposite side takes the weld.

Reading the side of the joint wrong puts the weld on the wrong face. So checking the standard first is important.

The basic weld symbols by joint

A handful of basic weld symbols cover most of what a shop welds.

Each one is a small shape that stands for a joint type, placed on the reference line.

They get lined up by a weld symbol chart side by side, which is the fastest way to learn the set.

• Fillet welds: a right-angled triangle, for a weld in the corner of two overlapping or perpendicular parts.
• Groove welds: square, V, bevel, U, and J shapes, for welds that fill a prepared gap between two edges. A square groove weld suits thin plate, while deeper grooves carry complete joint penetration on thicker steel.
• Plug welds and slot welds: a rectangle, for a weld that fills a hole or a slot to join overlapping plates.
• Spot welds and seam welds: a circle or an oval, for resistance welds that join the base metal of two sheets without an edge preparation.
• Surfacing and edge: for a build-up of weld metal on a surface or along an edge, rather than a joint.

Groove welds carry their own small family of weld symbols.

A square groove weld joins a simple butt, while a single bevel groove, a single V groove, and a J groove each show the edge preparation the joint needs.

So the groove weld symbol tells the shop how to cut the plate before a single arc is struck.

The fillet is the workhorse of structural fabrication and the most common type of weld. Most steel joints are fillet welds, so most weld symbols an estimator reads are that simple triangle with a size beside it.

The welding process behind each one, a groove weld or a plug weld, changes the labour more than the shape does.

The weld joints with a symbol

A weld symbol always sits on a joint, and the joint decides how the two parts meet.

The same fillet or groove weld symbols can appear on several of them, so it helps to know the common weld joints by name.

• Butt joint: two parts in the same plane, edge to edge, usually a groove weld with a prepared edge.
• Tee joint: one part meets another at a right angle, most often a fillet weld on each side.
• Lap joint: two parts overlap, joined by a fillet, a plug, or a slot weld.
• Corner joint: two parts meet at an outside corner, welded as a fillet or a groove.
• Edge joint: two parallel edges welded along their length, common on light sheet.

So the symbol names the weld, and the joint tells you how the metal comes together.

Reading both is what turns a drawing into a clear instruction for the shop.

How to read the dimensions

The numbers around the weld symbol turn it from a type into a spec.

Their position matters as much as their value, so read the size on one side and the length on the other.

• Weld size: the number to the left of the symbol sets the size, such as the leg length of a fillet.
• Length: the number to the right of the symbol gives the length of the weld along the joint.
• Pitch: a second number after a dash gives the spacing for intermittent fillet welds, centre to centre.
• Groove detail: a groove weld symbol adds the depth of preparation and the bevel angle, and whether the weld is complete or partial joint penetration.

So a fillet weld symbol marked six to the left and fifty to the right is a six-unit fillet, fifty long. Add a dash and a second number for intermittent fillet welds that repeat at that spacing.

Reading these numbers is where a quick glance becomes an accurate takeoff.

Supplementary symbols

A few supplementary weld symbols are around the reference line and change how the weld is made or finished.

They are small, but each one carries cost and process meaning.

• Weld all around: a circle at the junction of the arrow and the reference line means the weld runs the whole way round the joint.
• Field weld: a filled flag at the same junction, the field weld symbol, means the weld is made on site and not in the shop.
• Backing weld: a symbol on the reference line opposite the arrow calls for a backing weld or a backing bar behind a groove weld, so the root fuses fully.
• Contour: a straight, convex, or concave contour symbol over the weld sets the finished weld contour, the shape of the face.
• Melt-through: a melt-through symbol calls for full fusion to the back of the joint, visible on the far face.
• Finish method: a letter with the contour, such as G for grinding or M for machining, sets how to finish that face.

So a circle and a flag together tell the shop to weld all the way round and out on site.

Each mark shifts the labour, which is why they matter to a quote as much as to the welder.

Welding symbols versus welding positions

Readers often mix the weld symbol up with welding position codes like 1F or 3G. They are different things, and a drawing can carry both.

1. The weld symbol says what weld to make and where.
2. The position code, such as 1F for a flat fillet or 1G for a flat groove, says the orientation the weld is made in. Position codes belong to the welding procedure and not to the symbol on the drawing.

So read the symbol for the joint and the size, and treat the position code as a note about how the weld is carried out.

Both affect the cost, but only the symbol defines the weld itself.

Why welding symbols matter for cost and quoting

A welding symbol is where a drawing sets much of its own price.

The joint looks the same on paper, but the type of weld, the length, and the position decide the labour and the inspection.

• Weld volume drives labour: a larger fillet or a full-penetration groove takes more passes, more wire, and more time.
• Length and pitch add up: continuous welds cost more than intermittent ones, so the spacing changes the hours.
• Field and all-around welds cost more: a site weld or a weld-all-around adds access, setup, and handling time.

So an estimator cannot price a fabrication from its outline alone.

The weld symbols change the process and the cost, which is why reading them correctly is central to an accurate quote.

Automating that reading is a natural fit for agentic AI systems for manufacturing.

How AI reads welding symbols from a drawing

Most fabrication drawings still arrive as PDFs or scans, where a weld symbol is a graphic and not yet data.

Reading each one by hand is slow and easy to miss under a bid deadline.

AI can close that gap. Computer vision locates the weld symbols on the drawing, and a semantic layer reads what each one specifies.

This is what DAC.digital AI for technical drawings and CAD file analysis does.

• Extraction: weld symbols, dimensions, and notes come off the drawing as structured data.
• Meaning: the model links each specified weld to its joint, its side, and its size, rather than just transcribing it.
• Handover: the structured welds flow into a takeoff and a quote, ready for a welding engineer to check, so the same drawing prices the same way every time.

A quick routine for reading any weld symbol

The fastest way to get fluent is to read the symbol in the same order every time. So a short routine works on almost any drawing.

• Check the standard: confirm whether the drawing follows AWS or ISO before you read the side.
• Follow the arrow: find the joint the symbol points to, so you weld the right place.
• Read the side: decide arrow side or other side from where the symbol sits on the reference line.
• Read the weld and size: name the type of weld, then take the size on the left and the length on the right.
• Check the extras: note any weld-all-around, field weld, or contour that changes the job.

Do this on a few drawings and the symbols become a quick read of what to weld and what it will cost to make.

Frequently Asked Questions

Q1: What is a welding symbol?

A1: A welding symbol is a standardised notation on an engineering drawing that specifies a weld. It sets the weld type, size, length, and side of the joint in one graphic, so every fabricator reads the same instruction the same way. It is governed by AWS A2.4 and ISO 2553.

Q2: What is the difference between the arrow side and the other side?

A2: The arrow side is the side the arrow touches and the other side is opposite. Under AWS, a symbol below the line is the arrow side, above is the other side. Under ISO 2553, the solid line is the arrow side and the dashed line the other side.

Q3: How do you read a fillet weld symbol?

A3: A fillet is a triangle. The number to the left sets the leg size, and the number to the right sets the length. A second number after a dash gives the pitch for an intermittent weld. So six and fifty means a six-unit fillet, fifty long.

Q4: What do the circle and the flag mean?

A4: A circle at the junction of the arrow and the reference line means weld all around, so the weld runs the whole way round the joint. A filled flag there means a field weld, made on site. Both add labour and cost.

Q5: What is the difference between AWS A2.4 and ISO 2553?

A5: Both define welding symbols, but show the arrow side differently. AWS uses one reference line, with the arrow-side weld below it. ISO 2553 uses a solid line paired with a dashed line, and puts the arrow-side weld on the solid line. Europe follows this as EN ISO 2553.

Q6: What are welding positions like 1F and 1G?

A6: Welding positions describe the orientation of the weld and not the symbol. 1F is a flat fillet, 1G a flat groove, while higher numbers cover horizontal, vertical, and overhead work. Position codes belong to the welding procedure, so they sit alongside the symbol and not inside it.

Q7: Where are the weld dimensions placed on the symbol?

A7: The weld size sits to the left of the symbol, and the length to the right. A pitch for an intermittent weld follows the length after a dash. Groove welds add the preparation depth and bevel angle. The position of each number matters as much as the value.