Welding symbols on engineering drawings: a practical reading guide

That difference matters. The same basic mark can change meaning once you consider whether it is above or below the reference line, what numbers are placed around it, and whether the tail contains a process, specification or classification note .

The main parts of a welding symbol

First, check which standard applies

Before decoding the arrow and symbols, look at the title block, general notes or project documents for the applicable system, such as AWS, ISO or BS EN. AWS A2.4 covers symbols for welding, brazing and nondestructive examination . TWI notes that welding-symbol standards do not present all details in the same way, and that ISO 2553 and BS EN 22553 have display features that must be read within their own conventions .

In practice, a wall chart is useful only after you know which standard the drawing is using.

Arrow side and other side

Under the common AWS arrangement, the position of the weld symbol relative to the reference line tells you which side of the joint receives the weld :

• Symbol below the reference line: weld on the arrow side.
• Symbol above the reference line: weld on the other side of the joint.
• Symbols on both sides of the reference line: welds are generally required on both sides .

This is one of the fastest ways to orient yourself on a drawing, but it is not a substitute for checking the applicable standard. ISO and BS EN presentations can differ, so the project standard remains part of the interpretation .

How to read the numbers around the symbol

A common mistake is to treat every number beside a weld symbol as the weld size. Numbers around a welding symbol are positional information. Depending on the type of weld and where the number appears, it may refer to weld length, pitch or centre-to-centre spacing, groove angle, root opening, depth of filling, or the number of plug, slot, spot, seam, stud or projection welds .

For butt joints and butt welds, TWI explains that the letter S with a number to the left of the symbol refers to depth of penetration. TWI also notes that when a drawing using weld symbols gives no specific dimensional requirements for a butt weld, the usual assumption is a full-penetration butt weld .

Common welding symbols you will meet

1. Fillet welds

A fillet weld is shown with a right-triangle symbol on the reference line. The National Board notes that the perpendicular leg of the fillet-weld symbol is always on the left . After you determine the side from the symbol position, read the adjacent dimensions for the required size, length or pitch if those dimensions are shown .

2. Groove welds

Groove weld symbols are commonly used with butt joints. The joint may or may not require edge preparation before welding. If the weld is applied from one side only, it is a single groove weld; if it is applied from both sides, it is a double groove weld . In some groove-weld symbols, a break in the arrow helps identify which side of the joint must be prepared .

3. Plug and slot welds

Plug and slot weld symbols appear when the drawing calls for weld metal to fill a hole or slot to connect parts. Their dimensions can indicate information such as number of welds, countersink angle, depth of filling or centre-to-centre spacing, depending on the case .

4. Spot, seam, stud and projection welds

Welding-symbol charts are not limited to fillet and groove welds. Basic symbols also include spot welds, seam welds, stud welds and projection welds, which appear when those weld forms or processes are used on the drawing .

5. Back, backing, edge and other advanced symbols

More detailed drawings may include symbols for back welds, backing welds or edge welds. The tail can also carry mandatory information, such as a required class number in some applications, so it should not be treated as a casual note .

6. Nondestructive examination symbols

Some drawings include nondestructive examination requirements. AWS A2.4 is not limited to weld symbols; it also addresses brazing and nondestructive examination symbols . When these appear on a drawing, read them as part of the inspection and acceptance instructions, not as decoration.

A step-by-step method for reading any welding symbol

1. Identify the standard first. Look for AWS, ISO, BS EN or a project-specific standard, because the presentation can change how the symbol is read .
2. Follow the arrow to the joint. The arrow links the symbol to the joint or member it applies to .
3. Decide which side is being called out. In the common AWS layout, below the reference line means arrow side, above means other side, and symbols on both sides indicate both sides .
4. Identify the basic weld symbol. Determine whether the callout is for a fillet, groove, plug, slot, spot, seam, stud, projection or another weld type .
5. Interpret dimensions in context. Read each number according to its position and weld type; it may not be a weld size .
6. Read the tail. If a tail is present, it may specify a welding process, specification, classification or other reference that must be followed .
7. Check supplementary symbols. Field-weld, weld-all-around, contour, finish and nondestructive examination symbols can add important execution or inspection requirements .

Quick examples

Fillet symbol below the reference line: In the common AWS convention, this calls for a fillet weld on the arrow side. Any dimensions around the symbol then define details such as size, length or pitch if they are provided .

V-groove symbol above the reference line: If the drawing uses the AWS arrangement, the weld is required on the other side of the joint, not the arrow side .

Letter S with a number beside a butt-weld symbol: TWI explains that S with a number to the left of a butt-weld symbol indicates depth of penetration .

Information in the tail: A process, specification, classification or class number in the tail is part of the welding symbol and must be read before work starts .

Mistakes to avoid

• Using a generic chart without checking the standard. AWS, ISO and BS EN drawings can use different presentations, so the drawing standard matters .
• Reading every number as weld size. A number may indicate length, pitch, groove angle, root opening, depth of filling or depth of penetration .
• Ignoring above-versus-below placement. In the common AWS convention, symbol position relative to the reference line identifies the weld side .
• Skipping the tail. The tail may contain a required process, specification, classification or other reference .
• Missing inspection symbols. Nondestructive examination symbols, when present, are part of the drawing requirements .

Bottom line

Use a fixed reading order: standard, arrow, side, weld type, dimensions, tail and supplementary symbols. The above-or-below reference-line rule is very useful on AWS-style drawings, but it is not enough by itself. When there is ambiguity, the named standard and the project drawing requirements are the context that keeps the weld symbol from becoming guesswork .