Welding Symbols Explained: How to Read Weld Symbols

Welding symbols can look confusing at first—lines, arrows, triangles, numbers… all packed into a small space. But once you understand the system, they become a powerful and efficient way to communicate welding requirements.

In this guide, we’ll break down welding symbols step by step, based on standards from the American Welding Society (AWS A2.4), so you can confidently read and understand them.

1. What Are Welding Symbols?

Welding symbols are standardized graphical representations used on engineering drawings to specify how a weld should be made.

They communicate essential details such as:

The type of weld required
The size and dimensions of the weld
The location of the weld on the joint
Additional instructions such as finishing or process requirements

Instead of describing these requirements in text, welding symbols provide a universal shorthand that ensures consistency across teams, industries, and regions.

2. Structure of a Welding Symbol

Every welding symbol is built from a set of core elements. If you learn the "Big Three"—the Arrow, Reference Line, and Tail—you can decipher almost any drawing.

2.1 The Reference Line (The Anchor)

The horizontal reference line is the foundation. All information regarding the weld is placed on or around this line.

2.2 The Arrow (The Direction)

The arrow connects the reference line to the joint.

A "Broken" Arrow: If you see a break in the arrow line, it points specifically to the member of the joint that must be prepared (e.g., which plate gets the bevel).

2.3 The Tail (Additional Info)

The tail is used for supplementary information that isn't a standard symbol, such as:

Welding Process (e.g., GMAW, GTAW, SMAW)
Specification or procedure references
Filler metal types

2.4 Arrow Side vs. Other Side

This is the most critical rule in the AWS standard:

Below the Reference Line: Instructions for the Arrow Side (the side the arrow touches).
Above the Reference Line: Instructions for the Other Side (the opposite side of the joint).
Both Sides: If symbols appear on both sides of the line, weld both sides.

2.5 Essential Numbers and Variables

Numbers provide the "math" for the joint's structural integrity. Here is what the letters in a standard layout represent:

S (Size): The leg length of a fillet weld or the depth of a groove preparation.
E (Effective Throat): Usually in parentheses, this is the actual depth of fusion.
L (Length): How long the weld is.
P (Pitch): The center-to-center distance for intermittent (stitch) welds.
R (Root Opening): The gap between the two pieces of metal at the base.
A (Groove Angle): The total included angle of the bevel.
N (Number): The total count of welds required (common for spot or plug welds).

3. Primary Weld Symbol Types

Each weld type has a specific symbol that represents the shape of the weld in cross-section. These symbols tell the welder how to prepare and execute the joint.

3.1 Fillet Weld — The Most Common Weld

The fillet weld is widely used in T joints, lap joints, and corner joints.

Its symbol is a triangle, representing the triangular cross-section of the weld.

The size shown to the left refers to the leg length of the weld. If both legs are equal, only one dimension is given. If unequal, both values will be specified.

If no weld length is shown, the weld is assumed to run continuously along the joint.

For intermittent welds, the length and spacing are written as a pair (for example, 2-6, 2-inch weld with a 6-inch pitch).

3.2 Groove Welds — Used for Edge-to-Edge Joints

Groove welds are used when two parts are joined edge-to-edge. The type of groove depends on how the edges are prepared.

Square Groove：Used when edges are nearly square with little or no preparation.
Root opening (if required) may be specified.
V-Groove: Both edges are beveled. The symbol includes the groove angle and root opening. If the groove does not extend through the full thickness, the depth of preparation is specified.
Bevel Groove: Only one side is prepared. The arrow points to the side that must be beveled. A break in the arrow may be used to emphasize this requirement.
U & J Grooves: These grooves have curved profiles and are typically used for thicker materials. They allow deeper penetration with less filler material but require machining.
Flare-V & Flare-Bevel: Used when welding curved surfaces such as pipes. The weld size is often given as the effective depth.

3.3 Plug, Slot, Spot, and Seam Weld Symbols

Not all welds are edge joints. Some are used to join overlapping parts.

Plug and Slot Welds

These welds fill holes in one member to join it to another.

Plug weld → round hole
Slot weld → elongated hole

The symbol indicates:

Diameter or width
Number of welds
Spacing

Spot and Seam Welds

Used mainly in sheet metal:

Spot weld → individual weld points
Seam weld → continuous weld along a line

These symbols may appear centered on the reference line if the weld side is not important.

4. How to Read Fillet Weld Symbols in Practice

Reading a fillet weld symbol involves more than just recognizing the triangle; you must interpret the specific values surrounding it to ensure structural integrity.

4.1 The Left Side: Weld Size (S)

The value to the left of the triangle indicates the weld size, which refers to the leg length of the fillet.

Leg Length: For a standard fillet, the two legs (the sides touching the plates) are usually equal. A 1/4 notation means both legs must be 1/4".
Unequal Legs: If the design requires unequal legs (e.g., one side is 1/4" and the other is 1/2"), both dimensions will be noted, such as 1/4 x 1/2.
Standard Practice: If no size is specified, the weld size typically defaults to the thickness of the thinner plate in the joint. In structural work, welders use a fillet weld gauge to verify these dimensions post-welding.

4.2 The Right Side: Length (L) and Pitch (P)

Values to the right of the triangle define the weld's "footprint" along the joint.

Continuous Welds: If the right side is blank, the weld must run the full length of the joint without stopping.
Intermittent (Stitch) Welds: These are used to prevent heat distortion or save material. They are written as two numbers separated by a dash (e.g., 3 - 6).’

3 (Length): The length of each individual weld segment is 3 inches.

6 (Pitch): The Pitch is the distance from the center of one segment to the center of the next.

Staggered Intermittent Welds: If symbols appear on both sides of the reference line but are offset (not aligned), it indicates a staggered pattern. The welds on the "Other Side" are placed in the gaps of the "Arrow Side."

5. How to Read Groove Weld Symbols Correctly

Groove weld symbols are more complex because they dictate both how the metal is cut (preparation) and how it is filled (penetration).

5.1 Joint Geometry: Root Opening and Groove Angle

The "internal" numbers of the symbol define the gap and the shape:

Root Opening (R): The number inside the symbol (e.g., 1/8) specifies the gap between the parts at the base of the joint. This ensures proper root fusion.
Groove Angle (A): The degree value (e.g., 60°) represents the total included angle between the two members. For a standard V-groove, this usually means each plate is beveled to 30°.

5.2 The Left Side: Depth of Prep (S) vs. Effective Throat (E)

This is where many beginners get confused. AWS symbols often list two separate measurements for depth:

S (Depth of Preparation): This is the depth of the bevel or chamfer cut into the metal.
(E) (Effective Throat): Usually shown in parentheses, this represents the total required thickness of the weld.
Why the difference? A weld may penetrate deeper than the bevel cut. For example, a symbol showing 1/2 (5/8) tells the welder: "Grind a 1/2-inch deep groove, but use enough heat and penetration to achieve a total weld depth of 5/8-inch."

5.3 Importance of Accuracy

Understanding the difference between S and E is critical. If a welder only fills the prepared groove without achieving the required effective throat, the joint may fail under stress due to a "lack of fusion" at the root.

6.Penetration: CJP vs PJP

Weld penetration is often categorized as:

CJP (Complete Joint Penetration) → weld fully penetrates the joint
PJP (Partial Joint Penetration) → weld does not fully penetrate

CJP welds are typically required in structural or load-bearing applications, while PJP welds are used where full strength is not necessary.

These requirements are often specified in the tail of the welding symbol or in accompanying documentation.

7.Supplementary Symbols and Additional Information

Supplementary symbols provide extra instructions that refine the weld requirements.

Common examples include:

A circle at the arrow junction → weld all around
A flag → field weld (performed on-site)
Contour symbols → flat, convex, or concave finish
Finish symbols → grinding, machining, or other processes

These symbols ensure that the final weld meets both structural and visual requirements.

Conclusion

Welding symbols are not just marks on a drawing—they define how a weld should be made and how a structure will hold together. Learning to read them accurately helps you avoid mistakes, improve consistency, and work with greater confidence. For beginners, this may feel challenging at first, but with practice, these symbols will start to make sense and become second nature. Once you reach that point, you’re not just following drawings—you’re understanding them.