Arc welding symbols are standardized drawing marks used to communicate weld type, location, size, length, contour, and other fabrication requirements. More precisely, they are welding symbols: the notation is not limited to arc-welding processes. A drawing may use the same symbol framework for fillet, groove, plug, slot, spot, seam, edge, stud, surfacing, and other welds, with additional information identifying the process when needed.
Quick Answer
To read welding symbols correctly, first identify the governing standard, then follow the arrow to the joint, read the basic weld symbol, check which side of the reference line it occupies, and read the size, length, pitch, contour, and tail notes. Under AWS A2.4, below the reference line means arrow side and above means the other side.
Key Takeaways
- A weld symbol identifies a weld type; a complete welding symbol combines the arrow, reference line, weld symbol, dimensions, supplementary marks, and optional tail.
- Under AWS A2.4, information below the reference line applies to the arrow side; information above applies to the other side.
- Weld size is normally placed to the left of the basic weld symbol, while weld length and pitch are normally placed to the right.
- Common weld types include fillet, groove, plug, slot, spot, projection, seam, edge, stud, and surfacing welds.
- AWS and ISO systems are not visually identical, so always identify the governing standard before interpreting side significance.
- When a symbol is incomplete or ambiguous, the correct response is to obtain clarification from the drawing, specification, WPS, engineer, or other controlling document rather than guess.
What Are Arc Welding Symbols?
Arc welding symbols are often searched as if they belong only to SMAW, GMAW, GTAW, FCAW, or another arc-welding process. In practice, the standardized drawing language is broader. The AWS A2.4:2020 standard provides a system for communicating welding, brazing, and nondestructive-examination information through symbols.
The symbols allow a designer to state what weld is required and where it belongs without writing a long sentence beside every joint. Depending on the callout, the drawing may communicate weld type, side of joint, size, length, pitch, groove preparation, root opening, contour, finishing method, field welding, weld-all-around requirements, process references, or inspection information.
A spot weld symbol, for example, can identify a localized weld and carry information such as size or strength, spacing, and number of welds. Seam-weld notation has its own standardized symbol and can include size or strength, length, spacing, orientation, and contour information. Plug and slot welds have separate dimensional rules for the holes or slots used to join overlapping members.
The key point is that a symbol does not automatically tell you the welding process. A fillet weld symbol identifies the required weld geometry; the process may be stated in the tail, a drawing note, a welding procedure specification (WPS), or another project document.
Note: “Arc welding symbols” is a useful search term, but “welding symbols” is the more accurate technical term because the notation can be used for welds made by several different processes.
The Main Parts of a Welding Symbol
A welding symbol organizes fabrication information around a reference line and an arrow. The arrow connects the reference line to the joint or member being identified. Basic weld symbols and dimensions are then positioned according to the governing standard.
The main elements you may encounter are:
- Reference line: the main horizontal line that carries the weld information.
- Arrow: points from the reference line to the applicable joint or member.
- Basic weld symbol: identifies the required weld type, such as fillet, groove, plug, slot, spot, seam, or edge.
- Dimensions: specify values such as weld size, groove depth, effective throat, weld length, and pitch.
- Supplementary symbols: add requirements such as weld-all-around, field weld, contour, melt-through, backing, or consumable insert information.
- Tail: may identify a process, procedure, specification, or other reference. It can be omitted when no reference is required.
Weld Symbol vs. Welding Symbol
The terms sound similar but are not exactly the same. A weld symbol is the basic graphical mark that identifies a weld type. The welding symbol is the complete instruction built from the reference line, arrow, weld symbol, dimensions, supplementary symbols, and any required tail information.
For example, the triangle that identifies a fillet weld is the weld symbol. Once that triangle is placed on a reference line with an arrow, size, length, and other information, the entire callout is a welding symbol.
Common Welding Symbols Chart
The table below summarizes common categories. Because exact symbol geometry and placement are controlled by the governing standard, the official AWS or ISO chart should be used when drafting or interpreting production drawings.
| Weld or Symbol Type | What It Communicates | Typical Use |
|---|---|---|
| Fillet | Fillet weld at a joint | T-joints, lap joints, corner joints |
| Square groove | Square-edge groove weld | Butt joints where bevel preparation is not specified |
| V-groove | V-shaped joint preparation | Groove welding from one or both sides |
| Bevel groove | One member is beveled | Asymmetric groove preparation |
| U-groove / J-groove | Curved groove preparation | Thicker material or designs intended to reduce weld volume |
| Flare-V / flare-bevel groove | Groove created by curved member surfaces | Round bars, tubes, or formed shapes |
| Plug | Weld deposited through a round hole in one overlapping member | Joining overlapping plates or members |
| Slot | Weld deposited through an elongated slot | Overlapping members and load transfer |
| Spot / projection | Individual localized welds | Sheet-metal and production assemblies |
| Seam | Continuous or intermittent seam-type weld requirements | Sheet and overlapping joints |
| Edge | Weld along the edges of members | Edge joints and sheet fabrication |
| Stud | Stud welding requirement | Attaching studs to plate or structural members |
| Surfacing | Deposited weld metal on a surface | Build-up, hardfacing, or restoration |
| Back / backing | Backing-related or back-weld requirement | Groove-weld root control |
How to Read Arc Welding Symbols
Read a welding symbol in a fixed order instead of trying to understand every mark at once. This reduces the chance of confusing weld type, joint side, and dimensions.
- Identify the governing standard. Check the drawing title block, legend, contract, or project specification for AWS, ISO, or another system.
- Trace the arrow to the joint. The arrow tells you which joint or member the symbol applies to.
- Identify the basic weld symbol. Determine whether the drawing calls for a fillet, groove, plug, slot, spot, seam, edge, or another weld.
- Determine the required side. Under AWS A2.4, below the reference line means arrow side and above means other side.
- Read the dimensions. Check size or groove information to the left and length/pitch information to the right where applicable.
- Read supplementary symbols. Look for all-around, field-weld, contour, backing, melt-through, or other special requirements.
- Read the tail and drawing notes. Check for the welding process, WPS, specification, inspection requirement, or other controlling reference.
Warning: Do not fabricate from an ambiguous or incomplete welding symbol by assumption. When dimensions, side significance, joint preparation, process, or code requirements are unclear, obtain clarification from the responsible drawing, specification, WPS, engineer, inspector, or other authorized source.
Reference Line Basics
The reference line is the main baseline of the welding symbol. Basic weld symbols, dimensions, and supplementary information are positioned around it according to defined rules.
Under the AWS system, the location of information relative to the reference line is important. Information placed below the line applies to the arrow side of the joint. Information placed above the line applies to the other side. Symbols on both sides call for welding on both sides when the weld type allows side significance.
| AWS Placement | Meaning |
|---|---|
| Below reference line | Arrow side of the joint |
| Above reference line | Other side of the joint |
| Symbols above and below | Both sides, when applicable |
Pro Tip: Before memorizing any “above or below” rule, confirm the drawing uses AWS notation. ISO 2553 also recognizes a dual-reference-line approach, so blindly applying an AWS rule to an international drawing can put the weld on the wrong side.
Arrow And Tail Meaning
The arrow identifies the joint to which the callout applies. It does not matter whether the arrow approaches the joint from above, below, left, or right; side significance comes from the joint the arrow identifies and the symbol system being used.
For some asymmetric groove preparations, such as certain bevel- or J-groove applications, the arrow can also identify which member requires preparation. That detail should be read according to the governing standard rather than inferred from the drawing alone.
The tail is used when extra information must be referenced. Depending on the drawing and project requirements, it may contain a process abbreviation, WPS number, specification, procedure, or other note. When no such reference is required, the tail may be omitted.
| Element | Function | What to Check |
|---|---|---|
| Arrow | Identifies the applicable joint or member | Where the arrow tip lands |
| Reference line | Carries weld information | Symbol position and dimensions |
| Tail | Carries references or supplementary instructions | Process, WPS, specification, or note |
Symbol Placement Rules
Placement communicates more than location. Under AWS notation, cross-sectional dimensions such as fillet-weld size or groove information normally appear to the left of the basic weld symbol. Length and pitch information, when required, normally appear to the right.
The reference-line side controls the weld location. This is different from the left/right dimension rule, so the two should not be mixed together.
A circle at the junction of the arrow and reference line is used for a weld-all-around requirement. A field-weld symbol at that junction indicates welding is to be performed in the field rather than at the initial shop or fabrication location, subject to the governing project definition.
Fillet, Groove, and Edge Weld Symbols
Fillet, groove, and edge welds are among the most common callouts on fabrication drawings, but each carries different dimensional information.
Fillet Weld Symbol Meaning
The fillet weld symbol is triangular. It commonly appears on T-joints, lap joints, and corner joints. A fillet weld does not have to join members at an exact 90-degree angle; skewed joints can also use fillet welds when permitted by the design and applicable requirements.
For an equal-leg fillet weld, the required size is normally stated to the left of the symbol. When unequal legs are specified, the drawing must communicate the required dimensions clearly enough to identify the intended weld geometry.
When a fillet weld has a specified length, that length is shown to the right. For an intermittent fillet weld, the callout can include both segment length and pitch, with pitch referring to the center-to-center spacing of weld segments.
Groove And Edge Symbols
Groove-weld symbols communicate the required joint preparation or weld geometry. Common AWS groove categories include:
- Square groove
- V-groove
- Bevel groove
- U-groove
- J-groove
- Flare-V groove
- Flare-bevel groove
Depending on the joint, the callout can include depth of preparation, groove-weld size or effective throat, root opening, groove angle, contour, and finishing information. Complete-joint-penetration or partial-joint-penetration requirements should be interpreted from the complete design and applicable code rather than assumed from the basic groove shape alone.
An edge weld joins or builds up adjacent member edges. It has its own standardized weld symbol and dimensional rules. It should not be interpreted as “three parallel lines,” as the exact symbol geometry comes from the governing AWS or ISO standard.
Plug, Slot, Spot, and Seam Symbols
Plug, slot, spot, and seam symbols describe distinct forms of welding and require different dimensional information.
Plug Weld Symbols
A plug weld joins overlapping members through a generally round hole in one member. The callout can specify plug diameter, depth of filling, number of welds, pitch, countersink information, and contour or finish when required.
Under AWS A2.4:2020, the diameter sign is not part of the plug-weld symbol itself, although the diameter sign is still used when designating the plug dimension. This is one reason older symbol charts should not be treated as current without checking their edition.
Slot Weld Symbols
A slot weld is made through an elongated opening in one overlapping member. Its callout can include slot width, slot length, pitch, countersink angle, depth of filling, number of slots, and orientation information.
Plug and slot welds should not be confused with fillet welds made inside holes or slots. The design, preparation, and symbol on the drawing determine which weld is required.
Spot and Projection Weld Symbols
A spot weld is a localized weld at an individual point. Depending on the process and specification, a spot-weld callout can provide size or strength, spacing, number, extent, and contour information.
A projection weld uses related symbol conventions but represents a different weld configuration and process condition. The process should be confirmed from the drawing, tail, procedure, or production documentation.
Seam Weld Symbols
A seam weld is represented by its own standardized weld symbol; it is not simply “a line along the reference line.” A seam-weld callout can specify size or strength, length, intermittent spacing, number of welds, orientation, and contour.
A basic symbol identifies the weld category, but its dimensions and position are what turn that shape into a usable fabrication instruction.
Supplementary Welding Symbols
Supplementary welding symbols add information that the basic weld symbol alone cannot communicate. Common examples include contour, field weld, weld-all-around, melt-through, backing, spacer, and consumable-insert requirements.
| Supplementary Information | Meaning |
|---|---|
| Weld all around | Requires the applicable weld around the complete relevant joint perimeter identified by the symbol. |
| Field weld | Identifies welding to be performed in the field according to the project definition. |
| Contour | States the required finished weld profile, such as flush/flat, convex, or concave where applicable. |
| Finish method | When specified, indicates how the required contour is to be produced. |
| Melt-through | Indicates visible root reinforcement from complete penetration where the symbol and dimensions require it. |
| Backing | Indicates backing associated with the groove weld or joint. |
| Spacer | Indicates that a spacer is used as part of the joint configuration; it is not a generic “fill the gap” instruction. |
| Consumable insert | Indicates a specified consumable insert at the joint root when required by the joint design or procedure. |
AWS A2.4:2020 also changed an important contour rule. A contour requirement without a separately designated mechanical finishing method no longer automatically means that the contour must be achieved by welding alone. The 2020 edition also distinguishes the flat contour designation for fillet welds from the flush designation used with other weld types.
Weld Size, Length, and Placement
Dimensions around a welding symbol have fixed locations and should be read in combination with the basic weld symbol.
In the AWS system, information describing the weld cross section is generally placed on the left side of the weld symbol. Depending on weld type, that information may represent fillet size, groove depth, effective throat, plug diameter, slot width, or another cross-sectional dimension.
Information concerning weld length and pitch is generally placed to the right of the weld symbol. For an intermittent weld, pitch is the center-to-center distance between adjacent weld segments, not simply the unwelded gap.
| Information | Typical AWS Location |
|---|---|
| Fillet size / groove cross-sectional information | Left of the weld symbol |
| Weld length | Right of the weld symbol |
| Pitch for intermittent welds | Right of the symbol with the length information |
| Arrow-side information | Below the reference line |
| Other-side information | Above the reference line |
Continuous, Chain, and Staggered Intermittent Welds
A continuous weld runs for the required continuous extent. An intermittent weld is divided into separate segments. When intermittent welds occur on both sides of a joint, the drawing can specify a chain arrangement, where opposing segments line up, or a staggered arrangement, where segments on one side are offset from those on the other.
Do not treat pitch as the clear gap between weld segments. It is normally a center-to-center measurement, so confusing pitch with gap changes the amount and distribution of weld metal.
AWS, ASME, and ISO Standards
Welding notation must be interpreted using the standard named by the drawing, contract, specification, or governing code. Similar-looking symbols can have different placement rules under different systems.
AWS A2.4
For U.S.-style welding-symbol notation, the primary reference is AWS A2.4:2020, Standard Symbols for Welding, Brazing, and Nondestructive Examination. AWS also publishes welding-symbol charts that complement the standard.
One of the most important AWS rules is simple: below the reference line means arrow side; above means other side. Both-side welds are shown with applicable weld symbols on both sides of the reference line.
ISO 2553
ISO 2553:2019 is the current published international standard for symbolic representation of welded joints as of September 21, 2026. ISO recognizes two approaches to side designation: System A, based on a dual reference line, and System B, based on a single reference line.
ISO has also published ISO/DIS 2553 as a draft revision intended to replace ISO 2553:2019. Because the replacement is still under development, fabrication drawings should continue to follow the edition formally invoked by the project rather than assuming a draft has become mandatory.
ASME Codes and Welding Symbols
ASME codes govern many pressure-vessel, boiler, piping, qualification, and inspection requirements, but that does not make “ASME welding symbols” a separate universal drawing language equivalent to AWS A2.4. ASME’s welding training, for example, teaches standard AWS welding symbols while explaining how AWS, ASME, and API requirements interact.
For ASME-controlled work, the applicable construction code, Section IX qualification requirements where invoked, project drawings, specifications, and approved WPS documents must all be followed. A drawing symbol communicates the design requirement; it does not replace those controlling documents.
Note: Never convert an ISO drawing to an AWS interpretation from memory. Confirm the notation system in the title block or specification first, especially on drawings produced by international teams.
Why Clear Welding Symbols Matter
Clear welding symbols translate engineering intent into fabrication and inspection requirements. They tell the welder and inspector what joint is involved, what weld geometry is required, where the weld belongs, and which dimensions or additional instructions apply.
Good notation helps prevent several costly errors:
- Welding the correct joint on the wrong side.
- Using the wrong weld size or effective throat.
- Confusing weld length with pitch.
- Missing a weld-all-around or field-weld requirement.
- Preparing the wrong member for a bevel or J-groove.
- Using an incorrect process or WPS because the tail or drawing notes were ignored.
- Applying AWS side-placement rules to an ISO drawing.
- Assuming an unspecified contour, inspection method, or penetration requirement.
Standard notation also helps designers, fabricators, inspectors, and quality personnel discuss the same joint without relying on verbal descriptions. The symbol should provide enough information to communicate design intent, while the governing code, specification, WPS, and inspection documents supply any additional production and acceptance requirements.
Common Welding Symbol Reading Mistakes
Reversing Arrow Side and Other Side
Under AWS A2.4, the arrow-side information is below the reference line and other-side information is above it. The physical direction in which the arrow points on the sheet does not reverse this rule.
Reading the Wrong Standard
An AWS-trained reader can misinterpret an ISO System A drawing if the dashed/continuous reference-line system is ignored. Establish the governing notation before reading individual symbols.
Confusing Weld Size, Length, and Pitch
Cross-sectional dimensions and length dimensions occupy different positions. Intermittent-weld pitch is center-to-center spacing, not merely the empty distance between segments.
Ignoring the Tail
The tail can reference a WPS, process, specification, or other important requirement. A correct basic weld symbol can still be fabricated incorrectly if the referenced procedure is ignored.
Assuming a Missing Requirement
A basic weld symbol does not authorize the reader to invent a weld length, contour, inspection criterion, process, or penetration requirement. Use the full drawing package and obtain clarification when necessary.
Frequently Asked Questions
How Do Weld Symbols Differ on Metric and Imperial Drawings?
The basic symbol shapes may be similar, but dimensions are expressed in the units established by the drawing. Do not convert values unless the project requires it. More importantly, confirm the governing welding-symbol standard because differences between AWS and ISO side-designation systems matter more than simply whether the drawing uses inches or millimeters.
What Software Helps Create Welding Symbols Accurately?
Major CAD platforms such as AutoCAD and SOLIDWORKS include tools or workflows for adding welding symbols. The software does not replace standards knowledge: the drafter still needs to select the correct AWS, ISO, or project-specific convention and enter the correct size, side, length, pitch, contour, and reference information.
How Are Welding Symbols Used in Robotic Welding Programs?
Welding symbols communicate the engineering requirement to manufacturing personnel and digital workflows, but they are not complete robot programs. CAD/CAM data, fixture geometry, robot programming, WPS parameters, torch orientation, travel speed, sensing, collision checks, and other production data are used to turn the design requirement into an executable robotic weld path.
Can Welding Symbols Specify Inspection or Testing Requirements?
Yes. AWS A2.4 includes provisions for nondestructive-examination information, including examination method, frequency, and extent. Detailed acceptance criteria may still be controlled by the applicable code, specification, inspection plan, procedure, or drawing notes.
What Mistakes Cause Welding Symbol Misinterpretation on Blueprints?
Common mistakes include reversing arrow side and other side, using the wrong symbol standard, confusing size with length or pitch, ignoring the tail, overlooking supplementary symbols, misreading groove preparation, and assuming missing requirements. Checking the drawing legend and reading each symbol in a consistent order prevents many of these errors.
What Is the Difference Between a Weld Symbol and a Welding Symbol?
A weld symbol is the graphical mark for a particular weld type, such as the triangle used for a fillet weld. A welding symbol is the complete drawing instruction that combines the reference line, arrow, weld symbol, dimensions, supplementary symbols, and any tail information.
Does a Symbol Below the Reference Line Always Mean Arrow Side?
That is the AWS A2.4 convention: below means arrow side and above means other side. Do not apply the rule automatically to every international drawing, because ISO 2553 also permits a dual-reference-line system with different visual logic.
Which Welding Symbol Standard Should I Use?
Use the standard and edition specified by the drawing, contract, code, customer requirement, or project specification. AWS A2.4:2020 is the current published AWS symbol standard, while ISO 2553:2019 remains the current published ISO standard as of September 21, 2026.
Conclusion
Arc welding symbols are best understood as part of the broader standardized language of welding symbols. They communicate weld type, joint side, size, length, pitch, preparation, contour, and other requirements without forcing the designer to describe every weld in words.
For AWS drawings, remember the central placement rule: below the reference line is the arrow side and above is the other side. Then read the symbol systematically—joint, weld type, side, size, length or pitch, supplementary marks, and tail. On ISO or international drawings, confirm the governing system before applying that rule.
Accurate symbol reading improves fabrication and inspection because everyone works from the same design information. When the symbol, drawing note, code, or WPS is unclear, obtain clarification instead of filling in missing requirements by assumption.
Sources
- American Welding Society — AWS A2.4:2020 — current AWS standard for welding, brazing, and nondestructive-examination symbols.
- American Welding Society — Arrow-Side and Other-Side Symbol Guidance — confirms AWS placement rules for information below and above the reference line.
- American Welding Society — Symbols for Joining and Inspection — overview of welding-symbol components and common basic weld types.
- International Organization for Standardization — ISO 2553:2019 — current published international standard for symbolic representation of welded joints.
- International Organization for Standardization — ISO/DIS 2553 — draft revision under development to replace ISO 2553:2019.
- ASME — Applications of Welding — explains AWS welding symbols in the context of ASME, AWS, and API codes and standards.
