Fillet weld symbols tell a welder or fabricator where a fillet weld goes, how large it should be, how far it should extend, and whether it is continuous or intermittent. The basic rules are easy to learn, but the drawing standard matters: AWS and ISO drawings do not always show arrow-side and other-side information in the same way.
Quick Answer
Under common AWS A2.4 notation, read a fillet weld symbol from left to right: the number to the left gives the fillet-weld size, the triangle identifies a fillet weld, and dimensions to the right give weld length and, for intermittent welds, pitch. First confirm whether the drawing follows AWS or ISO conventions.
Key Takeaways
- The triangular fillet weld symbol identifies a fillet weld, commonly used at T-joints, lap joints, and corner joints.
- Under AWS notation, fillet-weld size is placed to the left of the triangle and weld length is placed to the right.
- For an AWS intermittent fillet weld, notation to the right normally gives weld-segment length followed by pitch, with pitch measured center to center.
- In the AWS system, a symbol below the reference line means arrow side; above means other side.
- Do not assume every drawing uses the AWS convention. ISO 2553 recognizes different symbol systems, so identify the governing standard first.
- Never assume an unlabeled dimension is inches. Follow the units stated on the drawing or in its general notes.
What Is a Fillet Weld Symbol?
A fillet weld symbol is the triangular basic weld symbol used to specify a fillet weld on an engineering or fabrication drawing. Fillet welds are commonly used where members form T-joints, lap joints, and corner joints.
The triangle is only one part of the complete welding symbol. The full welding symbol can include an arrow, reference line, fillet symbol, dimensions, supplementary symbols, and a tail containing additional information.
For U.S. drawings, the main reference is AWS A2.4:2020, Standard Symbols for Welding, Brazing, and Nondestructive Examination. It establishes conventions for showing weld location, dimensions, extent, and other information on drawings.
If you are still learning fabrication fundamentals, understanding symbols is just as important as developing effective welding techniques, because the symbol tells you what the drawing actually requires.
Fillet Weld Symbol Anatomy: What Each Part Means
Read the complete welding symbol in a consistent order instead of looking only at the triangle.
- Arrow: points to the joint or location being referenced.
- Reference line: carries the weld symbol and its dimensions.
- Fillet symbol: the triangular mark that identifies the required weld type.
- Size: normally shown to the left of the fillet symbol under AWS notation.
- Length: shown to the right when a specific weld length is required.
- Pitch: follows the weld length for an intermittent AWS fillet weld and describes spacing between adjacent segments.
- Supplementary symbols: can communicate requirements such as an all-around weld, field weld, or specified contour.
- Tail: may contain or reference additional information such as a welding process, specification, procedure, or other instruction.
Pro Tip: Read a welding symbol in the same sequence every time: identify the drawing standard, locate the arrow and joint, identify the weld type and side, read the size, read the length and pitch, then check supplementary symbols, the tail, and drawing notes.
How to Read Fillet Weld Size
Under AWS-style notation, the fillet weld size is placed to the left of the triangular fillet symbol. For a standard equal-leg fillet, a single dimension normally specifies the required fillet size.
For example, if a drawing using inch units places 1/4 to the left of the fillet symbol, it calls for a 1/4-inch fillet-weld size. On a metric drawing, a value such as 6 may indicate a 6 mm size. The drawing’s stated units control the interpretation.
Do not describe this dimension simply as “weld height.” Fillet-weld size is associated with the weld’s leg geometry. Theoretical or effective throat is a separate weld dimension used in design and inspection.
When the two legs are unequal, both dimensions are specified. The dimensions alone do not necessarily tell you which member receives the longer leg, so the drawing must make the required orientation clear.
Correctly understanding these weld size dimensions helps prevent a weld from being placed or sized differently from the design requirement.
Note: AWS A2.4:2020 does not require one measurement system. Never assume a bare number means inches; check the drawing’s unit designation, general notes, specifications, or project standard.
Arrow Side vs Other Side
For drawings using the AWS A2.4 convention, the position of the fillet symbol relative to the reference line identifies the side of the joint to be welded.
- Symbol below the reference line: weld on the arrow side of the joint.
- Symbol above the reference line: weld on the other side of the joint.
- Symbols above and below: weld on both sides of that joint.
The arrow side is the side of the joint to which the arrow points. The other side is the opposite face of that same joint.
This convention is particularly important on drawings containing several nearby joints. A double-fillet symbol applies to both sides of the specific joint identified by the arrow; it should not automatically be interpreted as calling for welds on every nearby face or member.
Knowing the intended joint before selecting welding processes prevents a correct-looking weld from being made in the wrong location.
AWS vs ISO Fillet Weld Symbols: Check the Standard First
The rule “below means arrow side and above means other side” should not be applied blindly to every welding drawing.
ISO 2553:2019 recognizes two approaches for identifying the arrow side and other side. System A uses a dual-reference-line approach, while System B uses a single-reference-line approach. This is one reason imported drawings or multinational projects can look different from typical AWS drawings.
As of September 2026, ISO 2553:2019 remains the published edition, while a new ISO/DIS 2553 revision is under development. AWS continues to list AWS A2.4:2020 as its current welding-symbol standard.
Warning: Identify the governing drawing standard before deciding which side of a joint the symbol specifies. If the standard, joint, or side is unclear, stop and obtain clarification from the responsible designer, engineer, inspector, or welding supervisor rather than guessing.
How to Read Weld Length and Pitch
Under AWS notation, a specific fillet weld length is placed to the right of the fillet symbol. This number describes how far that weld segment must extend along the joint.
If no length is given, do not automatically interpret the call as an unlimited weld across every connected edge. For common AWS-style fillet notation, the weld extends between specified dimension limits or between abrupt changes in the direction of welding unless the drawing gives other instructions.
For an intermittent fillet weld, the notation to the right normally includes two values separated by a hyphen:
weld length – pitch
For example, on a drawing using inch units, 2-6 means each weld segment is 2 inches long and the pitch is 6 inches. Pitch is measured from the center of one weld segment to the center of the next; it is not simply the clear unwelded gap between them.
Using the correct length and spacing is part of maintaining the required weld integrity specified by the drawing and design.
For an AWS intermittent fillet weld, think “length first, pitch second”: a call such as 2-6 specifies 2-unit weld segments at a 6-unit center-to-center pitch, using the drawing’s stated units.
Intermittent Fillet Weld Symbols
An intermittent fillet weld, sometimes called a skip or stitch weld in shop conversation, consists of separate weld segments rather than one uninterrupted bead along the entire joint.
The drawing controls both the length of each segment and the spacing. Intermittent welding should therefore never be laid out by estimating gaps visually.
- Segment length: the length of each individual welded portion.
- Pitch: center-to-center distance between adjacent segments on one side of the joint under AWS notation.
- Side: determined by the applicable symbol system and symbol placement.
- Extent: may also be controlled by dimension lines, end dimensions, or other drawing instructions.
These details matter regardless of whether the work is performed with TIG equipment or another process. Equipment considerations such as duty cycle and cooling do not change the drawing’s required segment length or pitch.
Chain vs Staggered Weld Layouts
When intermittent fillet welds are required on both sides of a joint, the drawing can specify either a chain or staggered arrangement.
Chain Intermittent Fillet Welds
In an AWS chain intermittent layout, the weld segments on opposite sides of the joint are positioned opposite one another. The dimensions for the intermittent welds are shown on both sides of the reference line as required.
Staggered Intermittent Fillet Welds
In an AWS staggered intermittent layout, the segments alternate along opposite sides of the joint instead of lining up directly across from each other. AWS represents this by offsetting the fillet symbols on opposite sides of the reference line.
ISO notation can represent intermittent welds differently, which is another reason the applicable standard must be identified before interpreting the pattern.
The designer—not the welder—determines whether a chain, staggered, or continuous weld is appropriate for the joint. Equipment features such as arc control features affect how the weld may be made, but they do not authorize changing the specified weld layout.
Other Symbols You May See With a Fillet Weld
A fillet symbol can appear with supplementary information that changes how the weld is executed. Common examples include:
- All-around symbol: indicates welding continuously around the applicable joint perimeter when properly used on the drawing.
- Field-weld symbol: indicates the weld is to be made in the field rather than at the normal shop location.
- Contour symbols: can specify a flush, convex, or concave finished contour where required.
- Finish information: may accompany a contour requirement when a specific finishing method is called for.
- Tail information: may reference a process, specification, procedure, or other requirement.
The Miller guide to AWS welding symbols provides visual examples of the reference line, side placement, fillet size, weld length, intermittent notation, field-weld symbol, all-around symbol, and tail.
Examples of Reading Fillet Weld Symbols
These examples assume an AWS-style drawing. The actual measurement unit comes from the drawing.
Example 1: Size Only
A value of 1/4 to the left of a fillet symbol below the reference line means a 1/4-unit fillet weld on the arrow side. If the drawing is dimensioned in inches, that means 1/4 inch.
Example 2: Size and Specific Length
A 1/4 size to the left and 3 to the right means a 1/4-unit fillet weld with a specified 3-unit weld length on the indicated side.
Example 3: Intermittent Fillet Weld
A size value to the left with 2-6 to the right means individual 2-unit weld segments at 6-unit center-to-center pitch.
Example 4: Both Sides
Fillet symbols above and below the AWS reference line call for fillet welds on both sides of the identified joint. Dimensions should be read on the same side of the reference line as the weld symbol they apply to.
Common Fillet Weld Symbol Mistakes to Avoid
Most interpretation errors come from reading one part of the symbol without checking the rest of the drawing.
- Assuming every drawing uses AWS notation: first identify the governing standard.
- Reversing arrow side and other side: under AWS, below is arrow side and above is other side.
- Assuming every dimension is in inches: use the drawing’s stated units.
- Reading pitch as the clear gap: AWS pitch is the center-to-center spacing between adjacent weld segments.
- Ignoring unequal-leg orientation: two leg dimensions require enough drawing information to show which leg goes where.
- Treating omitted length as unlimited: check dimension limits and abrupt changes in weld direction.
- Ignoring the tail and drawing notes: they may reference requirements not carried by the basic triangle and dimensions.
- Changing chain, staggered, or continuous welding in the shop: those are design requirements and should not be changed without authorization.
Careful interpretation helps prevent welds from being made in the wrong location, at the wrong size, or over the wrong length.
Safety: A welding-symbol guide does not replace the approved drawing, welding procedure specification, applicable fabrication code, or site safety program. Actual welding can involve arc radiation, hot metal, electrical hazards, fumes, and fire hazards. Follow applicable workplace requirements, including appropriate PPE and ventilation. See OSHA 29 CFR 1910.252 for U.S. general-industry welding requirements.
Frequently Asked Questions
How do you interpret the fillet weld symbol?
First identify the drawing standard. On an AWS-style drawing, find the arrow and reference line, determine whether the triangle is above or below the line, read the fillet size to the left, then read any weld length and pitch to the right. Finally, check supplementary symbols, the tail, dimensions, and drawing notes.
How do you measure the size of a fillet weld?
The specified symbol tells you the required weld size; inspection checks the completed weld. Fillet-weld size is associated with the leg dimensions of the weld cross-section. Depending on the inspection gauge and requirement, a gauge can be used to check leg size, throat-related dimensions, and weld profile.
How do you read weld symbols?
Start with the arrow and reference line, identify the basic weld symbol, determine the applicable side of the joint, then read dimensions such as size, length, and pitch. After that, check supplementary symbols, the tail, drawing dimensions, notes, and the governing welding-symbol standard.
How do you properly read a fillet weld gauge?
Use the instructions for the specific gauge because designs vary. Position the gauge squarely against the adjoining members and weld, then use the appropriate blade, arm, or scale for the dimension being checked. Adjustable gauges can measure fillet leg dimensions and throat-related geometry. Keep the contact surfaces clean and avoid tilting the gauge, which can create a false reading.
Does a fillet weld symbol use inches or millimeters?
Either may be used. AWS A2.4 does not require one measurement system. Read the drawing’s unit statement, general notes, specification, or project standard before interpreting a dimension.
Are AWS and ISO fillet weld symbols the same?
Not in every respect. AWS A2.4 uses the familiar single reference line where below indicates arrow side and above indicates other side. ISO 2553 recognizes System A and System B, including a dual-reference-line approach in System A. Always identify the standard used by the drawing before interpreting side placement.
Conclusion
Fillet weld symbols become much easier to read when you follow a fixed sequence. Identify the drawing standard and joint first, then read the side, fillet size, length, pitch, supplementary symbols, tail, and related drawing notes.
For AWS-style drawings, remember the core pattern: size goes to the left, specific length goes to the right, intermittent pitch follows the segment length, below the reference line means arrow side, and above means other side. Just as important, do not apply those side-placement rules automatically to an ISO drawing.
When a symbol is unclear, the correct response is to check the governing standard and project documents or obtain clarification—not to guess at the weld requirement.
Sources
- American Welding Society — AWS A2.4:2020 — primary U.S. standard for welding, brazing, and nondestructive-examination symbols.
- International Organization for Standardization — ISO 2553:2019 — international rules for symbolic representation of welded joints, including Systems A and B.
- ISO/DIS 2553, Edition 6 — 2026 draft revision showing the current development status of the next ISO 2553 edition.
- Miller Electric — How to Read and Understand Weld Symbols — practical AWS-based examples of side, fillet size, length, pitch, and supplementary symbols.
- U.S. Occupational Safety and Health Administration — 29 CFR 1910.252 — general-industry requirements covering welding, cutting, PPE, ventilation, and related safety controls.
- G.A.L. Gage Company — Adjustable Fillet Weld Gauge — manufacturer guidance for checking fillet-weld leg and throat-related dimensions.
