Fillet weld symbols appear on technical drawings and blueprints wherever two pieces of metal need to be joined at an angle. These standardized notations communicate weld size, location, and configuration without words.
The fillet weld symbol is a right triangle placed on the reference line with the perpendicular leg always positioned on the left side. This standardized symbol tells welders exactly where and how to create triangular cross-section welds that join two surfaces at approximately right angles.
- Key Feature: Right triangle symbol with perpendicular leg on left
- Used For: T-joints, lap joints, and corner joints
- Standard: AWS A2.4 Symbol Specification
I’ve seen these symbols on everything from structural steel beams to automotive frames. Understanding these symbols correctly prevents costly mistakes.
A welder I once worked with misread a 3/8-inch weld size as 1/4-inch, resulting in an entire batch of structural components being rejected. The cost of that mistake was three full days of rework and thousands of dollars in lost time.
Anatomy of a Fillet Weld Symbol
Quick Summary: Every fillet weld symbol consists of five main components: the reference line, arrow line, tail, weld symbol (triangle), and dimensions. Each element provides specific information about how the weld should be executed.
A fillet weld symbol isn’t just a triangle. It’s a complete communication system that includes several elements working together.
The Reference Line
The reference line is the horizontal foundation of the entire welding symbol. Think of it as the anchor point for all other information.
Reference Line: The horizontal line that forms the base of the welding symbol. All weld symbols, dimensions, and supplementary information are placed above, below, or on this line.
This line divides the symbol into two critical zones: the arrow side (below the line) and the other side (above the line).
The Arrow and Arrow Line
The arrow extends from the reference line and points directly to the joint that requires welding. This connection removes any ambiguity about which joint gets the weld.
In complex drawings with multiple joints, the arrow’s position is your definitive guide. I’ve seen confusion arise when welders assume rather than follow the arrow.
The Tail
The tail extends from the opposite end of the reference line and contains optional information. Not every symbol has a tail.
When present, the tail typically references welding procedures, specifications, or other notes. You might see “WPS” or specification numbers like “AWS D1.1” here.
The Fillet Weld Symbol (Triangle)
The triangular symbol represents the fillet weld itself. The perpendicular leg always appears on the left side of the triangle.
This triangle can appear above the reference line, below it, or on both sides. Its position tells you which side of the joint to weld.
Lap Joints
Corner Joints
Dimension Placement
Dimensions follow specific rules based on their position relative to the weld symbol. The size (leg length) always appears to the left of the triangle.
Length and pitch information appears to the right when specified. This consistent placement helps you read symbols quickly once you know the pattern.
Reading Fillet Weld Dimensions
Dimensions tell you the exact size and extent of the weld required. Reading them correctly is critical for meeting specifications.
Weld Size (Leg Size)
The weld size dimension appears to the left of the fillet weld symbol. This number represents the leg length of the triangular weld cross-section.
For example, if you see “3/8” to the left of the triangle, the weld legs should each measure 3/8 inch. The weld throat will be approximately 0.707 times the leg size.
3/8 inch
0.266 inch
Leg x 0.707
Weld Length
When a specific weld length is required, it appears to the right of the weld symbol. This dimension tells you the total length of the weld segment.
If no length is specified, the weld should run the full length of the joint. This default rule catches many beginners off guard.
Understanding Throat Measurements
The throat is the distance from the weld root to the weld face, measured through the weld cross-section. Two throat measurements matter.
Theoretical Throat: The distance from the root to the theoretical face of the weld, calculated as the leg size times 0.707 for equal-leg fillet welds.
Engineers use theoretical throat for strength calculations. Inspectors measure actual throat to verify the weld meets specifications.
I’ve seen projects delayed because welders focused on leg size while engineers specified throat requirements. Both measurements matter for different reasons.
Arrow Side vs Other Side Explained
This is the most common source of confusion for beginners. Getting it wrong means welding the wrong side of the joint.
Arrow Side Welds
When the fillet weld symbol appears below the reference line, the weld goes on the arrow side. This means the weld is placed on the side of the joint that the arrow points toward.
Think of it this way: the arrow points to one side of the joint material. The symbol below the line says “weld this side.”
Other Side Welds
When the fillet weld symbol appears above the reference line, the weld goes on the opposite side from the arrow. The arrow still points to the joint, but the weld goes on the other side.
This distinction matters most in T-joints and corner joints where welding on one side versus the other affects accessibility and appearance.
| Symbol Position | Weld Location | Quick Memory Trick |
|---|---|---|
| Below reference line | Arrow side | “Same side as arrow” |
| Above reference line | Other side | “Opposite from arrow” |
| Both sides | Double fillet weld | “Weld both sides” |
Double Fillet Welds
When the triangle symbol appears both above and below the reference line, you need to weld both sides of the joint. This is called a double fillet weld.
Double fillets provide symmetrical strength and are common in structural applications where balanced load distribution matters.
Intermittent Fillet Welds
Not all fillet welds run continuously. Intermittent welds consist of a series of weld segments with gaps between them.
Chain Intermittent Welding
Chain intermittent welds place segments directly opposite each other on both sides of the joint. The weld segments align in a chain-like pattern.
Both sides of the joint have welds at the same positions. This creates a symmetrical pattern that’s easy to inspect and provides balanced strength.
Staggered Intermittent Welding
Staggered intermittent welds alternate the weld segments between sides. The welds on one side are positioned in the gaps of the opposite side.
This pattern distributes stress more evenly across the joint. I’ve seen it used extensively in structural applications to reduce distortion while maintaining strength.
Pitch vs Spacing: The Critical Difference
This distinction trips up everyone at first. Pitch and spacing are NOT the same thing.
Pitch: The center-to-center distance between adjacent weld segments. Pitch includes both the weld length and the gap between welds. If pitch is 4 inches and weld length is 2 inches, the gap is 2 inches.
Spacing: The actual gap distance between weld segments. Spacing is the distance from the end of one weld to the start of the next.
Confusing these two terms leads to incorrect weld placement. A welder on a forum once admitted to scrapping a 500-part batch because they misunderstood pitch as spacing.
Remember: Pitch always equals weld length plus spacing. The dimension shown on drawings is almost always pitch, not spacing.
| Measurement | What It Measures | On Drawing |
|---|---|---|
| Weld Length | Length of each weld segment | Left of dash (e.g., 3-4) |
| Pitch | Center-to-center distance | Right of dash (e.g., 3-4) |
| Spacing | Gap between welds | Not shown directly (calculate: pitch minus weld length) |
Supplementary Weld Symbols
Additional symbols provide extra information about weld requirements. These supplementary symbols appear in combination with the basic fillet weld symbol.
All-Around Symbol
The all-around symbol is a circle that connects the arrow and reference line. It indicates the weld should be made completely around the joint.
This symbol eliminates the need for multiple weld symbols on circular or perimeter welds. One symbol with the circle covers the entire joint.
I’ve seen this on pipe connections, rectangular frames, and any application where continuous perimeter welding is required.
Field Weld Symbol
The field weld symbol is a small flag at the intersection of the reference line and arrow. It indicates the weld should be made at the installation site rather than in the fabrication shop.
This distinction matters because shop welding and field welding require different procedures, equipment, and quality control measures.
An inspector I know tells stories about expensive rework caused when welders missed field weld symbols and completed everything in the shop. The parts had to be disassembled and reassembled on site.
Contour Symbols
Contour symbols indicate the desired surface finish of the weld. These appear as lines above or below the fillet weld symbol.
- Flat contour: A straight line indicates the weld face should be approximately flush
- Convex contour: An arc curving outward indicates a raised, rounded weld face
- Concave contour: An arc curving inward indicates a recessed weld face
Convex (Raised)
Concave (Recessed)
Finish Method Symbols
Letters following the contour symbol indicate the required finishing method:
- C: Chipping
- G: Grinding
- H: Hammering
- M: Machining
- R: Rolling
These finishing methods ensure the weld meets specific surface requirements. Grinding is most common for smooth surfaces, while chipping removes slag before additional passes.
How to Read a Fillet Weld Symbol: Step-by-Step
Reading fillet weld symbols systematically prevents mistakes. Follow this process every time.
- Identify the reference line: Find the horizontal line that serves as the foundation of the symbol.
- Follow the arrow: Trace the arrow to see which joint it points to. This removes ambiguity about location.
- Check symbol position: Is the triangle below the line (arrow side), above the line (other side), or both (double weld)?
- Read the size dimension: Look to the left of the triangle for leg size. This is your primary weld size.
- Check for length and pitch: Look to the right of the triangle for intermittent weld specifications.
- Identify supplementary symbols: Look for all-around circles, field weld flags, or contour symbols.
- Check the tail: Note any specification references or procedure notes.
- Verify with the drawing: Cross-reference your interpretation with other views and notes on the drawing.
Reading Checklist
Use this checklist when reading fillet weld symbols on actual drawings:
| Check | What to Look For | Action |
|---|---|---|
| Weld side | Symbol above, below, or both | Determine which side to weld |
| Weld size | Dimension left of symbol | Set leg length dimension |
| Weld length | Dimension right of symbol | Mark weld segment length |
| Intermittent pattern | Dash notation (e.g., 3-4) | Calculate pitch and spacing |
| Supplementary symbols | Circle, flag, contour lines | Apply special requirements |
| Tail notes | Tail with text or specs | Review referenced procedures |
Common Mistakes When Reading Fillet Weld Symbols
After reviewing inspection reports and forum discussions, I’ve identified the most common mistakes welders make when interpreting fillet weld symbols.
Mistake 1: Confusing Arrow Side and Other Side
This mistake causes welders to weld the wrong side of the joint entirely. The result is often complete part rejection.
A Reddit user described making this exact error on an entire production run. The symbol showed welds on the other side, but they welded the arrow side instead.
Mistake 2: Misreading Leg Size
Reading the dimension to the right of the symbol instead of the left results in incorrect weld size. This common error compromises structural integrity.
I’ve seen 1/4-inch welds specified but 3/8-inch welds made, and vice versa. Both errors cause problems but for different reasons.
Mistake 3: Confusing Pitch and Spacing
When intermittent welds are specified, misunderstanding pitch as spacing results in incorrectly placed weld segments.
A welder on a forum shared how they calculated gaps using pitch values directly. The resulting welds didn’t meet specifications, and the entire assembly needed rework.
Mistake 4: Missing Field Weld Symbols
Overlooking the field weld flag means welding in the shop instead of at the installation site. This mistake creates expensive logistical problems.
One fabricator had to disassemble shop-welded structures and reweld them in the field because the field weld symbol was missed during initial fabrication.
Mistake 5: Ignoring Contour and Finish Symbols
Skipping contour and finish requirements leads to rejected welds during inspection. The weld might be structurally sound but fail surface finish requirements.
This matters most in applications where smooth surfaces are critical for appearance, fluid flow, or mechanical clearance.
Pro Tip: Always take time to verify your interpretation against other drawing views and notes. A few minutes of verification can prevent hours of rework.
Frequently Asked Questions
What is the symbol for fillet weld?
The fillet weld symbol is a right triangle placed on the reference line with the perpendicular leg always positioned on the left side. This triangle appears either above or below the horizontal reference line to indicate which side of the joint requires welding.
How do you read a fillet weld symbol?
Start by identifying the reference line and arrow. Check if the triangle is below the line (arrow side) or above (other side). Read the dimension to the left for weld size. Look to the right for length and pitch information. Finally, check for supplementary symbols like the all-around circle or field weld flag.
What is the difference between arrow side and other side in fillet weld symbols?
Arrow side means the weld is placed on the side of the joint that the arrow points toward, indicated by the symbol below the reference line. Other side means the weld is placed on the opposite side from the arrow, indicated by the symbol above the reference line. When the symbol appears on both sides, you need a double fillet weld.
What is pitch in intermittent fillet welds?
Pitch is the center-to-center distance between adjacent weld segments in an intermittent weld pattern. It includes both the weld length and the gap between welds. For example, a 3-4 designation means 3-inch weld segments with 4-inch pitch, resulting in 1-inch gaps between welds.
What are the parts of a fillet weld symbol?
The main components are the reference line (horizontal base), arrow line (points to the joint), tail (contains optional notes), weld symbol (the triangle), and dimensions. Each element provides specific information about weld location, size, length, and special requirements.
What does the circle symbol mean in welding?
The circle that connects the arrow and reference line is the all-around symbol. It indicates the weld should be made continuously around the entire joint. This symbol eliminates the need for multiple weld symbols when specifying perimeter welds on circular or closed joints.