What Are Welding Symbols?
Welding symbols communicate weld requirements on engineering drawings in a compact graphical language. Instead of writing "6 mm fillet weld, 100 mm long, every 200 mm on the arrow side," the drafter places a small symbol beside the joint. Anyone — fabricator, inspector, engineer — reads the same information instantly. Welding symbols are governed by two main standards:
- AWS A2.4 (American Welding Society) — used in USA, Americas, oil/gas industry
- ISO 2553 (International) — used in Europe, Asia, many international projects
This guide focuses on AWS symbols (most common in international engineering) and highlights the key differences with ISO 2553.
How to Read a Welding Symbol — Step by Step
- Find the reference line — the horizontal line is always present; everything else attaches to it.
- Check the arrow — the arrow points to the joint. A break in the arrow indicates which member needs preparation (bevel or J-groove).
- Look at the side of the line — symbol BELOW the line = weld on the arrow side; symbol ABOVE the line = weld on the other side; symbols on BOTH sides = welds on both sides.
- Read the basic weld symbol — triangle = fillet, V = V-groove, circle = plug/slot, etc.
- Read the dimensions — size (leg length) to the LEFT of the symbol, length to the RIGHT, pitch after a hyphen.
- Check supplementary symbols — all-around circle, field weld flag, finish marks, backing bar.
- Read the tail — process (SMAW, GMAW, GTAW), WPS reference, and acceptance criteria.
The Welding Symbol Structure
A complete welding symbol has:
<-- Tail (reference/spec)
/
----||---- <-- Reference line (horizontal)
\
<-- Arrow (points to joint)
Elements that may appear:
- Reference line (always present, horizontal)
- Arrow line (points to joint location)
- Basic weld symbol (triangle for fillet, V for V-groove, etc.)
- Dimensions (size, length, pitch)
- Supplementary symbols (field weld, all-around, etc.)
- Tail (reference to specification or process)
Welding Symbol Chart (Quick Reference)
| Symbol | Name | Meaning |
|---|---|---|
| △ | Fillet weld | Triangular weld at T, lap, or corner joints |
| V | V-groove weld | Both edges beveled to form a V |
| √ | Bevel groove weld | One edge beveled; arrow points to beveled member |
| U | U-groove weld | Both edges concave |
| J | J-groove weld | One edge concave |
| □ | Square groove weld | No bevel; edges squared off |
| ○ at junction | All-around weld | Weld extends around the entire joint |
| Flag | Field weld | Weld made at site, not in shop |
| Filled circle at junction | Plug/slot weld | Weld made in a hole or slot |
| Half circle | Backing weld | Backing run on the root side |
Basic Weld Symbols
| Symbol | Shape | Weld Type |
|---|---|---|
| △ (triangle) | Right triangle | Fillet weld (most common) |
| V | V shape | V-groove weld |
| √ (bevel) | One-sided V | Bevel groove |
| U | U shape | U-groove |
| J | J shape | J-groove |
| □ (square) | Two parallel lines | Square groove |
| ○ (circle) | Circle at reference/arrow junction | Plug/slot weld |
| (two vertical bars) | ||
| ☆ (flare-V) | Flared V | Flare bevel/V |
Fillet Welds (Most Common)
Fillet welds are triangular welds at T-, lap, and corner joints. The symbol is a right triangle.
Dimensioning Fillet Welds
Size: Leg length in mm (or inches) placed to the LEFT of the triangle.
- 6mm fillet = "6" left of triangle
- Equal-leg fillets are standard; unequal legs noted as two numbers
Length: Weld length in mm placed to the RIGHT of the triangle.
- "100" means 100mm long weld
Pitch/intermittent spacing: Center-to-center spacing of intermittent welds after a hyphen.
- "6-100-200" = 6mm fillet, 100mm long, every 200mm
Examples
6△— 6mm fillet, continuous, arrow side△150— fillet weld, 150mm long (size in separate note or both sides)8△ 8▲— 8mm fillet both sides (other side is mirror)6△200-400— 6mm intermittent fillet, 200mm long every 400mm staggered or chain
Groove Welds
Groove welds fill a prepared joint between two pieces:
| Type | Symbol | Preparation |
|---|---|---|
| Square | □ | No bevel, gap only; for t ≤ 6mm |
| V-groove | V | Both sides beveled; common for pipe |
| Bevel groove | √ (slant) | One side beveled; arrow points to beveled piece |
| U-groove | U | Both sides concave (J preparation) |
| J-groove | J | One side concave |
Groove Weld Dimensions
- Depth of bevel (left of symbol): how deep the bevel is cut
- Groove weld size (in parenthesis): required throat (usually = material thickness)
- Root opening: gap between parts; shown inside symbol or note
- Groove angle: angle of bevel; shown above/below symbol
Example: 20° V ¾ means V-groove with 20° angle, ¾" groove weld size, arrow side.
Supplementary Symbols
| Symbol | Location | Meaning |
|---|---|---|
| Solid circle at junction | At arrow/reference junction | Weld all around the joint |
| Flag (triangle) at angle | At junction | Field weld (site, not shop) |
| Concave curve over symbol | Above/below weld symbol | Concave finish |
| Convex curve over symbol | Convex finish | |
| Flush/flat bar | Finished flat (machined/ground) | |
| Melt-through (filled-in back) | Complete joint penetration from other side | |
| Backing bar symbol | Rectangle behind groove | Backing strip used |
AWS vs ISO 2553 — Key Differences
| Feature | AWS A2.4 | ISO 2553 |
|---|---|---|
| Arrow side location | Symbol below reference line | Symbol on continuous side (arrow side) |
| Other side | Symbol above reference line | Symbol on dashed side |
| Dashed line | Not normally used | Dashed line identifies other side |
| Fillet symbol | Triangle | Triangle (same) |
| Groove symbols | Similar shapes | Similar shapes, minor variants |
| Weld size units | mm or inches per project | mm |
| Dimension placement | Left of symbol (size), right (length) | Same convention |
Both standards are used worldwide; many international projects specify both. Always check the drawing title block for the governing standard.
Tail Reference — Process and Specification
The tail at the end of the reference line notes:
- Welding process: SMAW (stick), GMAW (MIG), GTAW (TIG), FCAW, SAW
- Specification: e.g., "WPS-P1" = Welding Procedure Specification P1
- Acceptance criteria: e.g., "UT per AWS D1.1"
Example: 6△ / SMAW WPS-101 = 6mm fillet using SMAW per WPS 101.
Common Joint Types
| Joint | Weld Type | Typical Symbol |
|---|---|---|
| Butt joint | V- or bevel-groove CJP | V with CJP note |
| T-joint | Fillet both sides | Double fillet triangle |
| Lap joint | Fillet weld ends | Fillet each end |
| Corner joint | Fillet or partial groove | |
| Edge joint | Edge weld (parallel lines) |
Common Welding Abbreviations
| Abbreviation | Meaning |
|---|---|
| CJP | Complete Joint Penetration (full penetration groove weld) |
| PJP | Partial Joint Penetration |
| GMAW | Gas Metal Arc Welding (MIG) |
| GTAW | Gas Tungsten Arc Welding (TIG) |
| SMAW | Shielded Metal Arc Welding (stick) |
| FCAW | Flux-Cored Arc Welding |
| SAW | Submerged Arc Welding |
| WPS | Welding Procedure Specification |
| PQR | Procedure Qualification Record |
| WPQ | Welder Performance Qualification |
| NDE | Non-Destructive Examination |
| UT | Ultrasonic Testing |
| RT | Radiographic Testing |
| MT | Magnetic Particle Testing |
| PT | Penetrant Testing |
Weld Size vs Strength
Fillet weld strength depends on effective throat:
For E70 electrodes (most common for structural steel), allowable shear ≈ 140 MPa (20 ksi).
Worked Example
A 6mm fillet weld 100mm long can carry:
- Throat = 6 × 0.707 = 4.24 mm
- Shear area = 4.24 × 100 = 424 mm²
- Allowable force = 424 × 140 = 59,400 N ≈ 60 kN per 100mm
Pipe Welding Symbols
For pipe butt welds (circumferential joints):
- V-groove or U-groove symbol with "all around" circle
- Often specifies root pass (TIG hot pass), fill, cap processes
- "100% RT" means full radiographic examination required
- Typical for process piping: GTAW root + SMAW fill/cap
Pipe welding symbols appear on piping isometrics and P&IDs. For related reading, see the Flange Rating Standards guide, Pipe Material Selection guide, and Pipe Support Spacing guide.
Common Mistakes When Reading Welding Symbols
- Confusing arrow side and other side — symbol below line means arrow side, not "underneath the drawing."
- Misreading the flag — a field weld flag means site welding, not a special process.
- Ignoring the break in the arrow — the break tells you which member to bevel.
- Using the same size for unequal-leg fillets — read two numbers carefully.
- Skipping supplementary symbols — all-around or melt-through changes the joint completely.
- Overlooking the tail — the WPS and process in the tail are mandatory requirements, not notes.
- Confusing AWS and ISO conventions on projects that mix standards.
Frequently Asked Questions
What does a triangle mean on a welding symbol? A triangle on the reference line is the AWS/ISO symbol for a fillet weld — the most common weld type used at T-joints, lap joints, and corner joints. The triangle points to the joint and is placed below (arrow side) or above (other side) the reference line.
How do you read welding symbols on drawings? Read the reference line and arrow first, then the symbol position (below = arrow side, above = other side), the basic weld symbol, dimensions (size left, length right, pitch after hyphen), supplementary symbols (all-around, field weld), and the tail (process/WPS).
What is the difference between AWS A2.4 and ISO 2553? Both describe welding symbols, but ISO 2553 uses a dashed reference line to show the "other side" while AWS places the symbol above the line. AWS is common in the Americas and oil/gas; ISO is used in Europe and Asia.
What does a circle at the welding symbol junction mean? A circle at the arrow/reference-line junction means the weld is made all around the joint. A filled circle means a plug or slot weld.
What is a field weld symbol? A flag at the arrow/reference junction indicates a field weld — welding performed at the construction site rather than in the fabrication shop. This affects cost, quality control, and inspection planning.
How is weld size shown on a symbol? Weld size (fillet leg length or groove depth) is placed to the LEFT of the weld symbol. Weld length is to the RIGHT, and pitch (center-to-center spacing for intermittent welds) follows after a hyphen.
What does CJP mean in welding? CJP stands for Complete Joint Penetration — a groove weld that extends fully through the joint thickness. It is the strongest type of groove weld and is specified for critical structural and pressure-retaining joints.
Weld Size Unit Conversion
Weld dimensions are called out in millimeters on ISO drawings and inches on many US projects — always check the title block. Common conversions for welding symbols:
| Quantity | Conversion |
|---|---|
| Length | 1 mm = 0.03937 in; 1 in = 25.4 mm |
| Fillet leg size | 6 mm ≈ 1/4 in; 8 mm ≈ 5/16 in; 10 mm ≈ 3/8 in |
| Throat | throat = leg × 0.707 (equal-leg fillet) |
| Weld length | 100 mm ≈ 4 in; 150 mm ≈ 6 in |
| Pitch | 200 mm ≈ 8 in spacing |
Example: a 1/4 in fillet weld callout converts to 6.35 mm leg length; a 6 mm fillet has a throat of 6 × 0.707 = 4.24 mm (0.167 in). When a drawing mixes units, convert every dimension before fabrication.
Related Calculators
- Material Volume Calculator — weld metal volume and filler consumption estimates
- Beam Deflection Calculator — deflection checks for welded structural members
Related Guides
- Steel Material Properties — steel grades and weldability
- Bolt Torque Calculation — bolted joint strength comparison
- Flange Rating Standards — flanged joint welding requirements
- Pipe Material Selection — pipe materials and joining methods
- Pipe Support Spacing — piping system supports and welds
- Valve Types and Applications — welded valve connections
- Flange Installation Best Practices — welded flanged joints
- Steam Trap Selection — welded steam piping details
- Fan Curve Selection — welded ductwork and fan connections
- Motor Efficiency Guide — welded baseplates and drives
Summary
Welding symbols follow AWS A2.4 or ISO 2553 standards. Reference line with symbols above/below identifies weld type and which side. Fillet welds (triangle) are most common — size to the left, length to the right. Groove welds specify bevel depth, angle, and CJP/PJP. Supplementary symbols denote field weld, all-around, finish, and backing. Always consult the project welding legend and WPS for process and electrode requirements. Incorrectly read symbols lead to welds on wrong side or wrong size — double-check arrow-side/other-side convention.