Weld Symbols & Blueprint Interpretation
Read AWS weld symbols by working through the reference line, arrow and other side, sizes and lengths, supplementary marks, tails and weld notes on a print.
Table of contents
A weld symbol packs a full welding instruction into a space the size of your thumb. It says what kind of weld, how big, how long, on which side, and sometimes how to finish it and where to do it. The symbols come from AWS A2.4, the standard for welding, brazing and nondestructive examination symbols. Once you can read the parts, you can walk from a drawing to a joint and know what the engineer wants.
What You Will Be Able to Do
- Name the parts of a weld symbol.
- Tell arrow side from other side.
- Read fillet and groove sizes, lengths and pitch.
- Read field, all-around, contour and finish marks.
- Turn a full callout into plain instructions.
The Parts of a Weld Symbol
OTHER SIDE information
tail --------------------------- arrow to the joint
ARROW SIDE information
| Part | What it does |
|---|---|
| Reference line | The horizontal line that holds all the information |
| Arrow | Points to the joint |
| Weld symbol | Shows the type of weld |
| Dimensions | Give size, length and spacing |
| Supplementary marks | Show field welds, all-around welds and contour |
| Tail | Holds process, WPS or note references, and is often left off |
Information is always read left to right on the reference line, wherever the arrow sits.
Arrow Side and Other Side
Beginners trip over this rule more than any other:
- A symbol below the reference line means the weld goes on the arrow side, the side of the joint the arrow touches.
- A symbol above the line means the weld goes on the other side, opposite the arrow.
- Symbols on both sides of the line mean you weld both sides.
Picture the reference line as a fold, with the arrow side hanging below it. The rule does not change, even if the arrow points up or the drawing is turned.
On a bevel or J groove, the arrow has a break in it when only one member gets prepared. The arrow points at the member that gets the bevel. If it is not broken, either member may be prepared.
Common Weld Symbols
| Weld | How the symbol looks | Used on |
|---|---|---|
| Fillet | Right triangle, vertical leg on the left | Tee, lap and corner joints |
| Square groove | Two short parallel lines | Thin butt joints |
| V-groove | A V | Butt joints, both members beveled |
| Bevel groove | Half a V, with a vertical leg on the left | One member beveled |
| U-groove | A U | Thick butt joints |
| J-groove | Half a U, with a vertical leg on the left | One member prepared |
| Plug or slot | A small rectangle | Lap joints through a hole or slot |
| Spot or seam | A circle | Resistance welds on sheet |
The vertical leg on fillet, bevel and J symbols is always drawn on the left.
Reading a Fillet Weld
Fillet dimensions sit on the same side of the line as the symbol.
| Position | What it gives | Example |
|---|---|---|
| Left of the triangle | Leg size | 1/4 means a 1/4 inch leg |
| Right of the triangle | Weld length | 6 means 6 inches long |
| After a dash | Pitch, the center-to-center spacing | 3-6 means 3 inches long on 6 inch centers |
If no length is given, the weld runs the full length of the joint. If the legs are not equal, the sizes are given like 1/4 x 3/8, and the drawing shows which leg goes where.
Intermittent Fillets
An intermittent weld is a series of short welds with gaps, instead of one continuous weld. A callout of 1/4 with 3-6 means a 1/4 inch fillet, each weld 3 inches long, with centers 6 inches apart. That leaves 3 inches between the welds.
Worked example: A 24 inch joint is called out 1/4, 3-6. Your first weld covers 0 to 3 inches. The second covers 6 to 9, the third 12 to 15, and so on. The last weld ends at the end of the joint, so check your spacing before you start. Pitch is measured center to center, never gap to gap.
When a joint gets intermittent welds on both sides, look at how the symbols sit:
- Lined up across the reference line: a chain pattern, with welds directly across from each other.
- Offset from each other: a staggered pattern, with welds opposite the gaps.
Reading a Groove Weld
Groove symbols tell you what shape to prepare and how much weld you need.
| Position | What it gives |
|---|---|
| Left of the symbol | Depth of groove preparation |
| In parentheses after it | Groove weld size, the effective depth of the weld |
| Inside the symbol | Root opening |
| Next to the groove angle | Included angle, such as 60 degrees |
Example: 1/2 (3/8) means a groove prepared 1/2 inch deep, for a weld with an effective size of 3/8 inch.
CJP stands for complete joint penetration, and it means the weld must go all the way through the joint. A callout may simply show CJP in the tail. PJP means partial joint penetration, and the size tells you how deep. A V-groove with no size at all is often read as CJP, but check the general notes for your project.
Other marks on grooves:
- Melt-through symbol: a filled-in shape on the opposite side of the line, which calls for visible root reinforcement.
- Backing symbol: a rectangle on the opposite side of the line. An R inside it can mean the backing must be removed.
- Back gouge note: usually in the tail or a note, for gouging to clean metal before welding the second side.
Plug and Slot Welds
These join a top plate to one below it through a hole or slot. The size to the left is the hole diameter or slot width. A number inside the symbol gives the fill depth, if it is less than full. Pitch goes on the right.
Supplementary Marks
| Mark | Where it sits | Meaning |
|---|---|---|
| Open circle | At the arrow and line junction | Weld all around the joint |
| Solid flag | At the arrow and line junction | Weld in the field, not the shop |
| Flat bar | Above or below the weld symbol | Flush contour |
| Arc curving out | Above or below the weld symbol | Convex contour |
| Arc curving in | Above or below the weld symbol | Concave contour |
A finish letter next to a contour mark tells you how to get that shape: C for chipping, G for grinding, H for hammering, M for machining, R for rolling and U for unspecified. A flush mark with a G means you grind the weld flush.
The field flag matters. A part welded in the shop that should have been left for the field may not fit at erection, and it may not ship.
The Tail
The tail is the V-shaped fork at the end of the reference line. It holds what will not fit anywhere else:
- A process, such as GMAW or SMAW
- A WPS number
- A specification, such as a code
- A note, such as "See Note 4"
If a tail says to see a note, go and read the note. Do not assume what it says.
Multiple Reference Lines
A symbol can have more than one reference line stacked on the same arrow. Each line shows one operation, and you read them starting with the line nearest the arrow. That order is the sequence of work.
Nondestructive examination has its own letters on the reference line: VT, PT, MT, UT and RT. A symbol may call for a test, with or without a weld, at that location.
Weld Information Elsewhere on the Print
The symbol is only part of the story. A welder also checks:
- The general notes. Project-wide rules such as "All fillets 1/4 inch unless noted" live here, and they give you the size for joints with no specific symbol.
- The title block. It shows units, material grades, revision level and sometimes the governing code.
- Sections and details. A section view may show the joint more clearly. Find the marker on the main view, and follow it to the detail.
- TYP. This means the callout is typical for similar joints.
- Revision clouds. A cloud shows what has changed. Make sure you have the current revision.
Worked Example: A Full Callout
Here is a callout described in words. The arrow points at a stiffener-to-plate joint. A solid flag sits at the junction. Below the line is a fillet with 1/4 on its left and 6-12 on its right. The tail says GMAW, WPS-22.
Work through it left to right:
- Flag: a field weld.
- Symbol below the line: the weld is on the arrow side.
- Triangle: a fillet weld.
- 1/4: a 1/4 inch leg.
- 6-12: intermittent, each weld 6 inches long on 12 inch centers.
- Tail: use GMAW, following WPS-22.
In plain words: "In the field, using GMAW under WPS-22, run 1/4 inch fillets on the arrow side, 6 inches long on 12 inch centers."
Common Mistakes
- Welding the wrong side. Below the line means arrow side.
- Missing the field flag. The part may not fit the next step.
- Missing the all-around circle. A partial weld on an all-around joint will be rejected.
- Reading pitch as a gap. 3-6 means 3 on, with 6 inches center to center.
- Skipping the notes. The general notes can change the weld.
Check Your Understanding
A fillet symbol sits above the reference line. The arrow points to the left side of a tee joint. Which side gets the weld?
The right side, which is the other side from the arrow.
You see a callout of 5/16 with 4-10 on a joint. What does that tell you?
A 5/16 inch fillet, intermittent, with each weld 4 inches long on 10 inch centers. That leaves 6 inches between welds.
A symbol shows a circle at the junction of the arrow and the reference line, on a tube welded to a base plate. What do you do?
Weld all the way around the tube.
A flush contour bar with the letter G sits on a groove weld symbol. What does the finished weld look like?
It is ground flat, level with the base metal.
Key Takeaways
- Below the line is the arrow side. Above the line is the other side.
- Size goes to the left of the symbol, and length and pitch go to the right.
- Pitch is measured center to center.
- A solid flag means field weld, and a circle means weld all around.
- CJP means the weld goes all the way through the joint.
- Read the tail, the notes and the details before you strike an arc.