Welding Symbols Explained — How to Read AWS A2.4 Weld Symbols on Drawings

Reading Weld Symbols

Weld symbols on engineering drawings tell the welder exactly what joint to make, where to put it, and how big it should be — without writing paragraphs of notes. The system is defined by AWS A2.4 and it’s more logical than it looks once you understand the structure.

The Weld Symbol Structure

Every weld symbol has the same basic anatomy:

Reference line — the horizontal line that everything hangs off of
Arrow — points to the joint being welded
Weld type symbol — the shape on/below the reference line
Dimensions — size, length, pitch of the weld
Tail — optional, contains process/specification notes

Arrow Side vs. Other Side

This is the #1 source of confusion.

Symbol below the reference line = weld on the arrow side (the side the arrow points to)
Symbol above the reference line = weld on the other side (opposite the arrow)
Symbols on both sides = weld both sides

Common Weld Types

Weld Type	Symbol Shape	Description	Common Use
Fillet	Triangle (△)	Triangular cross-section weld at a corner or lap joint	Most common weld — ~80% of all welds
Groove — V	V shape	V-shaped preparation for full penetration	Butt joints requiring full strength
Groove — Bevel	Half-V	One side beveled, other square	T-joints, corner joints
Groove — U	U shape	U-shaped preparation (for thick plates)	Thick sections, less filler metal than V
Groove — J	Half-U	One side J-shaped, other square	T-joints in thick material
Square	Two parallel lines	No preparation — square edges butted together	Thin sheet metal (< 3/16″)
Plug / Slot	Rectangle	Weld through a hole in one plate to the plate behind	Overlap joints where edge access is limited
Spot	Circle	Resistance spot weld or arc spot weld	Sheet metal assembly, automotive

Fillet Weld Sizing

Fillet welds are dimensioned by leg size (the length of each leg of the triangle). A 1/4″ fillet weld has two 1/4″ legs and a throat of approximately 0.177″ (leg × 0.707).

Minimum Fillet Weld Sizes (AWS D1.1)

Thicker Part (in)	Minimum Fillet Weld Size (in)
≤ 1/4	1/8
1/4 to 1/2	3/16
1/2 to 3/4	1/4
> 3/4	5/16

Key Supplementary Symbols

Symbol	Meaning	Notes
Flag on reference line	Field weld	Weld to be made at installation site, not in shop
Circle at arrow/line junction	Weld all around	Continuous weld around the entire joint
“CJP” in tail	Complete joint penetration	Full-strength weld through entire thickness
“PJP” in tail	Partial joint penetration	Weld doesn’t go all the way through

Practical Tips for Designers

Don’t over-weld. A 1/4″ fillet is adequate for most structural connections. Doubling the weld size roughly quadruples the cost (more passes, more filler, more distortion).
Intermittent welds save money. Specify “1/4 × 2-4″ (1/4″ fillet, 2″ long, 4” center-to-center) instead of continuous when full-length isn’t needed.
Think about access. Can the welder physically reach the joint? Is there room for the welding gun? Design for weldability.
Specify the process in the tail when it matters: GMAW (MIG), GTAW (TIG), SMAW (Stick), FCAW (Flux Core).