How to Read Welding Symbols & Blueprints

Why Welding Symbols Matter

Welding symbols are the universal shorthand for communicating exactly what weld goes where, how big, what process, and to what standard. If you work from blueprints, fabrication drawings, or even sketch plans from an engineer, you need to read these symbols. They eliminate ambiguity — instead of writing "put a 1/4-inch fillet weld on the far side of this joint, all around, using the TIG process," a single compact symbol conveys all of that information at a glance.

For hobbyists and home fabricators, the full AWS A2.4 symbol standard is overkill. But understanding the basic structure and the 5–6 most common symbols covers 90% of what you will encounter in practical fabrication work.

Anatomy of a Welding Symbol

Every welding symbol is built on the same framework:

• Reference line: A horizontal line that serves as the foundation. Everything attaches to this line.
• Arrow: Points from the reference line to the joint being welded. The arrow side (below the reference line) describes the weld on the side the arrow points to. The other side (above the reference line) describes the weld on the opposite side of the joint.
• Weld symbol: A small geometric shape placed on or below the reference line that tells you the weld type (fillet, groove, plug, etc.).
• Dimensions: Numbers placed near the weld symbol specifying size, length, pitch, and other measurements.
• Tail: An optional V-shaped tail on the end of the reference line opposite the arrow. Contains supplementary information: process (GMAW, GTAW), specification references, or special instructions.
• Supplementary symbols: Circles, flags, and other marks that indicate field welds, all-around welds, weld contour, and finishing requirements.

Common Weld Type Symbols

Fillet Weld (Triangle)

The most common weld in structural fabrication. The symbol is a right triangle placed on the reference line. The vertical leg is always on the left. A fillet weld creates a triangular cross-section joining two surfaces at roughly 90 degrees — think T-joints, lap joints, and corner joints. The number next to the triangle specifies the leg size (e.g., "1/4" means each leg of the fillet is 1/4 inch).

Groove Weld (V, U, J, Bevel)

Groove welds fill a prepared groove between two pieces. The symbol shape matches the groove profile: a V for V-groove, a U for U-groove, a half-V for single bevel. Groove welds are used for full-penetration butt joints where maximum strength is required. Dimensions specify the groove angle, root opening, and root face.

Plug and Slot Welds (Rectangle)

A plug weld fills a hole drilled in one piece to join it to another. The symbol is a rectangle on the reference line. Slot welds are elongated versions. These are common in sheet metal fabrication and automotive body repair.

Supplementary Symbols

• Circle at the junction of arrow and reference line = weld all around the joint.
• Flag (filled triangle) at the junction = field weld (done on-site, not in the shop).
• Contour symbols above the weld symbol = flush, convex, or concave finish required.

Reading Dimensions

Dimensions are placed to the left and right of the weld symbol:

• Left side: Weld size (leg size for fillets, depth of groove for groove welds).
• Right side: Weld length. If followed by a dash and another number, it means length-pitch (e.g., "2-5" means 2-inch welds spaced 5 inches apart center-to-center — an intermittent weld).

If no length is specified, the weld runs the full length of the joint. A circle at the reference line/arrow junction also means the weld continues all the way around the joint.