How to Read Welding Symbols【Reference Line · Arrow · Fillet · Groove Explained】

A welding symbol concisely specifies on a drawing "where, what weld, and what size" to join (JIS Z 3021). Grasp the symbol structure and the "arrow side / other side" rule and one symbol tells you the weld type, position and size. This guide covers the structure, how to distinguish sides, basic symbols and how to write dimensions — with a diagram.

Structure of a welding symbol

Fig. 1 — The elements. The arrow points to the weld (about 60° to the reference line), the basic symbol (weld type) sits on the reference line, with a dimension and, if needed, a tail (process, notes).

Arrow side and other side (below and above)

• A symbol below the reference line → weld the arrow side (the face the arrow points to).
• A symbol above the reference line → weld the other side.
• Mnemonic: "below = arrow side, above = other side." For both sides, put the symbol above and below.

Main basic symbols

Symbol	Weld type
Triangle (▷)	Fillet weld
V	V-groove
‖	I-groove (gap)
Slash	Bevel / half-V groove
U · J	U · J groove

Supplementary symbols

• All-around weld: a circle ○ at the junction of arrow and reference line. Weld the entire perimeter of the joint.
• Field weld: a flag symbol. Welded at the installation site, not the shop.
• Finish: specify a flat (F)/convex (C)/concave (V) finish.

How to write dimensions

• Fillet: write the leg length to the left of the symbol (e.g., a6 or 6▷).
• Groove: specify groove angle, root gap, throat thickness.
• Intermittent weld: write length-pitch-number to the right (e.g., 6▷ 50-100: length-pitch).

Common mistakes

• Confusing above/below (arrow side/other side) and welding the wrong face.
• Missing the all-around/field symbols.
• Skipping the leg-length/throat-thickness callout.
• Reading the "length-pitch" of an intermittent weld backwards.

Frequently asked questions (FAQ)

Q. What if there's a symbol both above and below the reference line? A. It means weld both sides. Read below = arrow side, above = other side separately.

Q. What is the fillet "leg length"? A. The length of the triangle's two right-angle sides. It expresses the weld size and ties directly to strength.

Q. What's the all-around symbol? A. A circle ○ at the arrow/reference-line junction. Weld the whole perimeter of the joint.

Q. Why distinguish a field weld? A. Conditions differ between the shop and the installation site. The flag symbol makes on-site work explicit.

Q. Do JIS and AWS differ? A. The position convention differs, so watch for standard differences. Check which standard the drawing follows.

Summary

A welding symbol is made of "arrow · reference line · basic symbol · dimension · tail," and the core of reading it is below = arrow side / above = other side. Read together with the basic symbols (fillet, groove), supplementary symbols (all-around, field) and dimensions like leg length to understand the weld instruction precisely.

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