Read every welding symbol from the reference line outward. AWS A2.4 Section 4.5 defines the basic elements; AWS A2.4 Section 6.2 sets below/above placement; AWS A2.4 Figure 4.3 shows where size, length, contour, and tail notes belong during drawing review.
A welding symbol is read in a fixed sequence: (1) find the reference line, (2) identify arrow side vs. other side, (3) read the weld type symbol, (4) read size and length dimensions, (5) check for finishing codes or tail notes. Per AWS A2.4:2020.
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Interactive Reading Guide
Explore the Symbol
Tap any chip to highlight that part of the welding symbol on the diagram. Each element is verified against AWS A2.4:2020.
Click a chip
Click any element above to highlight it on the diagram and read its definition from AWS A2.4:2020.
Quick Reference
You see…
It means…
Below ref line
Weld on the arrow side
Above ref line
Weld on the other side
Both sides of ref line
Weld on both sides
Open circle at junction
Weld all around the joint perimeter
Solid flag at junction
Field weld (made on site, not in shop)
Number left of weld symbol
Size (leg length for fillet; D(S) for groove)
Numbers right (e.g., 3–12)
Length–pitch for intermittent welds
Tail (V-shape opposite arrow)
Process / WPS reference / spec note
AWS A2.4:2020 §5 defines five basic welding joint types in Figure 5.1. Tap a joint to see the dominant weld symbols used on it.
Fit-up matters. Joint fit-up tolerance affects every downstream parameter — gap size drives preheat, root pass size, and consumable use. Tight fit means less filler metal, smaller HAZ, and fewer reworks.
Engineering drawings use a standardized set of line types — the “alphabet of lines” — where each line’s weight and pattern carries specific meaning. Master these 10 and you can decode almost any technical drawing.
Key rule: Line weight carries meaning. Thick lines show what you can see (visible edges, cutting planes). Thin lines carry information about what you measure or what’s hidden (dimensions, extension, centerlines, hidden features).
Per ASME Y14.2-2014 (R2020). Specific dimensions and dash patterns may vary slightly by jurisdiction — refer to ASME Y14.2 (US) or ISO 128-1 (international) for authoritative measurements.
Worked Example
Reading a Real Fillet Weld Symbol
Take a welding symbol showing: a right triangle below the reference line with the number 3/8 to its left and 3-12 to its right. Here is the full reading:
Symbol Reading
Step 1 — Reference line: Found. Horizontal line with arrow and triangle.
Step 2 — Arrow side: Triangle is below the reference line = arrow side weld.
Step 3 — Weld type: Right triangle with perpendicular leg on left = fillet weld.
Step 4 — Dimensions: 3/8 to the left = 3/8-inch fillet leg size. 3-12 to the right = 3-inch weld segments, 12-inch center-to-center pitch (intermittent fillet weld).
Step 5 — Special indicators: No flag, no circle, no tail. Standard shop weld, no special finishing required.
Full reading: Make 3/8-inch fillet welds on the arrow side of the joint. Each weld segment is 3 inches long, spaced 12 inches center-to-center along the joint.
Inspector check: When inspecting this weld, measure the actual fillet leg with a fillet weld gauge — verify each segment meets the specified 3-inch minimum length and that the spacing does not exceed 12 inches center-to-center. D1.1 specifies minimum segment length (§1046, 1-1/2 inch absolute floor); in practice fillet weld gauges read in 1/4-inch increments.
Common Questions
How to Read Weld Symbols FAQ
How do you read a welding symbol step by step?
Read a welding symbol in five steps per AWS A2.4:2020. Step 1: find the horizontal reference line — everything else attaches to it. Step 2: identify which side of the joint is the arrow side (where the arrow points) and which is the other side. Step 3: read the weld type symbol — a right triangle below the line means a fillet weld on the arrow side; an open-V means a V-groove. Step 4: read the dimensions — the number to the left is the weld size; numbers to the right are length and pitch for intermittent welds. Step 5: check for special indicators such as a field weld flag (solid triangle at the junction), a weld-all-around circle, or tail notes referencing a WPS number.
What does the number to the left of a weld symbol mean?
The number to the left of a weld symbol is the weld size. For a fillet weld, it is the leg length — for example, 5/16 means a 5/16-inch (8mm) leg. For groove welds, the groove depth D appears without parentheses and the weld size (S) appears in parentheses, written as D(S) per A2.4 section 7.2.6. If the size appears on the same side as the weld symbol (below for arrow side, above for other side), it applies to that side only. A CJP groove weld has no size dimension — the weld fills the full joint.
What does a circle on a welding symbol mean?
A small circle at the junction of the reference line and the arrow is the weld-all-around symbol per AWS A2.4:2020. It means the weld must be made completely around the joint perimeter — for example, all four sides of a tube-to-plate connection or the full circumference of a pipe sleeve penetration. The circle is placed at the reference line / arrow junction, not on the weld symbol itself. Do not confuse it with the field weld flag, which is a solid filled triangle at the same junction.
What does a flag on a welding symbol mean?
A solid triangular flag at the junction of the reference line and arrow is the field weld symbol per AWS A2.4:2020. It means the weld must be made in the field — at the erection or installation site — not in the fabrication shop. Field welds are significant because they affect inspection scheduling, preheat requirements (field conditions differ from shop conditions), and logistics. Per A2.4 §6.9, the flag may point in either direction. It can appear alongside a weld-all-around circle when the weld is both field-made and continuous around the joint.
"Reading weld symbols is the first skill every welding inspector learns — and the one they use every single day. If you cannot read the symbol, you cannot inspect the joint. I tell every CWI candidate: master A2.4 before you open D1.1."