Why weld symbols matter (and what you can ignore for now)
On simple projects and small repairs, weld symbols answer a few practical questions: where to weld, what type of weld to make, how big it should be (usually a fillet size), and sometimes whether it’s done in the shop or in the field. You do not need to memorize every symbol variation to start; you need recognition-level reading for the most common callouts you’ll see on brackets, frames, tabs, and patch plates.
Think of a weld symbol as a compact instruction label attached to a joint. Your job is to translate it into: (1) joint location, (2) weld type, (3) side of the joint, (4) approximate bead size/coverage, and (5) any special “do it all the way around / do it in the field” cues.
The parts of a basic welding symbol
1) Reference line
The reference line is the long horizontal line that everything attaches to. The weld symbol (triangle, V, etc.) sits on it, and numbers (size/length) are placed near it.
2) Arrow
The arrow points to the joint or the specific edge where the weld applies. If a drawing has multiple similar joints, the arrow is what tells you which one the note is talking about.
3) Basic weld symbol
This is the “shape icon” that tells you the weld type. For beginner work, the most common are:
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- Fillet weld: a small triangle.
- Groove (butt) weld indicators: V, bevel, U, etc. At a beginner recognition level, treat these as “this is a butt joint that may require edge prep and a groove fill,” even if you don’t yet execute advanced groove procedures.
4) Size and length (numbers)
Numbers around the symbol tell you how big and how long. For beginners, the most common is fillet size (leg length). Typical placement rules you’ll see:
- Size is usually to the left of the weld symbol (example:
1/4next to a fillet triangle). - Length is usually to the right of the weld symbol (example:
2meaning a 2-inch long weld segment). - If you see something like
2-6to the right, it often means intermittent welds (2-inch welds spaced on 6-inch pitch). For absolute beginner projects, many drawings simply call for continuous welds and won’t use this.
5) All-around circle
A small circle at the junction where the arrow meets the reference line means weld all the way around the joint (a continuous weld around the perimeter of a tab, pipe, or plate).
6) Field weld flag
A small flag at the arrow/reference-line junction means field weld—do it on site rather than in the shop. For home projects, interpret it as “this weld is intended to be made during installation/assembly, not pre-welded on the bench.”
Arrow side vs. other side (the most important beginner rule)
Weld symbols can be placed below or above the reference line:
- Below the line = weld on the arrow side of the joint.
- Above the line = weld on the other side of the joint.
- Both above and below = weld on both sides.
This “which side” cue is one of the easiest places to make a real-world mistake. Before striking an arc, physically point to the arrow side on the actual parts and confirm where the bead should go.
Common symbol #1: Fillet weld (triangle)
What it means in the shop
A fillet weld joins two surfaces that meet at roughly a right angle (T-joint, lap joint, corner). The triangle symbol tells you to place a bead in the corner where the two pieces meet.
Basic fillet size notation (recognition level)
When you see a number like 1/8, 3/16, or 1/4 to the left of the fillet symbol, it usually refers to the fillet leg size (approximate “width” of the weld measured along each plate surface). As a beginner, treat it as: “make a bead that reasonably matches that size,” rather than chasing perfect gauge measurements.
| Callout you might see | Interpretation | Action |
|---|---|---|
1/8 + fillet triangle | Small fillet | One controlled pass, modest bead, avoid overbuilding |
3/16 + fillet triangle | Medium fillet | One pass if your process/position allows; otherwise two passes may be needed |
1/4 + fillet triangle | Larger fillet | Often requires more heat and/or multiple passes; ensure tie-in to both legs |
Length on the right
If the right side shows 2, that’s typically a 2-inch long weld where the arrow points. If no length is shown, it often implies weld the full length of the joint (unless the drawing indicates otherwise).
Common symbol #2: Groove/butt weld indicators (recognition level)
Groove symbols (like a V shape or a single bevel) indicate a butt joint where the weld metal fills a groove between parts. For beginner reading, focus on these practical cues:
- It’s not a fillet in a corner; it’s a weld in a seam between aligned parts.
- The joint may require edge preparation (a bevel) and root gap depending on thickness and drawing notes.
- The weld is placed in the groove (on the indicated side), aiming for fusion to both edges.
If you see a groove symbol but you’re doing a small repair, you can still use it to decide: “I should align these edges and weld the seam,” even if your execution is a simple single-pass butt weld on thin material.
Guided translations: symbol → action
Example 1: Arrow-side fillet on a T-joint
Symbol: Fillet triangle below the reference line with 3/16 on the left, no length shown.
Translate it:
- Where to weld: the joint the arrow points to.
- Which side: arrow side (because the triangle is below the line).
- What type: fillet (bead in the inside corner of the T).
- How much: approximately a 3/16 fillet, likely along the full joint length.
Do it (step-by-step):
- Find the exact joint the arrow touches on the drawing, then locate it on the workpiece.
- Stand where you can see the corner clearly; point to the arrow side and confirm that’s the side you can access.
- Plan bead placement: aim the bead so it ties into both plates (equal attention to each leg).
- Run a steady bead along the joint length, keeping the bead size consistent with the callout.
Example 2: Fillet on the other side only
Symbol: Fillet triangle above the reference line with 1/8 on the left and 2 on the right.
Translate it:
- Which side: other side (triangle above the line).
- Length: make a 2-inch long weld (not necessarily the whole joint).
- Size: small 1/8 fillet.
Do it (step-by-step):
- Mark a 2-inch segment on the correct side of the joint (use a soapstone or scribe mark).
- Start slightly before your mark and stop slightly after, then blend/clean the ends so the effective welded length is about 2 inches.
- Keep the bead small; avoid making it “extra strong” by overbuilding—overwelding can warp thin parts.
Example 3: Fillet both sides
Symbol: Fillet triangles both above and below the reference line with 3/16 on the left.
Translate it: weld fillets on both sides of the joint, about 3/16 size.
Do it (step-by-step):
- Weld the more accessible side first while the part is stable.
- Flip/reposition safely and weld the opposite side.
- Try to keep bead size similar on both sides to balance heat and reduce pull.
Example 4: All-around fillet on a tab
Symbol: Fillet triangle with an all-around circle at the arrow/reference junction, size 1/8.
Translate it: weld a 1/8 fillet continuously around the entire perimeter where the tab meets the base plate.
Do it (step-by-step):
- Walk the perimeter and identify every edge that counts as part of the joint.
- Plan a sequence (short segments, moving around) to reduce distortion on thin parts.
- Connect the welds so there are no unwelded gaps around the tab.
Example 5: Field weld flag
Symbol: Fillet triangle with a field weld flag.
Translate it: this weld is intended to be made during installation/assembly. Expect limited access and plan for positioning, clamps, and power accordingly.
Quick sketch-based interpretation exercises
Use these as “read it, point to it, then describe the weld” drills. For each one: (1) identify the joint, (2) say arrow side/other side/both, (3) say weld type, (4) say size/length, (5) describe bead placement.
Exercise A: T-joint, arrow-side fillet
Plate (vertical) ↑ arrow points to the joint line on this side Reference line: -------------------- Symbol: 1/8 ▽ (triangle below line) (no length shown)Your action statement should sound like: “Make an approximately 1/8 fillet weld on the arrow side of the T-joint, likely full length unless otherwise limited.”
Exercise B: Lap joint, other-side fillet, short length
Top plate overlaps bottom plate Arrow points to overlap edge Reference line: -------------------- Symbol: 3/16 △ 2 (triangle above line, length 2)Your action statement should sound like: “On the far side of the lap joint, place a 3/16 fillet for 2 inches at the indicated location.”
Exercise C: Corner joint, both sides
Two plates form an L shape (corner) Arrow points to the corner seam Reference line: -------------------- Symbol: 1/8 △ and ▽ (triangles above and below)Your action statement should sound like: “Weld 1/8 fillets on both sides of the corner seam.”
Exercise D: Plate tab, all-around
Small tab welded to a base plate Arrow points to tab/base junction Reference line: -------------------- Symbol: ○ + 1/8 ▽ (circle at arrow junction)Your action statement should sound like: “Run a continuous 1/8 fillet all the way around the tab where it meets the base.”
Connecting symbol meaning to joint prep and bead placement
Fillet symbol → corner targeting
If the symbol is a fillet triangle, your physical goal is a bead that ties into both members of the joint. Before welding, visually identify the two “legs” the fillet will sit on. During welding, keep your aim so the puddle wets into both surfaces rather than riding only one plate.
Groove symbol → seam targeting
If the symbol is a groove/butt indicator, your physical goal is fusion along the two edges forming the seam. Check that the edges align as intended and that you can access the side specified by the symbol placement (above/below reference line). If the drawing implies a bevel, confirm which piece is beveled (the arrow can indicate the member to be prepared on some callouts), then place the bead into the groove rather than building a fillet on the outside corner.
Size callout → avoid underweld and overweld
Size is a target, not a suggestion. Too small can mean weak attachment; too large can mean distortion, burn-through on thin parts, or interference with fit. When you see 1/8 vs 1/4, treat it as a real difference in bead planning: travel speed, number of passes, and heat control.
Side callout → confirm access before you start
When the symbol specifies other side, don’t “just weld the side you’re on.” Reposition the part or yourself so you can weld the correct side. A good habit is to say out loud: arrow side or other side, then touch that side on the actual joint before welding.
