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U-groove weld symbol: what it means on a drawing

Fillet Gauge editorial teamSources checked 4 min read

The U-groove weld symbol is a rounded bowl sitting on a short stem on the reference line. It calls for a groove weld in which both members get a curved preparation, so the joint opens in a U with a rounded bottom. Below the line the U is on the arrow side; on both sides it is a double-U.

A butt joint drawn end on with both plates prepared in a curved, round-bottomed groove above a short root face, filled with weld metal, beside a U-groove symbol with 5/8 on its left and DETAIL A in the tail
Depth on the left. Radius and root face live in the detail the tail names. Numbers are invented.
Use this diagram

How to read it

  1. Find the side. Below the line, the U is cut and welded from the arrow side. Above the line, from the other side. A U on each side calls for a double-U, prepared and welded from both faces.
  2. Read the left side. The first number is the groove depth, how far down the preparation goes. A second number in parentheses is the weld size. 3/4 (5/8) means a 3/4 deep groove and a weld that fuses 5/8 deep.
  3. Read inside the U. A number there is the root opening, the gap between the members at the bottom of the groove.
  4. Read the tail. U and J preparations are machined to a set radius and root face, and those two dimensions aren't written on the symbol. The tail names a detail, section or note that gives them. Find it before you check the fit-up.
  5. Read the right side. A number on the right is the weld length. With none, the weld runs the whole joint.

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Symbols - Welding and NDE

On a welding symbol, what does a circle at the junction of the arrow and the reference line indicate?

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Why a U instead of a V

A V opens wider the deeper it goes, so on thick plate it needs a lot of filler. The U keeps its walls closer to upright and puts the width at the round bottom, where the root pass needs room. On heavy sections that can mean less weld metal and less distortion, at the cost of a machined or gouged edge on each member.

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Every answer comes with a full explanation.

Common mix-ups

  • U or J. The U mark is symmetric: a curve on both sides. The J-groove mark has one straight stroke and one curve, because only one member is prepared.
  • U or V. The V-groove mark is two straight sloping strokes meeting at a point. The U has a rounded bottom and a stem.
  • U or back weld. The open half-circle of a back or backing weld sits flat against the line with no stem, opposite a groove symbol. The U stands on a stem and is the groove symbol.
  • Depth or root face. The number on the left is how deep you cut. The root face is the land left uncut below it, and it comes from the detail.

U-grooves on the CWI exam

The symbols domain, Symbols, Welding and NDE, is at least 10% of CWI Part A, and Part A gives you no book, so the curved shapes have to be automatic. In Part B, where Utilization of Specifications and Drawings is at least 8% of the practical, knowing that the radius sits in a referenced detail is what sends you to the right page of the drawing.

Practice it

Build a U-groove in the welding symbol builder with a depth and a weld size, and read back how the parentheses work. Then line the U up against the other groove marks on the welding symbols chart.

Bowl and stem sorted? Check the rest of Part A

Take the free 10-question check. It's weighted to the Part A outline, scored by domain and needs no signup, so you see whether symbols or another domain costs you the most points. The Part A course gives you a lesson on reading welding and NDE symbols and on every other exam subject, two full-length timed practice exams, and a worked explanation with every answer.

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Facts checked 24 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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