Weld undercut: what it is and how to spot it

Use this diagram

Copy this HTML to show the diagram with a credit link<img src="https://filletgauge.com/charts/weld-discontinuities/weld-undercut.svg" alt="Two cross-sections of undercut. A: a T-joint fillet weld with a small notch melted into the base plate just past the toe. B: a groove weld with a dashed plate-surface line and a notch at each toe" width="100%"><p><a href="https://filletgauge.com/guides/weld-undercut?utm_source=embed&amp;utm_medium=referral&amp;utm_campaign=figure_embed&amp;utm_content=weld-undercut.svg">Depth is read from the original plate surface down to the bottom of the notch. Original drawing, not to scale. by Fillet Gauge</a></p>

How to recognize undercut

1. Go to the toe. Undercut is always at the boundary between weld and base metal. Run your eye, and a low-angle flashlight, along each toe.
2. Look for a shadow line. A sharp, continuous or intermittent shadow along the toe, with base metal visibly missing, is the usual look. A finger or fingernail drawn across the toe catches on it.
3. Decide which side the metal left. If the hollow is in the plate beside the weld, it is undercut. If the weld face itself is low, it is underfill.
4. Check the root too. On a weld made from one side, the back of the joint can be undercut along the root toes.

How to measure undercut

Depth is the number that matters. Set an undercut gauge, such as the pointer on a V-WAC gauge, on flat base metal next to the toe and zero it there, or note the reading if it can't be zeroed. Then slide it to the deepest part of the notch and read the depth from the original plate surface. A dial caliper's depth rod does the same job. Also record length, since many documents limit undercut by depth and accumulated length together, and the limit may change with the loading of the joint. Look that limit up in the governing document; don't carry a number in your head.

Common mix-ups

• Undercut or underfill. Undercut takes metal out of the plate at the toe. Underfill leaves the weld face below the plate surface. Same tool, different location.
• Undercut or overlap. Overlap adds unfused weld metal on top of the plate at the toe. Undercut removes plate metal.
• Undercut or a toe crack. A crack is a sharp, tight line with no visible width to its floor. Undercut has a rounded, open hollow you can measure.

Why it forms

Too much heat at the edge of the pool melts the plate faster than filler can replace it. The usual causes are excessive current, a long arc, too fast a travel speed, a poor electrode angle pointed at the plate, and on a weave, not pausing at each side. Fillet welds in the horizontal position are especially prone along the upper toe, where gravity pulls the pool down.

Undercut on the CWI exam

Undercut is a measuring task, so it is central to Part B: Welding Inspection and Discontinuities is at least 34% of the practical, and reading depth correctly on a replica is a core skill there. On Part A, it fits Visual Examination and Welding Application and Control. On Part C you look up the visual acceptance criteria for undercut in your code, D1.1 Clause 8 for the D1.1 endorsement, and read the conditions on loading in your own copy.

Practice it

Practice reading undercut depth on the weld gauge trainer. Then compare it with the two toe and face look-alikes: underfill and overlap. The weld discontinuities guide has every type in one table.

Check the rest of Part B

Take the free 10-question check. It's weighted to the Part B outline, scored by domain and needs no signup, so you see whether measuring or another domain is costing you points.

Sources

Show the list

• A general review of geometric shape imperfections, Part 1 (Job Knowledge 67), TWI. Retrieved 25 Oct 2026.
• CWI Part B Practical Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
• Discontinuity names follow AWS A3.0, Standard Welding Terms and Definitions. Every description here is our own wording.