Skip distance: what it means and why it matters

The formula

Draw the beam as a straight line crossing the plate. For a plate of thickness t and a beam angle θ measured from the normal, the horizontal distance covered going down is t × tan θ. That is the half skip. Going down and back up doubles it, so the full skip is 2 × t × tan θ.

Take a 25 mm plate and a 60 degree shear wave. Half skip is 25 × 1.732 = 43.3 mm, and full skip is 86.6 mm. The sound path on each leg is longer than the plate is thick: 25 / cos 60° = 50 mm, so the beam path to the bottom surface is 50 mm even though the depth is only 25 mm. Keep the three lengths apart: depth, surface distance and beam path.

What it is used for

In weld inspection the probe sits beside the weld and the beam is aimed across it. Knowing the half and full skip tells you where to place the probe so the beam reaches the root on the first leg and the cap region on the second leg. It also tells you how far back from the weld the probe has to travel to cover the volume with each leg. A reflector seen on the first leg is located from the index point with t × tan θ style geometry, and one seen on the second leg needs the reflected leg added in.

Where the numbers go wrong

• Angle from the surface instead of the normal. The formula above assumes the normal. If a source gives the angle from the surface, the surface distance for one leg is t / tan(angle). Check which convention the question uses before you start.
• Measuring from the wrong point. Skip distance is measured from the beam exit point on the wedge, called the index point, and not from the front edge of the probe.
• Mixing up legs. A 1.5 skip distance lands on the opposite surface again, so it is a full skip plus a half skip. Count the surface contacts, not the number of lines drawn.
• Using thickness for a curved part. The flat plate formula changes for pipe and other curved surfaces, so a question about a curved part will say how to treat it.

How the exam uses it

Skip distance sits under weldment evaluation and general evaluation, the topics that deal with beam geometry. Questions give a thickness and an angle and ask for a half skip, a full skip, a depth or a surface distance. The arithmetic is short once the sketch is right, so draw the plate and mark each leg before touching the calculator.

Topic names here follow the UT General outline for exams before 5 February 2027; the UT outline change guide shows how the later list lines up.

Practice it

Check your own numbers in the skip distance calculator, then work the geometry in the UT formulas guide. The angle that makes the geometry possible comes from critical angles, and the wave that travels it is a shear wave, covered in shear vs longitudinal waves. All twenty terms are in the NDT terms glossary.

Know the term? Check the rest of the UT General topics

Take the free 10-question check. It covers the UT General exam, needs no signup and explains every answer, so you see which topic to study next. The course then adds timed practice exams and a study plan built from your exam date.

Sources

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Background reading: Evident's published UT material and ASNT's published topic headings. Outline checked 25 October 2026.

• Evident, ultrasonic flaw detection tutorial
• Evident, ultrasonic transducer technical notes
• ASNT NDT Level II examinations, published topic headings.

Not affiliated with or endorsed by ASNT.