Skip distance formula in ultrasonic testing: worked examples

The three lengths

Draw a plate of thickness t. Put a probe on the top surface with a shear wave beam at angle θ, measured from the normal (a line straight down). The beam crosses to the bottom surface and bounces back up. Three lengths matter:

• Depth: how far below the scanning surface the beam is at a given point. At the first bounce it equals t.
• Surface distance: how far along the scanning surface that point is from the probe's index point (the beam exit point, not the front edge of the probe).
• Sound path: the distance the sound has traveled along the beam.

For the first leg, down to the bottom:

• Surface distance = t × tan θ. This is the half skip.
• Sound path = t ÷ cos θ.

After bouncing off the bottom surface, the beam comes up and meets the top surface at full skip = 2 × t × tan θ. The skip distance guide has the diagram.

Example 1: half and full skip

A 60° probe on 20 mm plate.

• Half skip = 20 × tan 60° = 20 × 1.732 = 34.6 mm
• Full skip = 2 × 34.6 = 69.3 mm
• Sound path to the bottom = 20 ÷ cos 60° = 20 ÷ 0.5 = 40.0 mm

The sound path is twice the plate thickness here. That is why a longer screen range is needed than the plate depth suggests.

Example 2: a steeper angle on thinner plate

A 70° probe on 12 mm plate.

• Half skip = 12 × tan 70° = 12 × 2.747 = 33.0 mm
• Full skip = 65.9 mm
• Sound path to the bottom = 12 ÷ cos 70° = 12 ÷ 0.342 = 35.1 mm

A steeper beam lands farther from the probe. The same 12 mm plate with a 45° probe would give a half skip of just 12 mm, so you pick the angle to put the beam where the weld is.

Example 3: where is a reflector on the second leg?

A 45° probe on 25 mm plate. A reflector gives an indication at a sound path of 60 mm.

1. Sound path to the bottom surface = 25 ÷ cos 45° = 35.4 mm.
2. 60 mm is past that, so the reflector is on the second leg, going up. Path left after the bounce = 60 − 35.4 = 24.6 mm.
3. Vertical travel on the way up = 24.6 × cos 45° = 17.4 mm.
4. Depth below the scanning surface = 25 − 17.4 = 7.6 mm.
5. Surface distance from the index point = 60 × sin 45° = 42.4 mm. That sits between the half skip (25 mm) and the full skip (50 mm), as it should.

The check in step 5 catches most errors. If your answer lands outside the leg you decided the reflector is on, redo it.

Example 4: 1.5 skip

A 60° probe on 20 mm plate. How far from the index point is the second bounce on the bottom surface, called 1.5 skip?

1.5 skip = 3 half skips = 3 × 34.6 = 103.9 mm.

Count surface contacts, not lines on the drawing. Half skip is the first bottom contact, full skip the first return to the top, 1.5 skip the second bottom contact.

Example 5: when the angle is given from the surface

Some sources, and some questions, give the angle from the scanning surface instead of the normal. A probe described as 30° from the surface is a 60° probe in the normal convention.

On 20 mm plate, using the surface angle α = 30°: half skip = t ÷ tan α = 20 ÷ 0.577 = 34.6 mm. Same answer as Example 1, because it is the same beam. Read which angle the question gives before you pick a formula. If it does not say, look for words like "from the normal" or "from the surface".

Where people lose points

• Using the angle from the wrong reference. See Example 5.
• Measuring from the front of the probe. Skip distance starts at the index point.
• Mixing depth with sound path. On the first leg they differ by a factor of cos θ.
• Counting legs wrong. Half, full, 1.5.
• Applying a flat plate formula to a curved part. A question about pipe or a cylinder will tell you how to treat it.
• Forgetting that the angle comes from the wedge and the material. The refracted angle is set by Snell's law and the velocities. The critical angles guide covers that step, and the Snell's law calculator checks your numbers.

Practice it, then check it

Work each example once by hand, then check yourself with the skip distance calculator. Do not use the calculator to learn the method. On exam day you will have a calculator but not the habit of sketching unless you built it first. The UT formulas guide places skip distance next to the other equations, and the math refresher rebuilds the trig.

Skip distance sits under weld evaluation and general evaluation in the UT General outline for exams before 5 February 2027, and the new outline covers the same geometry under its technique and interpretation headings. See what changes after that date.

See where you stand

Take the free 10-question UT check. It needs no signup, explains every answer and shows which topics to drill first. The course adds timed practice exams, a lesson for each published topic and a study plan built from your exam date, and Echo, the study assistant inside it, can walk through a geometry step you missed. See the plans.

FAQ

What is the skip distance formula?

For a flat plate of thickness t and a beam angle θ measured from the normal, half skip is t × tan θ and full skip is 2 × t × tan θ.

What is the difference between half skip and full skip?

Half skip is the surface distance to the first bounce off the opposite surface. Full skip is the surface distance to the point where the beam returns to the scanning surface.

Does skip distance depend on frequency?

No. It depends on thickness and beam angle. Frequency affects beam spread and resolution, not where the central beam lands.

Sources

Original arithmetic and examples, written for this article. The geometry is standard plane trigonometry, and the outline reference was read from ASNT's page on 26 October 2026.

• ASNT NDT Level II Examinations, ASNT Certification Services: UT General topic headings.
• Evident, ultrasonic flaw detection tutorial: background reading on angle beam geometry.