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Discontinuity Spacing Calculator

Calculates the mean discontinuity (joint) spacing along a scanline from the measured length and the number of discontinuities counted, plus the true (normal) spacing, the discontinuity frequency, and the ISRM spacing description. It supports rock-mass characterisation for engineering geology, slope and tunnel work.

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How to use this calculator

Enter Values

Minimum 1

Range 0–90

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the scanline length L in metres and the number of discontinuities N counted along it.
  2. Optionally enter the angle θ (decimal degrees) between the scanline and the discontinuity-set normal for a true-spacing correction.
  3. Read the mean spacing, true spacing, frequency λ and the ISRM spacing class.

How it works

The mean apparent spacing along the scanline is simply L ÷ N, and the discontinuity frequency is its reciprocal λ = N ÷ L per metre. This apparent spacing is only the true spacing when the scanline runs perpendicular to the set (θ = 0°).

When the scanline is oblique to the set, the true spacing measured normal to the joint planes is (L/N) × cos θ, where θ is the angle between the scanline and the set normal. The mean spacing is then classified using the ISRM (1978) scale from very close to extremely wide.

Worked example

10 m scanline, 40 joints, θ = 60°. Mean spacing = 10 ÷ 40 = 0.25 m (ISRM 'Wide', 0.2–0.6 m). Frequency λ = 40 ÷ 10 = 4 per m. With the scanline 60° from the set normal, true spacing = 0.25 × cos 60° = 0.125 m.

Common mistakes

  • Measuring the angle from the joint plane instead of the joint normal — θ is the angle to the set normal, so a scanline perpendicular to the set is θ = 0°.
  • Reporting apparent scanline spacing as true spacing when the scanline crosses the set obliquely.
  • Mixing units — keep the scanline length in metres so the spacing classes and frequency read correctly.

Frequently asked questions

How do I calculate mean joint spacing?

Divide the scanline length by the number of discontinuities intersected: mean spacing = L ÷ N. Its reciprocal is the discontinuity frequency λ = N ÷ L.

What is the difference between apparent and true spacing?

Apparent spacing is measured along the scanline. True spacing is measured perpendicular to the joint set and equals apparent spacing × cos θ, where θ is the angle between the scanline and the set normal.

What are the ISRM spacing classes?

Very close <0.02 m, Close 0.02–0.06 m, Moderate 0.06–0.2 m, Wide 0.2–0.6 m, Very wide 0.6–2 m, Extremely wide >2 m.

What is discontinuity frequency λ?

It is the number of discontinuities per metre along the scanline (λ = N ÷ L), the inverse of mean spacing, and it feeds parameters like RQD estimates.

Should I correct spacing for scanline orientation?

Yes, when the scanline is not perpendicular to the set. Apply the cos θ correction so the reported spacing reflects the true spacing normal to the joint planes.

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Cite this tool

Zerdly. (2026). Discontinuity Spacing Calculator. Retrieved from https://www.zerdly.com/tools/discontinuity-spacing-calculator/

Tip: Enter any known values to calculate the remaining results.

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