Zerdly

Downhole Depth Interval Calculator

Works out the length and midpoint of a logged downhole interval from its FROM and TO depths, and optionally the core recovery percentage. It is a quick everyday check for geologists and core loggers keeping drill logs tidy.

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How to use this calculator

Enter Values

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 FROM depth and the TO depth of the interval in metres (downhole, measured along the hole).
  2. Read off the interval length and its midpoint depth.
  3. Optionally enter the recovered core length to get recovery % for that interval.

How it works

The interval length is simply TO − FROM, and the midpoint is the average of the two depths, (FROM + TO) / 2. Both are measured downhole, i.e. along the drill hole, not corrected to true vertical depth.

If you also supply the length of core physically recovered from that run, recovery % = recovered length / interval length × 100. Recovery below about 90–95% flags core loss that should be noted against the interval.

Worked example

A 1.5 m run at 90% recovery. An interval logged FROM 12.30 m TO 13.80 m has a length of 13.80 − 12.30 = 1.50 m and a midpoint at (12.30 + 13.80) / 2 = 13.05 m. If 1.35 m of core was recovered, recovery = 1.35 / 1.50 × 100 = 90.0%.

Common mistakes

  • Swapping FROM and TO — TO must be the deeper (larger) depth or the interval is invalid.
  • Treating downhole depth as vertical depth in an inclined hole; a survey/deviation correction is needed for true vertical depth.
  • Confusing recovery (recovered / interval) with RQD, which counts only sound pieces ≥ 100 mm.

Frequently asked questions

Is this downhole or vertical depth?

Downhole — the depth measured along the hole. In an angled hole you need the survey data to convert to true vertical depth.

What is the midpoint depth used for?

It is the representative depth used when plotting an interval as a single point, e.g. for grade-vs-depth or geotech-vs-depth logs.

How is recovery percentage defined?

Recovery % = length of core recovered ÷ length of the drilled interval × 100. It measures how much core came back, not its quality.

What is a good recovery?

Around 95–100% is ideal in competent ground; low recovery often coincides with broken, weathered or mineralised zones and should always be recorded.

Can the interval length be negative?

No. If TO is less than or equal to FROM the tool returns an error — check you have entered the shallower depth as FROM.

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

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