Assay Interval Length Calculator
The assay interval length is simply the downhole distance between the start and end of the sample: length = To depth − From depth (both in metres measured along the hole).
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How to use this calculator
- Enter the 'From' (start) depth and 'To' (end) depth of the sampled interval in metres of downhole depth — the tool returns the interval length as To − From.
- Leave the angle blank for a plain downhole (apparent) interval length, e.g. to tally sample run lengths.
- Optionally enter the angle between the drillhole axis and the structure (90° = perpendicular) to also get an estimated true thickness = interval × sin(angle).
How it works
The assay interval length is simply the downhole distance between the start and end of the sample: length = To depth − From depth (both in metres measured along the hole). This is the length you would composite or send for assay, and the length used as a weighting when averaging grades over several intervals. The 'To' depth must be deeper than the 'From' depth, so the tool errors on equal or reversed depths rather than returning zero or a negative length.
Because holes rarely intersect a structure at right angles, the downhole length over-states the true thickness. When you supply the angle between the hole axis and the plane of the structure, the tool applies true thickness = interval × sin(angle): at 90° (hole perpendicular to the structure) sine is 1 so true equals apparent, and the true width shrinks as the intersection angle gets shallower. This is a first-order estimate — a rigorous true width uses the surveyed hole trend/plunge together with the structure's dip and dip-direction.
Worked example
A 47.4 m core interval at a 65° intersection angle. A diamond hole is sampled from 198.3 m to 245.7 m. Interval length = 245.7 − 198.3 = 47.4 m of core. If the angle between the hole axis and the mineralised structure is 65°, the estimated true thickness = 47.4 × sin(65°) = 42.96 m. The apparent (downhole) length always over-states the true width unless the hole meets the structure at 90°.
Common mistakes
- Swapping From and To. Downhole depth increases into the hole, so 'To' must be the deeper (larger) number — the tool rejects reversed or equal depths.
- Treating the downhole interval length as the true thickness. Unless the hole meets the structure at 90°, the apparent length is longer than the true width; use the angle input for an estimate.
- Using the core-axis angle or the dip alone as the 'angle between hole and structure'. It must be the angle between the hole and the structural plane (90° = perpendicular), which depends on both hole orientation and structure orientation.
Frequently asked questions
Is the interval length in true metres or downhole metres?
It is downhole (along-hole) metres — the distance measured along the drillhole from the From depth to the To depth. That is the length used for sampling and for grade weighting. Enter an angle to also estimate the true thickness perpendicular to the structure.
What angle should I enter for true thickness?
Enter the angle between the drillhole axis and the plane of the mineralised structure. If the hole is perpendicular to the structure that angle is 90° and true thickness equals the apparent length; as the hole becomes more parallel to the structure the angle drops and the true thickness shrinks by sin(angle). For a rigorous value use the surveyed hole orientation with the structure's dip and dip-direction.
Does 1 ppm equal 1 g/t for the grades over this interval?
Yes — by mass, 1 ppm equals 1 g/t (and 1% = 10,000 ppm). This tool only computes the interval length; use it together with the grade tools to length-weight assay grades over the sampled intervals.
Related tools
- Hole Angle Deviation Calculator
- Dipping Hole Toe Calculator
- Ore Waste Ratio Calculator
- Sample Spacing Calculator
- Grams per Tonne to Ounces per Tonne Converter
- PPM Percent Converter
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Cite this tool
Zerdly. (2026). Assay Interval Length Calculator. Retrieved from https://www.zerdly.com/tools/assay-interval-length-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.