Free Online Drillhole Assay Compositing Tool: Length-Weighted Grades

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Composites are length-weighted: each fixed interval averages the assays inside it, weighted by how much of each assay it contains. Composites covered below your minimum threshold are flagged, because a 2 m composite built from 0.4 m of sample is a guess wearing a number.

Making assays comparable

Drillhole samples arrive at whatever lengths the geology dictated: half a meter through a vein, two meters through waste. Resource estimation needs equal support, so the first step of every workflow is compositing. The tool above takes your from, to, grade rows and rebuilds them into fixed-length intervals, each grade weighted by the sample length it contains. Composites with poor sample coverage are flagged rather than silently reported, and the overall length-weighted mean grade summarizes the hole.

Diagram of raw drillhole assay intervals of unequal length being converted into fixed two meter composites with length-weighted grades
Raw samples of unequal length become fixed-length, length-weighted composite grades.

This opens the Mining and Resource Suite in the bar above: composite here, then feed the grades into the grade-tonnage and cutoff tools alongside.

Why length weighting matters

  • An unweighted average lets a 0.3 m bonanza sample swamp a 2 m barren one; length weighting keeps the metal honest.
  • Composite length should match the mining selectivity: 1 to 2 m for underground, bench height for open pits, because the composite pretends to be a mineable parcel.
  • The coverage flag catches the classic trap: a composite built from a sliver of sample at the end of a hole carries a grade but not the confidence of one.
  • Density weighting refines further when rock types differ; length weighting is the standard first pass.

What this tool is for

Preparing assay tables for statistics, checking what a commercial package did to your data, and teaching the first honest step between the lab certificate and the resource model.

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