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Composite Assay by Length Calculator

Calculates the length-weighted (composite) grade across several drill sample intervals — the standard way to combine assays over a mineralised zone. Exploration and resource geologists use it to report a single representative grade over a downhole intersection.

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How to use this calculator

Enter Values

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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. For each sample interval, enter its length in metres and its assay grade (g/t for gold, % for base metals) — up to four intervals.
  2. Keep all grades in the same unit; leave unused rows blank.
  3. Read the composite grade, the total composited length and how many intervals were combined.

How it works

The composite is a length-weighted average: composite grade = Σ(length × grade) ÷ Σ(length). Longer intervals pull the average toward their grade, so a thick low-grade sample counts more than a thin high-grade one — exactly how a drill intersection is reported (e.g. '4.5 m at 5.67 g/t').

Only intervals where you supply both a length and a grade are included. Where bulk density varies materially between intervals, geologists use the more advanced density-weighted (accumulation, or grade × thickness × density) composite; for most first-pass reporting the length-weighted result here is the accepted method.

Worked example

Three samples over 4.5 m. Intervals of 1.0 m at 5 g/t, 2.0 m at 8 g/t and 1.5 m at 3 g/t give Σ(l·g) = 5 + 16 + 4.5 = 25.5 and Σl = 4.5 m, so the composite = 25.5 / 4.5 = 5.67 g/t over 4.5 m. A plain average of 5, 8 and 3 would wrongly give 5.33 g/t.

Common mistakes

  • Taking a plain arithmetic average of the grades instead of weighting by length.
  • Mixing units — e.g. some grades in g/t and others in ppm or % — which corrupts the composite.
  • Entering a length with no grade (or vice versa); an incomplete pair is ignored, which can silently shorten the composited length.

Frequently asked questions

What is a length-weighted composite grade?

It is the average grade over several intervals, weighted by each interval's length: Σ(length × grade) ÷ Σ(length). It is how drill intersections are reported.

Why not just average the grades?

A simple average ignores interval length, so a thin sample would count the same as a thick one and misstate the true intersection grade.

What units should I use?

Any single consistent unit — g/t for precious metals, % for base metals. The result comes out in whatever unit you enter, so don't mix them.

When should I density-weight instead?

When bulk density changes significantly between intervals (e.g. massive sulphide vs oxide), a density-weighted (accumulation) composite is more accurate. Length-weighting assumes roughly constant density.

Can I composite more than four intervals?

This tool takes up to four. For a longer zone, composite in blocks and then length-weight the block composites together.

Does it apply a cut-off grade?

No — it composites exactly the intervals you enter. Apply your cut-off first, then composite the intervals that pass.

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Cite this tool

Zerdly. (2026). Composite Assay by Length Calculator. Retrieved from https://www.zerdly.com/tools/composite-assay-by-length-calculator/

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

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