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.
Enter Values
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
- For each sample interval, enter its length in metres and its assay grade (g/t for gold, % for base metals) — up to four intervals.
- Keep all grades in the same unit; leave unused rows blank.
- 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.
Related tools
- Downhole Depth Interval Calculator
- Logging Interval Totals Calculator
- Core Recovery Calculator
- RQD Calculator
- True Width from Intercept Length Calculator
- Dip-Corrected Interval Calculator
Explore more in Geology, Geotechnical & Ground Engineering.
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.