How to Read Drill Results
What a Drill Hole Really Tells You
Decoding intercepts, grades and widths in a company's news release.
- A drill result describes a sample through the ground. It is not a resource estimate, and it is not proof of a mine.
- Read grade, interval length, depth and location together. The biggest number rarely tells the whole story.
- A reported drill interval can be longer than the deposit's true thickness.
- Check whether a headline interval includes a much smaller, higher grade section.
- The useful question is whether the result strengthens the case for a continuous, potentially mineable deposit.
Start With the Sentence
Say a company announces "20 metres at 2 grams per tonne gold from 100 metres." That means the sampled interval starts 100 metres down the drill hole, runs for 20 metres, and averages 2 grams of gold per tonne over that stretch. It does not mean the deposit is 20 metres thick everywhere.
Think of drilling as pushing a thin straw through a layered cake. The straw tells you what it passed through. You still need more samples to understand the shape and size of the whole cake.
The Four Numbers to Read Together
- Grade. How much metal the sample contains. Gold and silver are usually reported in grams per tonne, written g/t. Copper is often reported as a percentage.
- Length. How much of the drill hole the result covers. A long interval matters, but only in the context of the grade and the geology around it.
- Depth. Where the interval starts along the hole. This isn't necessarily how deep it sits below the surface.
- Location. Where the hole sits relative to earlier drilling. A result inside a known deposit answers a different question than one far beyond it.
Beginner Benchmark
What counts as an economically interesting grade. There is no universal grade that makes a drill result economic. Compare the same commodity, mining method, true width, depth, continuity and expected recovery, then consider costs and infrastructure. A promising intercept is evidence for further work, not proof of a profitable mine. Use the project’s dated technical report and its assumptions rather than a general grade threshold.Drill Length Is Not Always True Width
A hole that cuts across a mineralized zone at an angle can travel through more mineralization than the zone's actual thickness. Look for the company's statement about true width, or an acknowledgment that true width isn't yet known. The BCSC's mining disclosure guidance explains why this distinction matters.
Watch the Word "Including"
Take a hypothetical result like "20 metres at 2 g/t, including 2 metres at 15 g/t." The higher grade 2 metres is already inside the 20 metre interval. It is not extra metal stacked on top.
Using simple length weighted math, the remaining 18 metres averages about 0.56 g/t. That comes from (20 × 2) minus (2 × 15), divided by 18. This ignores rounding and sampling detail, but it shows how a narrow, rich interval can lift an overall average on its own. Read both numbers, not just the headline one.
Look Beyond One Impressive Hole
Compare nearby holes, maps and cross sections. Are results repeating across an area, or is the headline built on one isolated intercept? Check sampling methods, lab testing and quality control information too. A photo of visible mineralization is not a substitute for an assay.
Before drawing a conclusion, write down what actually changed. More evidence of continuity, a possible extension, better geological confidence, or just another data point. The next drilling program should help test that read.Want more mining news explained this way?
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Next in the Mining 101 Series
Grade, Tonnage & Scale
Why a little metal over a lot of rock can still add up to a real mine.
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