Ore Reserve Calculator
Estimate proven and probable reserves from drill data, cutoff grade, and metal price for mine planning.
About this calculator
This calculator provides a simplified, planning-level estimate of ore reserve tonnage and contained metal, not a formal reserve statement. "Proven" and "Probable" are real reserve-classification terms used across the major international reporting codes (JORC in Australia, NI 43-101 in Canada, SEC S-K 1300 in the United States), which are broadly aligned through the CRIRSCO family of standards -- but under those codes, classifying a reserve as Proven or Probable requires a Qualified or Competent Person's formal assessment of geological confidence plus a full set of "modifying factors" (mining, processing, economic, legal, environmental, and social factors demonstrating the material can be economically extracted), documented in an auditable technical report. This calculator only splits Economic Tonnage between Proven and Probable using a single user-entered Proven Confidence percentage, so it cannot itself produce a compliant reserve statement -- it is a quick estimation tool for early- stage planning, not a substitute for the geological and economic work a real reserve report requires.
Economic Tonnage is estimated by assuming a simplified, roughly linear relationship between how far Average Grade sits above Cutoff Grade and the fraction of drilled tonnage that clears the cutoff; a wider margin above cutoff grade raises the economic fraction toward 100%, while grade at or below cutoff yields zero economic tonnage. Recoverable Metal and Gross Metal Value then apply Mill Recovery and Metal Price to the contained-metal estimate.
Inputs
Results
Economic Tonnage
857,143 t
How to Use This Calculator
- Enter drilled tonnage and average grade (ppm or g/t) from drill hole assay data.
- Set the cutoff grade (ppm) — the minimum grade at which mining is economic.
- Enter proven confidence (%) to split reserves into proven and probable categories.
- Input metal price ($/oz) and mill recovery (%) for economic value calculation.
- Review Economic Tonnage, Proven Reserve (t), and Contained Metal as a quick planning-level estimate — not a substitute for a Qualified/Competent Person's formal JORC or NI 43-101 reserve report.
How the result changes with Drilled Tonnage
| Drilled Tonnage | Economic Tonnage |
|---|---|
| 500,000 | 428,571 t |
| 750,000 | 642,857 t |
| 1,500,000 | 1,285,714 t |
| 2,500,000 | 2,142,857 t |
What each input means
- Drilled Tonnage
- Total tonnage estimated from drill data
- Average Grade
- Average metal grade in grams per tonne
- Cutoff Grade
- Minimum economic grade for ore classification
- Proven Confidence
- Percentage of reserve classified as proven (rest is probable)
- Metal Price
- Current metal price per troy ounce
- Mill Recovery
- Expected metal recovery percentage in processing
How this is calculated
Worked example, using the default values
- Identify Input Parameters6 parametersDrilled Tonnage = 1000000, Average Grade = 3.5, Cutoff Grade = 1, Proven Confidence = 60, Metal Price = 1900, Mill Recovery = 90 = 6 input(s) provided
- Calculate Economic TonnageEconomic Tonnage857143 = 857143
- Calculate Proven ReserveProven Reserve514286 = 514286
- Calculate Probable ReserveProbable Reserve342857 = 342857
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does this calculator produce a JORC or NI 43-101 compliant reserve statement?
No. "Proven" and "Probable" are real terms from those reporting codes, but a compliant reserve statement requires a Qualified or Competent Person's formal sign-off on geological confidence plus documented modifying factors (mining, processing, economic, legal, and other factors) in an auditable technical report. This calculator is a simplified planning estimate using a single user-entered confidence split, not a substitute for that formal process.
Why does raising the Cutoff Grade lower the Economic Tonnage?
Cutoff Grade is the minimum grade at which material is considered economic to mine. Raising it while Average Grade stays fixed narrows the margin between the two, which this calculator's simplified formula translates into a smaller fraction of the Drilled Tonnage counted as economic -- fewer tonnes clear a higher bar.
How is the Proven vs. Probable split determined?
This calculator splits the estimated Economic Tonnage directly using the Proven Confidence percentage entered by the user -- for example, a 60% entry assigns 60% of economic tonnage to Proven and the remaining 40% to Probable. In a real reserve estimate, this split would instead come from the underlying geological confidence category (Measured vs. Indicated resources) rather than a single input value.
Does Metal Price affect how much reserve tonnage is classified as economic?
No. Metal Price only enters the calculation after tonnage has already been split into Economic Tonnage and Proven/Probable categories -- it is used solely to convert Recoverable Metal (ounces) into Gross Metal Value (dollars). It has no effect on the Economic Tonnage or the Proven/Probable split themselves.
What does Mill Recovery represent in the Recoverable Metal calculation?
Mill Recovery is the percentage of contained metal actually extracted during processing -- real ore processing never recovers 100% of the metal present in the rock, so Recoverable Metal multiplies Contained Metal by this recovery percentage to estimate the metal that will actually reach saleable form, which is always less than the total metal physically present in the economic tonnage.
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