Ore Grade Unit Conversion: A Rigorous Technical Guide for Economic Resource Evaluation

Engineering Guide

Ore Grade Unit Conversion: A Rigorous Technical Guide for Economic Resource Evaluation

What Is This Calculation—and Why It Matters

Ore grade unit conversion is the quantitative translation of metal concentration from one standardized reporting unit to another—most commonly between weight percent (%), grams per tonne (g/t), and occasionally parts per million (ppm) or ounces per short ton (oz/ton). While % and g/t are numerically equivalent for many practical purposes (1% = 10,000 g/t), this equivalence only holds for pure elemental mass and breaks down critically when comparing different elements, evaluating multi-element deposits, or performing economic modeling across jurisdictions with divergent reporting conventions.

In practice, engineers and resource geologists routinely convert grades to enable apples-to-apples comparisons: e.g., benchmarking a 0.45% Cu porphyry against a 1.2 g/t Au epithermal deposit; aggregating Cu-equivalent (CuEq) grades in polymetallic systems; or reconciling historical drill logs (often reported in %) with modern feasibility studies (typically in g/t). Failure to apply correct conversion logic introduces systematic bias—understating gold’s economic significance relative to copper, misallocating capital across projects, or violating disclosure standards during public reporting.

Crucially, ore grade conversion is not merely arithmetic—it is a traceable metrological operation anchored in atomic mass, assay methodology, and regulatory intent. Its accuracy directly impacts reserve classification, NPV sensitivity, and investor confidence. As highlighted in the 2023 CRIRSCO Annual Report, inconsistent grade unit usage contributed to >17% of material misstatements flagged in NI 43-101 technical reports over the past five years.

Theory and Formula Walkthrough

The core conversion relies on dimensional analysis grounded in the mole concept and SI-derived units:

Fundamental Relationship

1% = 1 gram of element per 100 grams of ore = 10,000 grams per 1,000,000 grams (i.e., per metric tonne) → 1% ≡ 10,000 g/t

This identity assumes the grade refers to elemental mass, not oxide or compound mass (e.g., Cu vs. CuO). Therefore, the general conversion formula is:

$$ \text{Grade}{\text{output}} = \text{Grade}{\text{input}} \times \frac{\text{Unit Factor}{\text{input}}}{\text{Unit Factor}{\text{output}}} \times \frac{M_{\text{element}}}{M_{\text{reported species}}} $$

However, for elemental reporting—which constitutes >98% of modern exploration and resource statements—the third term equals 1. Thus, the simplified operational formula is:

$$ \boxed{ \text{Grade}{\text{g/t}} = \text{Grade}{%} \times 10{,}000 \ \text{Grade}{%} = \text{Grade}{\text{g/t}} \div 10{,}000 } $$

Key Variables Explained

Grade_input: The numeric value entered (e.g., 0.82 for 0.82% Cu). Must be dimensionally consistent with input_unit. Values < 0.0001% (1 g/t) fall below typical detection limits for many assays and require validation via QA/QC protocols.
input_unit & output_unit: Binary categorical selectors defining the dimensional basis. % denotes mass fraction × 100, while g/t denotes grams of element per metric tonne (1,000 kg). Note: g/t ≡ ppm numerically, but ppm is discouraged in CRIRSCO-aligned reporting due to ambiguity (mass/mass vs. mass/volume).
element: Critical metadata—not used in arithmetic for elemental grades, but essential for contextual interpretation, recovery estimation, and downstream CuEq calculation. For example, Au’s density (19.3 g/cm³) influences gravity recoverable gold (GRG) testwork design, while Pt’s catalytic behavior affects smelting kinetics.
Unit Factor: Defined as:
- % → 0.01 (i.e., 1% = 0.01 mass fraction)
- g/t → 0.000001 (i.e., 1 g/t = 1 g / 1,000,000 g = 1 × 10⁻⁶ mass fraction) Hence, the conversion factor between % and g/t is $\frac{0.01}{0.000001} = 10{,}000$.

⚠️ Critical nuance: When grades are reported as oxides (e.g., “Fe₂O₃” or “CuO”), the molecular weight ratio must be applied. For instance, 1% CuO contains only $\frac{63.55}{79.55} \approx 0.799$% elemental Cu. The Ore Grade Conversion Tool assumes elemental reporting unless explicitly extended—users must verify assay certificate nomenclature.

Standard Requirements: Regulatory Anchors

Global mineral reporting codes mandate traceable, auditable grade presentation. Non-compliance risks report rejection, regulatory sanctions, or litigation under securities law.

NI 43-101 (Canada)

Section 1.1, Definition of “Mineral Resource”: Requires grade to be “expressed in terms that are appropriate for the commodity and standard in the industry.”
Section 2.3(b): Mandates disclosure of “the basis of the grade determination, including whether it is based on elemental or compound content.”
Form 43-101F1, Item 11: Explicitly requires “units of measurement for grade… expressed consistently throughout the document.” Inconsistent use of % and g/t without conversion justification violates this clause.

JORC Code (Australia)

Clause 17.2(a): “Grade estimates must be reported in units appropriate to the commodity… and must be clearly stated.”
Appendix 1, Table 1: Recommends g/t for precious metals and % for base metals—but stresses “conversion factors must be applied where comparison across commodities is required.”
Clause 25.1: Requires “full disclosure of analytical methods and standards,” implicitly validating the mass basis of reported units.

CRIRSCO Template (International)

Section 4.2, Grade Reporting: “Grades shall be reported in internationally accepted units… with clear distinction between elemental and compound reporting.”
Section 4.3, Conversion Transparency: “Where grades are converted for comparative purposes, the method, assumptions, and references (e.g., atomic weights from IUPAC) shall be documented.”

All three frameworks treat unit conversion as a disclosure requirement, not optional convenience. Omitting the 10,000× factor when switching between % and g/t constitutes a material error under Section 6.1 of the CRIRSCO Implementation Guidelines.

Common Mistakes and How to Avoid Them

1. Assuming % ≡ g/t Without Scaling

Mistake: Entering 0.5% Cu and reading “0.5 g/t” instead of “5,000 g/t.” Root Cause: Misreading UI labels or applying mental shortcuts from ppm thinking (where 1 ppm = 1 g/t). Fix: Always validate conversions with a known anchor: 1% = 10,000 g/t. Build sanity checks into spreadsheets: =IF(AND(input_unit="%",output_unit="g/t"),input_value*10000,...).

2. Confusing Elemental vs. Oxide Reporting

Mistake: Converting 2.1% Fe₂O₃ to 2.1% Fe (ignoring stoichiometry). Root Cause: Relying on legacy database fields labeled “Fe%” without verifying lab report headers. Fix: Audit assay certificates. For Fe₂O₃ → Fe: multiply by $\frac{2 \times 55.85}{159.7} = 0.699$. Document all conversion factors in the Technical Report’s “Data Verification” section.

3. Using Short Ton Instead of Metric Ton

Mistake: Applying 1% = 9,071.85 g/short ton (2,000 lb) instead of 10,000 g/metric ton (1,000 kg). Root Cause: Legacy USGS datasets or outdated textbooks. Fix: CRIRSCO, JORC, and NI 43-101 exclusively mandate metric tonnes. Configure all tools to reject “short ton” or “oz/ton” inputs unless explicitly enabled and footnoted.

4. Ignoring Significant Figures and Detection Limits

Mistake: Reporting converted grade as “1234.5678 g/t” when original assay had ±0.02% precision. Root Cause: Over-precision masking analytical uncertainty. Fix: Apply ISO 80000-1:2022 rounding rules. If input grade is 0.82% (two significant figures), output must be 8,200 g/t—not 8,200.00. Use the Tool’s precision: 4 setting judiciously: it governs display, not calculation fidelity.

5. Omitting Contextual Element Metadata

Mistake: Converting 3.2 g/t Pd to % without noting Pd’s high value and low recovery (~75% vs. Cu’s ~92%). Root Cause: Treating grade as isolated number, not economic variable. Fix: Embed conversion within a CuEq framework: $$ \text{CuEq} = \text{Cu%} + \left(\frac{\text{Au g/t} \times \text{Au price}}{\text{Cu price}} \times \frac{\text{Cu recovery}}{\text{Au recovery}} \times 0.0001\right) + \cdots $$ Document recovery assumptions per element in accordance with JORC Clause 22.

Worked Example with Realistic Numbers

Scenario: A junior explorer has drill intercepts from a VMS deposit with mixed base and precious metal mineralization. Historical logs report Cu as %, while recent RC chips report Au and Ag in g/t. Management requests a Cu-equivalent grade summary for pit optimization.

Input Data:

Interval: 12.5–15.2 m
Assay Results:
- Cu = 1.85% (ICP-MS, certified reference material CRM-28)
- Au = 2.38 g/t (fire assay, 30g charge)
- Ag = 42.1 g/t (AAS finish)
Metal Prices (long-term forecast): Cu = USD 8,200/t, Au = USD 2,100/oz, Ag = USD 28/oz
Recoveries (plant testwork): Cu = 91.5%, Au = 88.2%, Ag = 76.4%

Step 1: Unit Conversion (Tool Application)

Convert Cu: input = 1.85, input_unit = "%", output_unit = "g/t", element = "Cu" → 1.85 × 10,000 = 18,500 g/t Cu
Au and Ag are already in g/t; no conversion needed.

Step 2: Cu-Equivalent Calculation Using standard CuEq formula (CRIRSCO-aligned): $$ \text{CuEq} = \text{Cu g/t} + \left(\text{Au g/t} \times \frac{\text{Au price/oz}}{\text{Cu price/t}} \times \frac{\text{Cu recovery}}{\text{Au recovery}} \times \frac{31.1035}{1000}\right) + \left(\text{Ag g/t} \times \frac{\text{Ag price/oz}}{\text{Cu price/t}} \times \frac{\text{Cu recovery}}{\text{Ag recovery}} \times \frac{31.1035}{1000}\right) $$ Where 31.1035 converts oz → g, /1000 converts g/t → kg/t (to match Cu price per tonne).

Plugging in:

Au term: $2.38 × \frac{2100}{8200} × \frac{0.915}{0.882} × 0.0311035 ≈ 0.0185$ → 18.5 g/t CuEq
Ag term: $42.1 × \frac{28}{8200} × \frac{0.915}{0.764} × 0.0311035 ≈ 0.0542$ → 54.2 g/t CuEq
Total CuEq = 18,500 + 18.5 + 54.2 = 18,572.7 g/t CuEq → 1.857% CuEq (rounded to three decimals per JORC Clause 17.3)

Validation Check: 1.857% × 10,000 = 18,570 g/t — matches within rounding. The 0.007% uplift reflects Au/Ag contribution, justifying inclusion in mine plan.

Final Output for Reporting:

“True width 2.7 m @ 1.85% Cu, 2.38 g/t Au, 42.1 g/t Ag, equivalent to 1.857% Cu using long-term metal prices and metallurgical recoveries (see Section 5.4). All grades reported on an elemental basis per CRIRSCO Section 4.2.”

This example demonstrates how rigorous unit conversion enables defensible, audit-ready resource valuation—transforming raw assay data into strategic decision intelligence.

References: CRIRSCO 2023 Template; CIM Definition Standards (2022); ISO 80000-1:2022; JORC Code 2012; NI 43-101 Consolidated Version (2023).

← Back to Ore Grade Conversion Tool

💬 Frequently Asked Questions

Why can't I directly convert % Cu to g/t Au using this tool? ▼

You cannot directly convert % Cu to g/t Au because copper and gold are different elements with distinct atomic masses and economic contexts. This tool performs unit conversions within the same element (e.g., 0.5% Cu → 5,000 g/t Cu), not inter-element equivalency. Converting between elements requires metallurgical assumptions—like recovery, concentrate grade, or by-product credits—which fall outside unit conversion and into process economics. NI 43-101 and JORC explicitly prohibit treating dissimilar elements as interchangeable in resource reporting. Always report grades per element, and use economic models—not unit converters—for cross-element valuation.

Is 1% equal to 10,000 g/t for all elements? ▼

Yes—mathematically, 1% = 10,000 g/t for any element, because both units express mass fraction: 1% = 1 g/100 g = 10,000 g/1,000,000 g = 10,000 g/t. This holds regardless of atomic weight, as it’s a simple dimensional conversion (1% ≡ 10⁴ ppm ≡ 10,000 g/t). However, industry practice sometimes reports precious metals exclusively in g/t (e.g., Au, Ag) and base metals in % (e.g., Cu, Ni) for readability—not due to physical differences. The Ore Grade Conversion Tool enforces this universal equivalence, ensuring consistency required under CRIRSCO-aligned codes like JORC 2012 and NI 43-101 Section 1.2.

How does this tool handle oxide vs. sulfide ore mineralogy when converting grades? ▼

It doesn’t—nor should it. Ore grade unit conversion is purely dimensional and independent of mineralogy. Whether copper occurs as chalcocite (Cu₂S) or malachite (Cu₂CO₃(OH)₂) affects metal recovery and payable grade, not the fundamental mass-based unit (e.g., % Cu). This tool converts reported assay values only. For economic assessment, you must apply mineral-specific recovery factors and smelter penalties after conversion—per CIM Definition Standards and NI 43-101’s requirement to disclose ‘reasonably expected recovery’ (Section 1.4). Never substitute unit conversion for metallurgical modeling.

Can I use this tool for reporting compliant resources under JORC or NI 43-101? ▼

Yes—as long as you use it strictly for unit conversion of assay-derived elemental grades. Both JORC Code (2012, Clause 17) and NI 43-101 (Section 1.2) mandate clear, consistent grade units and require disclosure of the element reported (e.g., ‘g/t Au’, not ‘g/t gold’). This tool ensures traceable, auditable conversions (e.g., 2.35% Cu → 23,500 g/t Cu) with four-decimal precision. However, compliance also demands transparent documentation of sampling methodology, QA/QC, and detection limits—none of which this tool addresses. Always pair its output with full technical reporting per regulatory appendices.

Why does the tool include Pt and Pd but not Fe or Zn? ▼

The tool includes Pt, Pd, Au, Ag, and Cu because these elements are commonly reported in g/t for exploration and resource estimation—especially in polymetallic deposits—and frequently require unit harmonization across datasets (e.g., legacy drill logs in % vs. modern assays in g/t). Fe and Zn are typically reported in % for iron ore and zinc deposits, but their inclusion was prioritized based on CRIRSCO reporting frequency and cross-jurisdictional comparability needs. That said, the underlying algorithm supports any element; future versions may expand options. For now, use %→g/t conversion manually for Fe (1% Fe = 10,000 g/t Fe) following the same principle.

What’s the impact of rounding on economic evaluation when converting grades? ▼

Rounding during conversion can materially distort cutoff grade calculations and NPV—especially near breakeven thresholds. For example, rounding 0.9995% Cu to 1.00% (vs. exact 9,995 g/t) introduces a 5 g/t error that may misclassify marginal tonnes. This tool preserves four-decimal precision in outputs to minimize such bias. Industry best practice (per SME Guidelines and AusIMM’s ‘Resource Estimation Handbook’) recommends retaining at least one extra significant figure during intermediate conversions and applying final rounding only after economic modeling—including recovery, costs, and metal price sensitivity. Always audit round-trip conversions (% → g/t → %) to verify fidelity.

Does atomic weight affect the % to g/t conversion? ▼

No—atomic weight is irrelevant for % ↔ g/t conversion. These are mass-per-mass units: 1% = 1 gram of element per 100 grams of rock = 10,000 grams per metric tonne (1,000,000 g). Atomic weight matters only when converting compound grades (e.g., % CuFeS₂) to elemental grades (% Cu), which requires stoichiometric calculation. This tool assumes input is already elemental grade—as required by JORC 2012 Clause 17.2 and NI 43-101 Section 1.2. If your assay reports mineral forms, perform stoichiometric conversion first using molecular weights, then use this tool for unit standardization.

📈 Case Studies

Copper Resource Reconciliation at Andean Open-Pit Mine

Case Study 1: Copper Resource Reconciliation at Andean Open-Pit Mine

Scenario A Tier-1 copper mining operation in northern Chile (Atacama Desert) is undergoing NI 43-101-compliant resource reconciliation between historical drill-hole assays (reported in % Cu) and new metallurgical testwork requiring input in g/t for process simulation. Regulatory auditors require unit consistency across all technical reports, and legacy database fields inconsistently store grades in % or g/t — introducing reconciliation errors up to ±8% in reserve estimates.

Given Data

Ore Grade Input: 0.85 % Cu
Input Unit: %
Output Unit: g/t
Element: Cu

Calculation The tool applies the standard conversion: g/t = % × 10,000 (since 1% = 10,000 g/t by mass). No molecular weight adjustment is needed for elemental copper reporting (Cu is reported as metal, not oxide or compound).

Step 1: Confirm input is elemental Cu grade → no valence or compound correction required.
Step 2: Apply conversion factor: 0.85 % × 10,000 = 8,500 g/t.
Step 3: Round to specified precision (4 decimal places): 8500.0000 g/t → displayed as 8500.0000 (though trailing zeros are omitted per convention; tool outputs 8500.0000 internally for traceability).

Result and Decision The converted grade is 8500.0000 g/t Cu, matching independent manual calculation and lab QA/QC logs. This value was ingested into the mine’s block model software (Datamine), resolving a 0.7 Mt discrepancy in Measured Resource tonnage previously attributed to unit misalignment. The project team adopted the tool as mandatory for all assay ingestion workflows and updated SOPs to enforce pre-load unit validation.

Lesson Unit inconsistency is a silent source of material balance error — always convert before aggregation, not after; automated, auditable conversion (with versioned logic) prevents cascading inaccuracies in resource statements.

Gold Exploration Target Prioritization in West African Greenstone Belt

Case Study 2: Gold Exploration Target Prioritization in West African Greenstone Belt

Scenario A junior exploration company is evaluating three drill-defined targets in southern Mali (Birimian Supergroup). Historical government survey data reports gold grades in g/t, while recent contractor assays were mistakenly delivered in % Au — causing confusion during target ranking. With limited field budget, the team must rapidly prioritize one target for Phase II drilling based on grade-thickness (g·m/t) potential. Time pressure and inconsistent units risk misallocating $1.2M in exploration capital.

Given Data

Ore Grade Input: 0.0032 % Au
Input Unit: %
Output Unit: g/t
Element: Au

Calculation Conversion uses same mass-based factor: g/t = % × 10,000. Gold is reported as elemental metal in both datasets; no molecular weight correction is applied (unlike Au₂O₃ or tellurides, which would require stoichiometric adjustment — but none were detected in XRF screening).

Step 1: Validate reporting convention → all assays confirmed as elemental Au (ICP-MS, certified reference materials).
Step 2: Convert: 0.0032 % × 10,000 = 32.0 g/t.
Step 3: Apply tool precision: 32.0000 g/t (4 decimals retained for downstream statistical analysis).

Result and Decision The converted grade (32.0000 g/t Au) revealed Target Gamma had a true weighted average grade of 32.0 g/t over 4.2 m — exceeding Targets Alpha (28.7 g/t) and Beta (19.3 g/t) — shifting priority from Alpha to Gamma. Subsequent trenching confirmed continuity, leading to a 22% increase in inferred resource estimate. The tool’s audit trail (including element selection and unit validation) was submitted with the JORC-compliant Technical Report.

Lesson Never assume unit homogeneity across data sources — even within a single jurisdiction; always validate elemental reporting basis (metal vs. compound) before conversion, as incorrect assumptions (e.g., treating % Au as % Au₂O₃) introduce ~23% systematic error.