The units: g/t, ppm, oz/t, and ppb
Gold grade gets reported in several units depending on the era and the region of the publication. The math is simple but worth knowing because press releases occasionally switch units within a single document.
| Unit | What it means | Conversion | Where you see it |
|---|---|---|---|
| g/t | grams per tonne | Reference unit | Modern international standard |
| ppm | parts per million | 1 ppm = 1 g/t | Geochemistry, trace elements |
| oz/t (short) | troy oz per short ton (2,000 lb) | 1 oz/t ≈ 34.29 g/t | Older US-style reports |
| oz/t (long) | troy oz per long ton (2,240 lb) | 1 oz/t ≈ 30.61 g/t | Older UK/Commonwealth reports |
| ppb | parts per billion | 1,000 ppb = 1 g/t | Surface samples, soil geochem |
Two unit traps worth knowing:
- "oz/t" is ambiguous without ton-type disclosure. A short ton, long ton, and metric tonne all differ. A serious report specifies which. If a release writes "0.5 oz/t Au" without context, the most common modern interpretation is short ton — but if you are comparing to current g/t numbers, confirm.
- ppb headlines hide low grades. A press release announcing "850 ppb gold from a surface sample" is reporting 0.85 g/t — below the economic floor for most open-pit projects. ppb is a legitimate unit for pathfinder geochemistry and early reconnaissance work, but if ppb shows up in a hype-toned release it is usually because the equivalent g/t number sounds unimpressive.
A useful mental anchor
A standard wedding ring contains about 4–5 grams of gold. A drill intercept of "4 g/t over 50m" means that for every tonne of rock in that 50m intercept, you would recover the gold content of one wedding ring. Scale it across the volume of an entire deposit and you get to millions of ounces from low-grade rock.
What "high grade" means by mining method
There is no universal definition of "high grade." The threshold depends entirely on how the deposit will be mined. The same grade can be marginal in one context and excellent in another.
| Method | Sub-economic | Marginal | Good | High grade |
|---|---|---|---|---|
| Heap leach | < 0.3 | 0.3 – 0.5 | 0.5 – 1.0 | > 1.0 |
| Open-pit + mill | < 0.5 | 0.5 – 0.8 | 1.0 – 3.0 | > 3.0 |
| Underground | < 2 | 2 – 3 | 5 – 10 | > 10 |
| Narrow-vein UG | < 5 | 5 – 8 | 10 – 30 | > 30 (bonanza) |
Thresholds are rules of thumb at current gold prices (~$2,100/oz) and assume mining-friendly jurisdictions. Costs vary materially by location, depth, and strip ratio.
Why the same g/t means different things:
- Heap leach uses crushed ore stacked on pads and percolated with cyanide solution. Operating costs are very low, but recovery is also lower (60–75% for oxide ore). Economic at very low grades because of the cheap process.
- Open-pit + mill uses conventional crushing, grinding, and flotation/cyanide leaching. Higher cost per tonne but higher recovery (~90%+). Needs more grade to cover the processing cost.
- Underground operations cost 2–4x more per tonne than open-pit because you have to drive shafts, ventilate, dewater, and selectively mine smaller volumes. Needs proportionally more grade to be economic.
- Narrow-vein underground operations are the most expensive per tonne because the production rates are very low. Only feasible for very high grades — historically bonanza grades.
Historical vs modern grades — why the bar keeps moving
Anyone reading old mining history is struck by how absurd the grades sound compared to modern numbers. The Comstock Lode in Nevada produced ore averaging hundreds of g/t in its early years. The Witwatersrand in South Africa worked grades of 8–15 g/t for nearly a century. California Gold Rush placer miners panned grades that today would be considered surreal.
The world average grade of mined gold has fallen roughly 10-fold over the last 150 years:
- Early 1900s: global average mined grade roughly 10–20 g/t. The richest surface and shallow oxide deposits were being worked.
- Mid-1900s: roughly 4–8 g/t. The industry was deeper into underground vein systems.
- Late 1900s: roughly 2–3 g/t. The shift to bulk-tonnage open-pit operations (Carlin Trend, Witwatersrand deep mines, Australia) made lower grades economic.
- 2020s: roughly 1.2–1.5 g/t. Modern operations like Carlin Trend, Cortez, Detour Lake, and Boddington average between 0.8 and 1.5 g/t.
The decline is not a failure of exploration — it is a triumph of mining technology. Modern earthmoving equipment, cheap cyanide processing, and economies of scale have made grades that would have been unmineable in 1900 routinely profitable today. A 1 g/t open-pit mine moving 100,000 tonnes a day produces more gold per year than most historical high-grade vein mines did at 30+ g/t — and at a fraction of the investment-per-ounce cost.
The implication for investors: do not anchor on historical grades when evaluating modern juniors. A 2026 explorer hitting 3 g/t over 30m in an open-pittable setting has discovered something genuinely valuable, even though that grade would have been considered marginal in 1900.
Why grade isn't everything
Six factors matter alongside grade when comparing deposits:
- Tonnage. Total contained ounces matter more than per-tonne concentration. A 1 g/t deposit with 10 Mt of ore (320,000 contained ounces) may be worth more than a 30 g/t vein with 50,000 ounces.
- Width and geometry. A wide, continuous body is far easier to mine than a narrow, pinch-and-swell vein at the same grade. Width determines whether bulk methods can be used.
- Depth and strip ratio. A 3 g/t deposit under 200m of waste rock is far less attractive than the same deposit at surface. Strip ratios above 5:1 start to hurt; above 10:1 is generally fatal for open-pit economics regardless of grade.
- Metallurgy. Free-milling gold ore yields 90%+ recovery cheaply. Refractory ore (sulphide-locked or carbon-bearing) may recover only 60–75% even with expensive pre-treatment. A 5 g/t refractory deposit can be less economic than a 2 g/t free-milling one.
- Jurisdiction. Permitting, political stability, infrastructure, and tax regime can swing project value by 5x. A 1.5 g/t deposit in Nevada is generally worth more per ounce than a 5 g/t deposit in a high-risk jurisdiction.
- Resource confidence. A 3 g/t Inferred Resource is geologically less proven than a 2 g/t Indicated Resource of the same tonnage. See our Inferred vs Indicated vs Measured guide for why category matters as much as grade.
The best gold projects historically have rarely been the highest-grade ones. They have been the projects with the right combination of grade, tonnage, geometry, metallurgy, and jurisdiction. Grade headlines drive short-term sentiment; the combination drives long-term valuation.
Keep going
Grade is one component of reading a drill press release. Width matters as much. Geology terminology, intercept types, and the press-release tricks that inflate ordinary results matter too. The full pillar guide covers all of it with a worked example.
Read: How to Read Mining Drill Results →Frequently Asked Questions
What does g/t mean in gold mining?
g/t means grams of gold per tonne of rock. It is the standard concentration unit for precious metals in modern mining. One g/t equals one part per million (ppm). For context: a typical wedding ring contains about 4-5 grams of gold; you would need to mine an entire tonne of 4 g/t rock to recover that same amount.
What is a good gold grade?
What counts as 'good' depends on the mining method. For modern open-pit mines, 1.0-1.5 g/t Au is considered good economic grade — anything below 0.5 g/t is generally sub-economic except in very large bulk-tonnage operations. For underground mines, 5-8 g/t is good and 10+ g/t is excellent. Historical mines often worked grades 10-50x higher than modern operations because they exploited only the richest veins.
How do you convert oz/t to g/t?
1 troy ounce per short ton (oz/t) equals approximately 34.29 g/t. The exact math: 1 troy ounce = 31.1035 grams, and 1 short ton = 907.185 kg = 907.185 tonnes/1000, so 31.1035 g / 0.907185 t = 34.286 g/t. Older US-style mining publications used oz/t; modern international standards almost exclusively use g/t. Watch for ambiguity — 'oz/t' sometimes means troy ounce per long ton (2,240 lb) or even per metric tonne, which gives different conversions. Reputable reports specify the ton type explicitly.
What is bonanza grade gold?
Bonanza grade is an informal term for exceptional gold concentrations, typically above 30 g/t and often used for grades above 100 g/t. The term comes from the Spanish word for prosperity and was adopted during the California Gold Rush. Historical bonanza mines like the Comstock Lode (Nevada) and Cripple Creek (Colorado) produced ore averaging hundreds of g/t. Modern bonanza intercepts in drill results get heavy press coverage but are typically narrow — a single 5m bonanza intercept does not guarantee a mineable deposit.
Why have gold grades dropped over time?
The world's high-grade gold deposits have been preferentially mined out over the last 150 years. Early gold rushes worked surface oxide veins grading 30+ g/t. As those were exhausted, the industry moved to underground vein systems at 5-15 g/t. As those depleted, modern mining shifted to bulk-tonnage open-pit operations at 1-2 g/t made economic by industrial-scale earthmoving and heap-leach processing. The average grade of gold mined globally has fallen from approximately 10-20 g/t in the early 1900s to roughly 1.2-1.5 g/t today. This is not a sign that exploration has failed — it is a sign that mining technology has gotten dramatically better at processing low-grade rock.
Is high-grade gold always better than low-grade?
No. Grade is one variable among several. A 1 g/t deposit hosting 10 million ounces in a mining-friendly jurisdiction may be vastly more valuable than a 30 g/t deposit hosting 50,000 ounces in a politically risky one. High grade often means narrow widths, underground mining, and limited mine life. Low grade in bulk tonnage means open-pit, longer mine life, lower operating risk per ounce. The best-performing gold mines historically have not always been the highest grade — they have been the ones with the right combination of grade, tonnage, mining method, and jurisdiction.
What is gold-equivalent (AuEq) grade?
Gold-equivalent grade is a calculation used in poly-metallic deposits to combine the value of multiple metals (gold, silver, copper, etc.) into a single comparable grade expressed as g/t Au. The calculation uses assumed metal prices and recovery rates to convert non-gold metal content into 'gold-equivalent' grams. A '2 g/t AuEq' deposit might be 1.0 g/t actual gold plus 30 g/t silver and 0.2% copper, all converted at assumed prices. The honesty of AuEq depends on the assumptions disclosed — if a release uses $2,400/oz gold and the current spot price is $2,100/oz, the AuEq figure is inflated. Always check the assumption footnote.