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Grams per tonne and other grade units

A grade says how much metal is in a tonne of rock. Precious metals come in grams per tonne, base metals in per cent, and some metals as an oxide. The units look different but they convert simply.

The common units

Unit	Means	Used for
g/t	grams per metric tonne (= ppm)	Gold, silver, platinum group metals
%	per cent by weight (1% = 10,000 g/t)	Copper, zinc, lead, nickel, tin, iron
ppm	parts per million (= g/t)	Rare earths (as TREO), uranium (as U₃O₈), tantalum, lithium in clays and brines
% Li₂O	lithium oxide, per cent	Hard-rock lithium (spodumene pegmatites)
% WO₃	tungsten trioxide, per cent	Tungsten
oz/t	troy ounces per short ton	Older US gold and silver reports

What 2.4 g/t means: in a tonne of rock (about 72 cm a side at 2.7 t/m³), 2.4 g of gold would make a cube about 5 mm a side. In real ore it is spread through the rock, often as grains too small to see.

Conversions

Converts using the factors in the table below. Oxide factors are ratios of molecular weights.

From	To	Multiply by
%	g/t or ppm	10,000
g/t	%	0.0001
troy oz per short ton	g/t	34.2857
troy oz per metric tonne	g/t	31.1035
grams	troy ounces	0.032151
% Li₂O	% Li	0.4645
Li₂O	lithium carbonate equivalent (LCE)	2.473
Li	LCE	5.323
WO₃	W	0.7930
Ta₂O₅	Ta	0.8190
Nb₂O₅	Nb	0.6990
SnO₂	Sn	0.7877
U₃O₈	U	0.8480

The oxide factors are ratios of molecular weights, so they hold for any grade or tonnage.

From grade to contained metal

Tonnes times grade gives contained metal. A resource of 10 million tonnes at 1.5 g/t gold holds 15 million grams, which is about 482,000 troy ounces (15,000,000 × 0.032151). A copper resource of 200 million tonnes at 0.45% Cu holds 900,000 tonnes of copper. Contained metal is in the ground; how much is recovered depends on mining and processing, which is what a reserve accounts for.

Worked example: a lithium intercept

A release reports 20 m at 1.45% Li₂O. In lithium metal that is 1.45 × 0.4645 = 0.67% Li; in lithium carbonate equivalent it is 1.45 × 2.473 = 3.6% LCE. All three describe the same rock, which is why it matters which one a headline uses.

Metal equivalents

When a deposit has more than one valuable metal, companies may combine them into one figure such as gold equivalent (AuEq) or copper equivalent (CuEq). The result depends on the metal prices and the metallurgical recoveries assumed. The JORC Code and NI 43-101 both require those assumptions to be disclosed, so look for them next to the figure.

Questions

What does g/t mean in mining?

Grams per tonne: grams of metal in each metric tonne (1,000 kg) of rock. 1 g/t is one part per million, so 2.4 g/t gold means 2.4 grams of gold in every tonne.

How many grams per tonne is 1%?

10,000 g/t. One per cent is 10 kg in a tonne, which is 10,000 grams, or 10,000 ppm.

How do you convert ounces per ton to grams per tonne?

Multiply by 34.2857 for troy ounces per short ton (the old US unit). For troy ounces per metric tonne, multiply by 31.1035.

Why is lithium reported as Li2O?

Hard-rock lithium is reported as lithium oxide, the convention for spodumene. Multiply % Li2O by 0.4645 to get % lithium metal, or by about 2.473 to get lithium carbonate equivalent (LCE).

Sources

An explainer of how mining companies report their work, written from the public codes and rules linked above. It describes the documents and does not rate any company, project or result. Not investment advice.