per tonne

Prices per tonne are how the world's bulk trades speak: iron ore, wheat, cement, scrap steel, fertiliser and ocean freight are all quoted this way, and the arithmetic down to the kitchen scale is mercifully clean, since a dollar per tonne is exactly a tenth of a cent per kilogram. This converter's usual job is carrying a per-tonne quote down to the per-kilogram or per-pound figure a smaller purchase actually happens at, and as with every cost rate on the site the price moves opposite to the unit: smaller mass, smaller number.

Two habits protect a comparison. The first is in the caution above, since North American quotes frequently mean the 2000-pound short ton, about 9 percent less material than the metric tonne. The second is to ask what the tonne is a tonne of: ores, wood chips and treatment sludges trade per dry tonne, with moisture measured and deducted, so a per-tonne price on wet material is not comparable with one quoted on a dry basis.

The most consequential commodity now priced in this unit is invisible. Carbon markets and carbon taxes price emissions per tonne of CO₂, and a tonne of CO₂ at room conditions occupies about 500 cubic metres, a cube roughly eight metres on a side. Anyone who checks that figure once never again pictures a tonne of gas as a small thing.