In American English, a ton is a unit of measurement equaling 2,000 pounds. In non-U.S. measurements, a ton equals 2,240 pounds. A tonne, also known as a metric ton, is a unit of mass equaling 1,000 kilograms.
American English speakers generally have no use for tonne, so the spelling rarely appears in U.S. publications. Elsewhere, fastidious publications use the appropriate spellings for the units of measurement. And ton (often pluralized) is used informally as a noun meaning a large extent, amount, or number.
British, Canadian, and Australian publications generally reserve tonne for very narrow uses (i.e., in reference to the metric ton)—for example:
Almost 30 firefighters tackled a blaze involving 2,000 tonnes of rubbish at a recycling centre in Southampton. [BBC News]
His own crop was reduced from 4000 tonnes to 3000 tonnes because of the floods. [Herald Sun]
B.C.’s carbon tax is a weensy $20 a tonne, or about four-and-a-half cents per litre at the gas pump. [Vancouver Sun]
All use ton (or tons) in contexts unrelated to measurement—for example:
Many people are interviewed in the series, but a dominant voice is that of Robbie Robertson, who has tons of stories. [Globe and Mail]
A Denver medical marijuana dispensary says it is literally trading a ton of joints for a ton of food. [Denver Post]
The feelings of love he’d expected to hit him like a ton of bricks when little Phoebe arrived simply didn’t come. [Mirror]
Better yet, Klugt says, her iPhone has tons of cool apps and the digital jukebox, iTunes. [Sydney Morning Herald]