One short ton is exactly 907.18474 kilograms. It is the American ton, defined as 2,000 pounds, and it is about nine per cent lighter than the metric tonne it is constantly confused with.
About the units
The short ton is 2,000 avoirdupois pounds, a round number in its own system that becomes an awkward one in metric. It is standard in the United States for freight, mining, agriculture and waste, where it is usually written simply as "ton". The metric tonne is exactly 1,000 kg, and the long ton, the older British unit, is 2,240 pounds or 1,016.05 kg. All three share a name in ordinary English. The 2,000-pound value came from the hundredweight: the American hundredweight is 100 pounds, and twenty of them make the short ton, while the British hundredweight is 112 pounds, which is where the long ton's 2,240 comes from.
The exact factor
The derivation is exact: 2,000 × 0.45359237 kg = 907.18474 kg. In tonnes that is 0.90718474, so a short ton is a little over nine tenths of a tonne. The differences between the three tons are large enough to matter commercially. On a 10,000-unit shipment, short tons versus tonnes is a discrepancy of 928 tonnes, and short tons versus long tons is over 1,000 tonnes. Any contract or manifest that says only "tons" without qualification is a real financial risk.
Where you meet this conversion
American freight and bulk commodity trade is the primary user: coal, grain, aggregate, scrap metal and municipal waste are all quoted in short tons. Air conditioning capacity is quoted in "tons of refrigeration", which is a rate of heat transfer rather than a mass and is a different unit entirely despite the name. Vehicle ratings in the United States use fractions of a short ton, which is why a half-ton pickup is a size class rather than a payload figure.