One tonne is exactly 1,000 kilograms. The metric tonne is unambiguous, but the word "ton" without the final e is not — it can mean three different masses depending on the country, and the difference reaches ten per cent.
About the units
The tonne, sometimes written as the metric ton, is defined as exactly 1,000 kg. It is not formally an SI unit but is accepted for use with SI, and it fills an obvious gap: a megagram is the correct SI name and almost nobody uses it. The confusion comes from two older units with similar names. The short ton, standard in the United States, is 2,000 pounds or 907.18474 kg. The long ton, the traditional British unit, is 2,240 pounds or 1,016.0469088 kg. All three are called "ton" in ordinary English, and a shipment quoted at 500 tons could be 453.6, 500 or 508 tonnes depending on who wrote the document.
The exact factor
The factor is exactly 1,000, a decimal shift. 3.5 t is 3,500 kg; 250 kg is 0.25 t. The relationships to the other tons are worth having in the same place: a short ton is 0.90718474 t, a long ton is 1.0160469088 t, and a long ton is exactly 1.12 short tons. In freight, the practical consequence is that the metric tonne sits between the two, closer to the long ton — which is why British and metric figures often agree well enough to hide an error while American ones do not.
Where you meet this conversion
Freight, construction and agriculture are the main users. Shipping manifests, truck load limits and bulk commodity prices are quoted per tonne almost everywhere outside the United States. Carbon emissions are reported in tonnes of CO₂ equivalent, and national statistics in tonnes of oil equivalent. Structural engineering specifies loads in tonnes or kilonewtons, and confusing the two is its own separate trap since one is mass and the other force. In vehicle specifications, gross weight is in kilograms for cars and tonnes for trucks.