Weight versus mass
Mass is the amount of matter in an object and is measured in kilograms. Weight is the force gravity puts on that mass. In everyday use the two words mean the same thing, and scales are calibrated to show mass, so this converter treats them together.
The international pound has been defined as exactly 0.45359237 kg since 1959. An ounce is one sixteenth of a pound (28.3495 g) and a stone is 14 pounds (6.35 kg).
Tons are not all the same
A metric ton (tonne) is 1,000 kg. A US short ton is 2,000 lb (907.18 kg) and a UK long ton is 2,240 lb (1,016.05 kg). Precious metals use the troy ounce (31.1035 g), which is heavier than the everyday ounce, while gold in South Asia is often quoted in tola (11.664 g).