Pounds, kilograms and the stone
Like length, mass conversions between imperial and metric are exact by definition: one pound is exactly 453.59237 grams, agreed internationally in 1959. Everything else follows — an ounce is a sixteenth of a pound, a stone is fourteen pounds, and a US ton is two thousand pounds.
The stone survives almost exclusively for body weight in the UK and Ireland. A weight of 11 stone 4 pounds is 158 pounds, or 71.7 kilograms — the conversion needs the pounds folded in before you start, which is the step people usually miss.
Tons are three different things
A metric tonne is 1000 kg. A US short ton is 2000 pounds, about 907 kg. A UK long ton is 2240 pounds, about 1016 kg. The three differ by up to 12%, so shipping, freight and commodity figures are worth checking before you rely on them.
Precious metals add another layer: gold and silver are priced per troy ounce, which is 31.1035 grams rather than the 28.3495 grams of an ordinary avoirdupois ounce — about 10% heavier. A troy pound is only twelve troy ounces, so it is lighter than a normal pound despite the heavier ounce.