Short ton, long ton, metric tonne: why 'a ton' means three different things

"A ton" sounds like it should mean one specific weight. It doesn't. Order a ton of gravel in the US and a ton in the UK, and you'll get two different amounts of gravel, plus a third amount if whoever's selling it means the metric tonne. Nobody involved is being imprecise on purpose; there are genuinely three different units all sharing one name.

Short tons, long tons, and where the split happened

Both the US short ton and the UK long ton grew out of the same starting point, the hundredweight, but the two countries ended up defining a hundredweight differently. The US settled on a hundredweight of 100 pounds, which makes a ton (20 hundredweight) come out to a clean 2,000 pounds, hence "short" ton. The UK kept an older hundredweight of 112 pounds, making its ton 2,240 pounds, the "long" ton. Then there's the metric tonne, defined simply as 1,000 kilograms, which converts to about 2,204.6 pounds, sitting almost exactly between the other two. Depending on which one gets used, the same word can mean anywhere from 2,000 to 2,240 pounds, a spread of over 10%.

US short ton   = 2,000 lb   = 907.18 kg
Metric tonne   = 2,204.6 lb = 1,000 kg
UK long ton    = 2,240 lb   = 1,016.05 kg

Shipping and freight are where this actually causes problems in practice, since bills of lading, weight limits, and customs paperwork don't always specify which ton they mean, and a 10%+ error on a heavy shipment isn't trivial.

The pound itself comes from Rome

The abbreviation "lb" doesn't look like it should stand for "pound," and it doesn't, it's short for the Latin libra, the Roman unit of weight the English pound descended from. That same root is also where the £ symbol for British currency comes from (a pound sterling was originally, in fact, a pound of silver). The modern avoirdupois pound, the one used for body weight, groceries, and just about everything else, was pinned to exactly 453.59237 grams in that same 1959 international agreement that fixed the yard.

Why gold isn't priced in that pound

Here's a detail that trips people up when they start paying attention to gold and silver prices: those markets use troy ounces, not the regular (avoirdupois) ounce this converter uses for everyday weight. A troy ounce is about 31.1 grams, versus 28.35 grams for a normal ounce, a difference of roughly 10%. A troy pound is also lighter than a regular pound (12 troy ounces instead of 16). The troy system is an old European jewelers' and mint standard that never got replaced for precious metals specifically, so it just kept running in parallel with the everyday pound and ounce, and still does.

And the stone, while we're here

In the UK and Ireland, body weight often gets quoted in stone, where 1 stone equals 14 pounds. Someone saying they weigh "11 stone 6" means 11 stone plus 6 pounds, or 160 pounds total. It's essentially unused outside those two countries, but common enough there that it's worth recognizing if you run into it.