Carats (ct) to Long Tons (long-ton) conversion

What is the Carat?

The carat is a unit of mass used to weigh gemstones and pearls. It should not be confused with the "karat," which measures gold purity.

Definition

The metric carat is defined as exactly 200 milligrams:

$$1\ \text{ct} = 0.0002\ \text{kg}$$

That is, 1 carat = 200 mg = 0.2 g exactly. Each carat is further divided into 100 "points," so a 0.25 ct stone is called a 25-point diamond.

Origin and History

The word derives from the carob seed (Greek keration), historically used as a counterweight in balancing scales because its mass was thought to be remarkably consistent. To end regional variation, the metric carat of exactly 200 mg was adopted internationally in the early 20th century, around 1907–1914.

Law and Notable Facts

The metric carat is legally recognized in most countries for the gem trade. A common misconception is that carat measures a diamond's size; it actually measures mass, so two stones of equal carat can look different depending on cut and shape.

Real-World Examples and Conversions

A typical engagement-ring diamond is around 1 carat (200 mg). The Hope Diamond weighs about 45.52 carats (≈9.10 g). Five carats equal exactly one gram.

What is the Long Ton?

The long ton is a unit of mass in the British imperial system, historically used for weighing bulk commodities such as coal, ore, and shipping cargo. It is equal to 2,240 pounds and remains familiar in the United Kingdom and in maritime contexts.

Definition

The long ton is defined as exactly 2,240 pounds avoirdupois. Since one pound equals 0.45359237 kg exactly, its SI value is:

$$1\ \text{long-ton} = 1016.05\ \text{kg}$$

Equivalently, the long ton equals 20 hundredweight (cwt) of 112 lb each, distinguishing it from the 2,000 lb US short ton and the 1,000 kg metric tonne.

Origin and History

The long ton descends from the medieval English "tun," a large cask used to ship wine, whose full weight was standardized in trade. The avoirdupois system fixed the hundredweight at 112 pounds (eight stone of 14 lb), and twenty of these made the ton, giving 2,240 lb. This "long" hundredweight and ton became the standard for British commerce and were carried throughout the Commonwealth and colonial trade.

Law and Notable Facts

The long ton is the British/imperial ton, contrasted with the 2,000 lb US short ton and the 1,000 kg metric tonne (megagram). It is legally recognized in the United Kingdom, though the metric tonne has largely replaced it for official trade. Notably, the long ton (1016 kg) sits remarkably close to the metric tonne (1000 kg), differing by only about 1.6%, which is why "ton" and "tonne" are often used loosely as near-equivalents in shipping.

Real-World Examples and Conversions

• A long ton equals 1016.05 kg, about 16 kg heavier than a metric tonne.
• Ship displacement and cargo were traditionally quoted in long tons; a 10,000 long ton vessel carries roughly 10,160 metric tonnes.
• One long ton (2,240 lb) is 240 lb heavier than one US short ton (2,000 lb), a 12% difference.
• A long ton of coal (1016 kg) is a typical unit still cited in older British mining and heating records.