Ton
The ton is a unit of mass or weight primarily used in the avoirdupois system, with variations including the short ton (2,000 pounds or approximately 907.18 kilograms) common in the United States, the long ton (2,240 pounds or approximately 1,016.05 kilograms) historically used in the United Kingdom and some Commonwealth countries, and the metric ton or tonne (1,000 kilograms or approximately 2,204.62 pounds) adopted internationally under the metric system.[1][2] The term originates from the Old English "tunne," referring to a large cask or barrel for wine or ale, whose capacity and typical filled weight established the unit's foundational value in medieval trade and shipping.[3] Historically, the ton evolved from ancient volumetric measures tied to cargo like barrels of goods, with the short and long tons emerging in the 19th century to standardize imperial weights for commerce, coal, and shipping, while the metric ton was formalized in 1875 via the Metric Convention to align with the kilogram for global consistency.[1] These distinctions persist due to lingering imperial influences, leading to potential confusion in international trade; for instance, one short ton is about 0.907 metric tons, while one long ton is roughly 1.016 metric tons.[4] Beyond mass, "ton" also denotes derived units like the ton of refrigeration (capacity to freeze one short ton of water in 24 hours) in engineering and the displacement ton in naval architecture for ship volume.[3] In modern usage, the metric ton dominates scientific, industrial, and global contexts, reflecting the widespread adoption of the International System of Units (SI).[2]Introduction
Definition
A ton is a unit of measurement historically employed to quantify large quantities of mass or volume, with its exact value varying by regional standards and context. In terms of mass, it typically represents between 2,000 and 2,240 pounds (approximately 907 to 1,016 kilograms), serving as a non-metric imperial unit primarily in trade, shipping, and industry.[5] For volume, particularly in maritime freight, a ton often denotes 40 to 100 cubic feet (approximately 1.13 to 2.83 cubic meters), though specific applications like the measurement ton equate to 40 cubic feet.[6] The ton is not an official unit of the International System of Units (SI), but the metric variant—known as the tonne and defined as exactly 1,000 kilograms—is accepted for use alongside SI units due to its alignment with the kilogram and the SI prefix "kilo-".[7] This acceptance stems from the tonne's adoption by the International Committee for Weights and Measures (CIPM) in 1879, facilitating its integration in scientific and commercial contexts where SI compatibility is essential.[7] Non-metric tons, such as the short ton (2,000 pounds) common in the United States and the long ton (2,240 pounds) used in the United Kingdom, lack formal SI endorsement but remain prevalent in customary systems.[5] Symbols for the ton vary to distinguish variants: the general symbol is "t" for the tonne, while "tn" or "sh tn" may denote the short ton, and "lt" or "LT" the long ton.[8] In volume measurements, terms like "freight ton" or "measurement ton" emphasize the cubic capacity rather than weight, often applied to assess cargo space in shipping without direct mass correlation.[6]Scope and Variations
The long ton predominates in the United Kingdom and other Commonwealth nations, where it serves as the standard unit for mass in various applications, while the short ton is the primary mass unit in the United States. In contrast, the metric tonne—defined as exactly 1,000 kilograms—has emerged as the global standard for mass measurements, influenced by the widespread adoption of the International System of Units (SI) in most countries outside the traditional imperial spheres.[9] Ton units encompass both mass-based and volume-based variants, with mass tons quantifying weight for commodities like cargo, and volume tons assessing capacity in contexts such as ship holds, where the two lack direct equivalence due to differing physical properties. For instance, the register ton, a volume measure, equates to 100 cubic feet of enclosed space, independent of the actual mass it might contain. Throughout the 20th century, international trade and scientific practices shifted toward the metric tonne as the preferred unit, aligning with broader metrication efforts to streamline global exchanges and precision in research. Imperial variants like the long and short tons persist in legacy sectors, including mining operations and maritime shipping, where historical conventions maintain compatibility with existing infrastructure.[10] The United Nations Economic Commission for Europe (UNECE) provides codes for units of measure used in international trade, incorporating standardized definitions for various tons to reduce discrepancies and support fair global transactions.[11]Etymology and History
Etymology
The term "ton" originates from the Old English word tunne, referring to a large cask or barrel used for storing wine, ale, or beer in Anglo-Saxon England, where it symbolized a standard unit of volume.[12] This usage traces back further to pre-12th-century influences, including Roman adoption of wooden barrel technology from the Gauls, whose Celtic term tondā—meaning "surface" or "skin"—evolved into the Latin tunna for a large container or tun.[13] North Sea Germanic languages, including those spoken by Viking traders, also contributed to the spread of similar barrel standards across Europe, reinforcing the term's association with large-scale liquid measures.[12] By the 12th century, Medieval Latin tunna had been integrated into English trade practices, where the barrel's contents began to denote a corresponding weight measure, marking the transition from volume to mass in commercial contexts.[14] The spelling shifted from "tun" to "ton" by the late 14th century, reflecting broader Middle English adaptations for a defined capacity in commodities like timber, stone, and iron.[15] In the 18th century, the French variant tonne—itself derived from the same Latin root—influenced the metric system's adoption of a standardized mass unit, distinguishing it from imperial forms.[16] Linguistic variations highlight the term's Indo-European roots tied to barrel measures: German employs Tonne for both volume and the metric ton, while Spanish uses tonelada, stemming directly from tonel (a large cask akin to the tun).[15] These cognates underscore a shared heritage in ancient container-based quantification across Romance and Germanic languages.[17]Historical Development
The ton as a unit of mass originated in medieval England, where it was established in the 13th century primarily for the wool trade, England's key export at the time. It was defined as 20 hundredweights, each consisting of 112 pounds, totaling 2,240 pounds, to standardize shipments of wool sacks that typically weighed around 364 pounds. This avoirdupois system emerged from earlier Roman and local measures but was formalized to facilitate commerce and taxation. These weights and measures aimed to reduce discrepancies in trade across regions and curb fraud in the wool market.[18] During the colonial period, the American colonies diverged from British standards by adopting the short ton of 2,000 pounds in the 18th century, simplifying calculations for local trades like coal and grain where a hundredweight of 100 pounds proved more practical than the imperial 112 pounds. This short ton persisted after independence, embedded in early U.S. commercial practices, while the long ton of 2,240 pounds remained standard in the British Empire for international shipping and bulk goods. The 19th century saw the rise of the metric tonne, defined during the French Revolution in 1795 as 1,000 kilograms—the mass of one cubic meter of water at 4°C—to create a decimal-based system independent of commodities. Although Britain permitted metric units in trade via the 1864 Weights and Measures Act following earlier discussions in the 1850s, imperial tons resisted widespread replacement due to entrenched industrial use.[18][19] In the 20th century, legal shifts accelerated metrication in some regions while others clung to customary units. The UK's Weights and Measures Act 1985, implementing EU directives, retained definitions for both the long ton and the metric tonne but supported ongoing metrication efforts that limited non-metric units in trade, though supplementary imperial use lingered in legacy sectors.[20] In the United States, the short ton was retained through laws like the 1893 Mendenhall Order, which aligned yard and pound with metric prototypes but preserved customary units for domestic commerce, reinforced by ongoing federal standards. Entering the 21st century, EU mandates under Directive 80/181/EEC (amended in 2000) required primary use of SI units like the metric tonne for measurements in trade to ensure uniformity across borders. Meanwhile, imperial tons, particularly the short ton, persist in outdated U.S. aviation contexts for payload calculations, despite global shifts toward metric tonnes in international cargo operations.Units of Mass
Long Ton and Short Ton
The long ton, also known as the imperial ton or British ton, is a unit of mass equal to 2,240 avoirdupois pounds (1,016.0469 kilograms). It is defined as 20 hundredweight, with each hundredweight consisting of 112 pounds. This unit originated from the imperial system and remains recognized under UK regulations for supplementary indications alongside metric units.[21][22][23] The short ton, also called the US ton or net ton, is a unit of mass equal to 2,000 avoirdupois pounds (907.18474 kilograms). It comprises 20 hundredweight, each weighing 100 pounds, and forms part of the US customary system standardized by federal authorities.[21][22][24] These units differ primarily in their base hundredweight values, leading to the long ton being approximately 12% heavier than the short ton, which affects non-metric trade and measurement contexts where precision in imperial systems persists. The long ton's symbol is typically LT or long tn, while the short ton uses ST or tn (with "US" qualifiers to distinguish it internationally).[24][25] Traditionally, the long ton has been applied in British naval contexts for ship displacement calculations and in railway operations for freight loading, particularly in historical bulk transport. It continues to see use in the UK, Canada, and India for commodities like coal and steel, especially in legacy trade documentation and international contracts influenced by imperial standards. In the 2020s, despite the UK's shift to metrication, the long ton persists in niche heritage industries, such as preserved railways and industrial museums, where it supports authentic recreations of traditional weighing practices.[26][27] The short ton serves as the standard in the United States for freight and mining operations, including coal production reporting and mineral output assessments. It is also prevalent in American automotive sectors for vehicle payload ratings and in agriculture for measuring crop yields and commodity shipments.[28][29]Metric Tonne
The metric tonne, also spelled tonne, is a unit of mass defined as exactly 1,000 kilograms (kg), or equivalently 1 megagram (Mg), in the International System of Units (SI).[7] This definition aligns with the metric system's base unit of mass, the kilogram, and traces its formal establishment to the Metre Convention signed on 20 May 1875 in Paris, which created the International Bureau of Weights and Measures (BIPM) to standardize metric units globally. The metric tonne is approximately equivalent to the mass of 1,000 liters of pure water at its maximum density of 4 °C and standard atmospheric pressure, where water's density is 0.99997 kg/L, underscoring its practical relation to volume in scientific and industrial contexts. To distinguish it from imperial tons, the metric tonne uses the symbol "t" and the spelling "tonne" (or "metric ton" in some regions), as recommended by international standards bodies.[30] Although not an SI base or derived unit, it is an accepted non-SI unit compatible with the SI for expressing large masses.[31] The metric tonne serves as the international standard for mass measurements in the European Union (EU), where SI units are legally mandated under Directive 80/181/EEC, most Asian countries that have adopted the metric system since the early 20th century, and global scientific research adhering to SI conventions.[32] In international trade, it is required for consistency in commodities like grains, metals, and chemicals under United Nations Economic Commission for Europe (UNECE) recommendations, which specify "TNE" as the code for tonne in trade documentation to ensure uniformity across borders.[11] Its decimal structure provides key advantages for scalability in large-scale applications; for instance, multiples like the kilotonne (kt = 1,000 tonnes) and megatonne (Mt = 1,000,000 tonnes) enable straightforward calculations without fractional conversions, supporting efficiency in engineering, logistics, and environmental assessments. In the 2020s, the metric tonne has become central to climate reporting for quantifying greenhouse gas emissions, particularly carbon dioxide (CO₂), with global inventories tracking billions of tonnes annually; for example, 1 tonne of CO₂ emissions roughly corresponds to the combustion of fossil fuels providing about 10 to 20 GJ of energy, depending on the fuel type and standard emission factors (e.g., ~50 kg CO₂/GJ for natural gas).[33][34] This usage highlights its role in international agreements like the Paris Accord, where emissions are reported in metric tonnes to facilitate comparable, precise tracking of mitigation efforts.Subdivisions and Other Variants
The long ton is subdivided into 20 long hundredweights, with each hundredweight equal to 112 pounds, yielding a total mass of 2,240 pounds. Similarly, the short ton divides into 20 short hundredweights, each consisting of 100 pounds, for a total of 2,000 pounds.[35] The metric tonne, in contrast, is simply subdivided into 1,000 kilograms as part of the International System of Units. Specialized variants of the ton include the assay ton, employed in ore sampling to simplify precious metal quantification. The short assay ton equals 29.166 grams, calibrated such that the milligrams of extracted metal directly indicate troy ounces per short ton of ore. A long assay ton variant measures 32.667 grams, aligning with the long ton's structure for analogous calculations in legacy imperial contexts. In shipping, the gross ton serves as an alternative designation for the long ton (approximately 1.016 metric tonnes) in some historical mass measurements, distinct from volume-based gross tonnage.[26] The deadweight ton denotes a vessel's total carrying capacity in mass terms, encompassing cargo, fuel, and provisions, and is conventionally expressed in metric tonnes. Rare applications persist in mining, where a "ton of ore" historically equates to 32,000 avoirdupois ounces (based on the short ton's 2,000 pounds at 16 ounces per pound), facilitating grade reporting in ounces per ton despite the shift to SI units.[26] Such non-standard variants, including phased-out regional forms like the Spanish tonelada (around 920 kilograms), are largely avoided in modern metrology but linger in legacy assays and documentation.[36]Comparisons and Conversions
The short ton, long ton, and metric tonne are interrelated units of mass, with precise conversion factors derived from international standards. One short ton equals exactly 907.18474 kilograms, or 0.90718474 metric tonnes, based on the definition of 2,000 pounds where 1 pound = 0.45359237 kilograms exactly. Similarly, one long ton equals exactly 1,016.0469088 kilograms, or 1.0160469088 metric tonnes, from 2,240 pounds using the same pound-kilogram factor.[22] Conversely, one metric tonne equals 1,000 kilograms exactly, which is 1.10231131 short tons or 0.9842065276 long tons. To derive these conversions, begin with the metric tonne as the base unit: 1 metric tonne = 1,000 kg. For the short ton, multiply the pounds definition by the exact pound-to-kilogram factor: 2,000 lb × 0.45359237 kg/lb = 907.18474 kg, then divide by 1,000 to get metric tonnes (907.18474 / 1,000 = 0.90718474). For the long ton: 2,240 lb × 0.45359237 kg/lb = 1,016.0469088 kg (1,016.0469088 / 1,000 = 1.0160469088 metric tonnes). These exact factors, established since the 1959 international yard and pound agreement, ensure consistency across scientific and commercial applications.| Unit | Pounds (lb) | Kilograms (kg) | Relation to Metric Tonne |
|---|---|---|---|
| Short Ton | 2,000 | 907.18474 | 0.90718474 (9.28% lighter) |
| Long Ton | 2,240 | 1,016.0469088 | 1.0160469088 (1.60% heavier) |
| Metric Tonne | 2,204.6226218 | 1,000 | 1 (base) |