Kilograms to Tonnes (kg to t)

Since 20 May 2019 the kilogram (kg) is defined by fixing the numerical value of the Planck constant h at exactly 6.62607015 × 10⁻³⁴ when expressed in J·s, which is equivalent to kg·m²·s⁻¹. Because the metre and second appearing in that expression are themselves anchored to the speed of light c and the caesium-133 hyperfine transition frequency Δν_Cs, the kilogram ultimately rides on three fixed constants of nature and can be realised in any sufficiently equipped laboratory without reference to a physical artefact. National metrology institutes do so by one of two routes: a Kibble balance (renamed in 2016 in honour of the late NPL physicist Bryan Kibble, having previously been called the watt balance), which equates electrical and mechanical power to relate mass to the Planck constant via a precisely-measured electromagnetic force; or the X-ray crystal density method, which counts the atoms in a near-perfect spherical single crystal of silicon-28 enriched to roughly 99.995% purity. By international convention the kilogram is the only base unit defined with a prefix in its name, and decimal multiples are formed from the root "gram" rather than "kilogram" — so one million grams is a megagram, not a "kilokilogram".

The kilogram is unique among the seven SI base units in carrying a metric prefix in its very name — a relic of its eighteenth-century origins, when the gramme was defined first and the unit a thousand times larger happened to be the convenient size for everyday weighing. The original legal definition came in the Loi du 18 germinal an III (7 April 1795), the metric law passed during the French Revolution, which fixed the gramme as the mass of one cubic centimetre of water at the temperature of melting ice; the kilogramme was simply its thousand-fold multiple. To realise that abstract definition the French Academy of Sciences commissioned a platinum cylinder, the Kilogramme des Archives, completed in 1799 and held in the National Archives in Paris. The unit's role moved onto the international stage with the Convention of the Metre in 1875, which established the Bureau International des Poids et Mesures (BIPM) at Sèvres just outside Paris. At the 1st General Conference on Weights and Measures (CGPM) in 1889, a new artefact — the International Prototype of the Kilogram (IPK), informally called Le Grand K — was adopted as the world standard: a cylinder roughly 39 mm in both height and diameter cast from a 90% platinum, 10% iridium alloy, with iridium added because pure platinum had proved too soft for an artefact intended to last centuries. For the next 130 years Le Grand K had no measurement uncertainty, because by definition it was one kilogram. The trouble was that periodic verifications against its sister copies — held in 1889, 1948 and 1989 — showed the official copies and the IPK appearing to drift apart by something on the order of fifty micrograms over a century, with the cause never satisfactorily identified. On 16 November 2018 the 26th CGPM voted at Versailles to redefine the unit, and on 20 May 2019 — World Metrology Day — the new definition came into force, ending a 220-year reliance on a single physical artefact.

The kilogram is the legal unit of mass in nearly every country on Earth, recognised by all signatories of the Convention of the Metre as the standard for trade and metrology. Across the European Union it is mandatory for trade, labelling and scientific work under directive 80/181/EEC. The United Kingdom completed its statutory metrication of trade in 2000, with the well-known carve-outs for draught beer and milk sold in returnable containers (still legal in pints) and for road distance and speed signage (still legal in miles and miles per hour). The United States, never officially metricated for everyday commerce, nonetheless requires kilograms or grams alongside customary units on consumer packaging via FDA labelling rules. Healthcare worldwide runs on kilograms regardless of regional preferences for body weight: patients are charted in kg even in American hospitals, because medication dosing is overwhelmingly expressed in milligrams per kilogram of body mass — a convention so universal in paediatrics that any deviation triggers patient-safety review. Olympic sports use kilograms for weight classes apart from boxing, wrestling and mixed martial arts, which inherited their imperial classes from American and British origins. International freight outside US domestic routes, scientific publishing and global commodity markets all denominate mass in kilograms, with the metric tonne (1,000 kg) standard for bulk goods.

The tonne (t) is exactly 1,000 kilograms — equivalently 1 megagram (Mg) in coherent SI notation, though the megagram form is rarely seen outside metrology textbooks. The official symbol is the lowercase "t" per the SI Brochure (9th edition, 2019); the older "T" was sometimes used in mid-twentieth-century engineering literature but conflicts with the SI symbol for tesla and is no longer recognised. Decimal multiples are formed with standard SI prefixes: a kilotonne (kt) is 10⁶ kg, a megatonne (Mt) is 10⁹ kg (a billion kilograms), and a gigatonne (Gt) is 10¹² kg, with the megatonne and gigatonne in routine use for industrial-commodity statistics, atmospheric carbon-emission accounting and yield estimates for nuclear weapons. The tonne is not part of the SI proper but is listed in CIPM Recommendation 1 (2003) and the SI Brochure as a non-SI unit accepted for use with the SI, alongside the litre, hectare and minute. The unit's relationship to the metric volume system is exact: one tonne of pure water at 4 °C occupies one cubic metre — a definitional cleanness inherited from the 1795 system that survives as a useful working approximation in shipping, water-treatment engineering and bulk-liquid trade.

The tonne takes its name from the medieval French tonneau — the cargo cask whose volume defined ship-cargo capacity in Atlantic and Mediterranean shipping. The Bordeaux wine and cargo tonneau (~900 litres) gave its name first to the volume in which ocean-going ships were rated, and the unit migrated from volume to mass through nineteenth-century commercial practice as cargo manifests shifted from cask-counting to weight-counting. The metric tonne (tonneau métrique) of exactly 1,000 kilograms entered widespread French and continental European commercial use through the mid-nineteenth century, though it was not part of the original 1795 metric law. Twentieth-century metrology had to decide what to do with a 1,000 kg unit that industry used universally but that did not fit the SI's prefix-of-a-base-unit convention. The SI's coherent name for the quantity is the megagram (Mg) — one million grams, formed by the mega prefix on the SI's "gram" root — but megagram never gained traction outside metrology textbooks because shipping, agriculture and industry had settled on tonne. CIPM Recommendation 1 (2003) and the SI Brochure (9th edition, 2019) list the tonne as a non-SI unit accepted for use with the SI, with lowercase "t" as the symbol. The international and most-of-world spelling is "tonne"; American English uses "metric ton" for the unit, distinguishing it from the unqualified "ton" of US-customary use. Three different units share the English word "ton" without a modifier — the tonne (1,000 kg), the US short ton (2,000 lb / 907.18 kg), and the UK long ton (2,240 lb / 1,016.05 kg) — and international contracts in steel, grain, scrap and bulk-shipping trade must specify which is meant or default by jurisdiction. The 9.3% gap between tonne and short ton, with the 1.6% gap to long ton, produces multi-million-dollar disputes when commodity contracts are read across systems.

International shipping runs on the tonne. The International Maritime Organization's Cargo Statistics database, vessel deadweight tonnage (DWT) ratings, and ISO 668 container weight specifications are all denominated in tonnes, and ship-class names are derived from tonnage thresholds: Panamax (now Neopanamax) for vessels fitting the new Panama Canal locks, Capesize for vessels too large for either canal and routed around the Cape of Good Hope or Cape Horn, and very-large and ultra-large crude carriers (VLCC, ULCC) at 200,000+ DWT. The IMO's Carbon Intensity Indicator (CII) framework, applicable to all vessels of 5,000 GT and above since January 2023, denominates carbon intensity in grams of CO₂ per tonne-mile. Agriculture and grain trade: the FAO and USDA report global production and trade in tonnes, and the major commodity exchanges split by system — Euronext Paris milling-wheat futures (the EU benchmark) trade in euros per tonne, while CBOT Chicago wheat, corn and soybean futures trade in US-cents per bushel — a metric/imperial split that propagates into every cross-border grain contract. Industrial commodities: the World Steel Association's annual production figures are reported in million tonnes (Mt) — China at roughly 1,000 Mt against a global figure near 1.9 Gt — and the London Metal Exchange (LME) settles copper, aluminium, zinc, lead, tin and nickel futures in dollars per tonne. Greenhouse-gas accounting: the IPCC, UNFCCC and Paris Agreement reporting frameworks denominate emissions in tonnes of CO₂ equivalent (tCO₂e), with country National Inventory Reports submitted in megatonnes and gigatonnes. The EU Emissions Trading System and the UK ETS both price each tCO₂e allowance in euros and pounds respectively. Vehicle and road law: the UK Driver and Vehicle Standards Agency C1 licence covers rigid lorries between 3.5 and 7.5 tonnes gross weight, with the full Category C HGV licence required above 7.5 tonnes; the maximum gross weight for a 6-axle articulated lorry on UK roads is 44 tonnes, set by the Construction and Use Regulations.