The tonne (/tʌn/ (listen) or /tɒn/; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI.[1] It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States customary units), and the long ton (British imperial units).[2][3] It is equivalent to approximately 2204.6 pounds,[4] 1.102 short tons, and 0.984 long tons. The official SI unit is the megagram (symbol: Mg), a less common way to express the same mass.

A one-tonne (1000-kilogram) concrete block
General information
Unit ofmass
In SI units:1000 kg

Symbol and abbreviations

The BIPM symbol for the tonne is t, adopted at the same time as the unit in 1879.[5] Its use is also official for the metric ton in the United States, having been adopted by the United States National Institute of Standards and Technology (NIST).[6][7] It is a symbol, not an abbreviation, and should not be followed by a period. Use of minuscule letter case is significant, and use of other letter combinations can lead to ambiguity. For example, T, MT, mT, Mt and mt are the SI symbols for the tesla, megatesla, millitesla, megatonne (one teragram), and millitonne (one kilogram), respectively. If describing TNT equivalent units of energy, one megatonne of TNT is equivalent to approximately 4.184 petajoules.

Origin and spelling

In English, tonne is an established spelling alternative to metric ton.[8] In the United States and United Kingdom, tonne is usually pronounced the same as ton (/tʌn/), but the final "e" can also be pronounced, i.e. "tunnie" (/ˈtʌni/).[9] In Australia, the common and recommended pronunciation is /tɒn/.[10][11] In the United States, metric ton is the name for this unit used and recommended by NIST;[6] an unqualified mention of a ton almost invariably refers to a short ton of 2000 pounds (907 kg) and to a lesser extent to a long ton of 2240 pounds (1020 kg), the tonne is rarely used in speech or writing. Both terms are acceptable in Canadian usage.

Ton and tonne are both derived from a Germanic word in general use in the North Sea area since the Middle Ages (cf. Old English and Old Frisian tunne, Old High German and Medieval Latin tunna, German and French tonne) to designate a large cask, or tun.[12] A full tun, standing about a metre high, could easily weigh a tonne.

The spelling tonne pre-dates the introduction of the SI in 1960; it has been used with this meaning in France since 1842,[13] when there were no metric prefixes for multiples of 106 and above, and is now used as the standard spelling for the metric mass measurement in most English-speaking countries.[14][15][16][17] In the United States, the unit was originally referred to using the French words millier or tonneau,[18] but these terms are now obsolete.[3] The Imperial and US customary units comparable to the tonne are both spelled ton in English, though they differ in mass.


One tonne is equivalent to:

  • In kilograms: 1000 kilograms (kg) by definition.[1]
  • In grams: 1000000 grams (g) or 1 megagram (Mg). Megagram is the corresponding official SI unit with the same mass. Mg is distinct from mg, milligram.
  • In pounds: Exactly 1000/0.45359237 pounds (lb) by definition of the pound,[19] or approximately 2204.622622 lb.
  • In short tons: Exactly 1/0.90718474 short tons (ST), or approximately 1.102311311 ST.
    • One short ton is exactly 0.90718474 t.[20]
  • In long tons: Exactly 1/1.0160469088 long tons (LT), or approximately 0.9842065276 LT.
    • One long ton is exactly 1.0160469088 t.[20]

A tonne is the mass of one cubic metre of pure water: at 4 °C one thousand litres of pure water has an absolute mass of one tonne.[21]

Derived units

As a non-SI unit, the use of SI metric prefixes with the tonne does not fall within the SI standard. For multiples of the tonne, it is more usual to speak of thousands or millions of tonnes. Kilotonne, megatonne, and gigatonne are more usually used for the energy of nuclear explosions and other events in equivalent mass of TNT, often loosely as approximate figures. When used in this context, there is little need to distinguish between metric and other tons, and the unit is spelled either as ton or tonne with the relevant prefix attached.[22]

Tonnes Grams Equivalents*
NameSymbol NameSymbol Tonnes (t)US/short tons (ST)Imperial/long tons (LT)
tonnet megagramMg 1 t1.1023 ST0.98421 LT
kilotonnektǂ gigagramGg 1000 t1102.3 ST984.21 LT
megatonneMt teragramTg 1 million t1.1023 million ST984210 LT
gigatonneGt petagramPg 1 billion t1.1023 billion ST984.21 million LT

*The equivalent units columns use the short scale large-number naming system currently used in most English-language countries, e.g. 1 billion = 1000 million = 1000000000.
Values in the equivalent short and long tons columns are rounded to five significant figures. See Conversions for exact values.
ǂThough non-standard, the symbol "kt" is also used (instead of the standard symbol "kn") for knot, a unit of speed for aircraft and sea-going vessels, and should not be confused with kilotonne.

Alternative usages

Metric ton units

A metric ton unit (mtu) can mean 10 kg (approximately 22 lb) within metal trading, particularly within the United States. It traditionally referred to a metric ton of ore containing 1% (i.e. 10 kg) of metal.[23][24] The following excerpt from a mining geology textbook describes its usage in the particular case of tungsten:

Tungsten concentrates are usually traded in metric tonne units (originally designating one tonne of ore containing 1% of WO3, today used to measure WO3 quantities in 10 kg units. One metric tonne unit (mtu) of tungsten (VI) contains 7.93 kilograms of tungsten.

Walter L Pohl, Economic Geology: Principles and Practices, English edition, 2011, p 183.

In the case of uranium, MTU is sometimes used in the sense of metric ton of uranium (1,000 kg).[25][26][27][28]

Use of mass as proxy for energy

The tonne of trinitrotoluene (TNT) is used as a proxy for energy, usually of explosions (TNT is a common high explosive). Prefixes are used: kiloton(ne), megaton(ne), gigaton(ne), especially for expressing nuclear weapon yield, based on a specific combustion energy of TNT of about 4.2 MJ/kg (or one thermochemical calorie per milligram). Hence, 1 t TNT = approx. 4.2 GJ, 1 kt TNT = approx. 4.2 TJ, 1 Mt TNT = approx. 4.2 PJ.

The SI unit of energy is the joule. One tonne of TNT is approximately equivalent to 4.2 gigajoules.

In the petroleum industry the tonne of oil equivalent (toe) is a unit of energy: the amount of energy released by burning one tonne of crude oil, approx, 42 GJ. There are several slightly different definitions. This is ten times as much as a tonne of TNT because atmospheric oxygen is used.

Unit of force

Like the gram and the kilogram, the tonne gave rise to a (now obsolete) force unit of the same name, the tonne-force, equivalent to about 9.8 kilonewtons. The unit is also often called simply "tonne" or "metric ton" without identifying it as a unit of force. In contrast to the tonne as a mass unit, the tonne-force is not accepted for use with SI.

See also

Notes and references

  1. Le Système international d’unités [The International System of Units] (PDF) (in French and English) (9th ed.), International Bureau of Weights and Measures, 2019, ISBN 978-92-822-2272-0
  2. Government of Canada, Innovation. "Field Inspection Manual – Automatic Weighing Devices". Retrieved 2021-08-05.
  3. "Metric System of Measurement: Interpretation of the International System of Units for the United States" (PDF). Federal Register. 63 (144): 40338. July 28, 1998. 63 FR 40333. Archived from the original (PDF) on October 15, 2011.
  4. United States National Bureau of Standards (1959-06-25). "Notices "Refinement of values for the yard and the pound"" (PDF). Retrieved 2006-08-12.
  5. Table 6 Archived 2009-10-01 at the Wayback Machine. BIPM. Retrieved on 2011-07-10.
  6. Metric System of Measurement: Interpretation of the International System of Units for the United States Archived 2008-04-09 at the Wayback Machine (PDF). See corrections in the Errata section of Archived 2008-04-18 at the Wayback Machine.
  7. NIST Special Publication 330, 2019 edition states "The name of the unit with symbol t and defined according to 1 t = 103 kg is called 'metric ton' rather than 'tonne.'".
  8. "tonne, n". OED. Retrieved 6 August 2021.
  9. The Oxford English dictionary 2nd ed. lists both /tʌn/ and /ˈtʌni/
  10. Macquarie Dictionary (fifth ed.). Sydney: Macquarie Dictionary Publishers Pty Ltd. 2009.
  11. "How To Pronounce Metrics Units (advertisement by Australian Metric Conversion Board)". The Age. 1972-11-21. p. 14. Retrieved 2021-11-01.
  12. Harper, Douglas. "tonne". Online Etymology Dictionary.
  13. "Recherche d'un mot".
  14. "Guidance Note on the use of Metric Units of Measurement by the Public Sector" (PDF). National Measurement Office. 2007. Archived from the original (PDF) on 2011-02-07. Retrieved 2010-02-13. "Tonne" is listed under "The Principal Metric Units of Measurement" on p. 7.
  15. "National Measurement Regulations 1999 |". Australian Government. 1999. Retrieved 2010-02-13. "Tonne" is listed under Schedule 1, Part 3 as a non-SI unit of measurement used with SI units of measurement.
  16. "Appendix 4: Units of Measurement and Conversion Factors". MAF (Ministry of Agriculture and Forestry (New Zealand)). Retrieved 2010-02-13.
  17. "Canada Gazette". Government of Canada. 1998–2007. Retrieved 2010-02-13. The Corporation shall pay to producers selling and delivering wheat produced in the designated area to the Corporation the following sums certain per tonne basis...
  18. Act of July 28, 1866, codified in 15 U.S.C. § 205
  19. Barbrow, L.E.; Judson, L.V. (1976). Weights and measures standards of the United States – A brief history. Archived from the original on 2008-05-11.
  20. National Institute of Standards and Technology. Butcher, Tina; Crown, Linda; Harshman, Rick; Williams, Juana, eds. (October 2013). "Appendix C – General Tables of Units of Measurement" (PDF). Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. NIST Handbook. Vol. 44 (2014 ed.). Washington, D.C.: U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology. p. C-13. ISSN 0271-4027. OCLC 58927093. Retrieved 10 December 2013.
  21. To within 0.003%.
  22. The Oxford English Dictionary 2nd ed. gives both megaton and megatonne and adds "The unit may be calculated in either imperial or metric tons; the form megatonne generally implies the metric unit". The use for energy is the first definition; use for mass or weight is the third definition.
  23. "Platt's Metals Guide to Specifications – Conversion Tables". 8 September 2008. Archived from the original on 8 September 2008.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
  24. How Many? A Dictionary of Units of Measurement Archived 2011-09-04 at the Wayback Machine. Retrieved on 2011-07-10.
  25. Reference.Pdf. (PDF) . Retrieved on 2011-07-10.
  26. "Glossary". (June 2000). Disposition of Surplus Hanford Site Uranium, Hanford Site, Richland, Washington. US Department of Energy.
  27. "Acronyms". Y-12 National Security Complex.
  28. NRC Collection of Abbreviations (NUREG-0544, Rev. 4), United States Nuclear Regulatory Commission. (2011-03-13). Retrieved on 2011-07-10.
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