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My suggestion for WP:UNITS, up to and including MOS:CONVERSIONS.

Note that the existing "SI Standard subsection" gets moved to the "Unit names and symbols" section.

Which units to use[edit]

In general, quantities should be specified in systems of measurement that are prevalent in context in English-speaking countries: in most cases, that means the SI, US Customary and imperial systems of measurement. Note that in practice, the US customary and imperial quantities will normally be the same - in which case, one quantity can serve for both; see Imperial and US customary measurement systems.

The general rule is that the quantity that should be placed first (the "primary quantity") is that expressed in the most prevalent units in the field internationally. Generally this will be SI and non-SI units officially accepted for use with the SI. Other values should be provided as conversions (see MOS:CONVERSIONS). Exceptions to this rule are detailed below.

The choice of which quantity should the primary quantity should be guided by the following principles:

  • Use the same units for main quantities in a given context consistently throughout a given article.
  • Units should not change in definition for different quantities in an article.
  • Units used should be familiar to the reader (e.g. yards or metres as opposed to chains or rods) and unambiguous (e.g "US gallons" and "imperial gallons" as opposed to "gallons").

Quantities should be accompanied by a proper citation of the source using a method described at the style guide for citation.

General exceptions[edit]

  • Nominal and defined quantities should take the original quantity as primary quantity, even if this makes the article inconsistent: for example, When the Republic of Ireland adopted the metric system, the road speed limit in built-up areas was changed from 30 miles per hour (48 km/h) to 50 kilometres per hour (31 mph). (The focus is on the change of units, not on the 3.6% increase.)
  • In a quotation, always keep the original quantity as primary quantity.

Science-related articles[edit]

In science-related articles, the primary quantity is that expressed in the most prevalent units in the field internationally (most commonly SI and non-SI units officially accepted for use with the SI).

However, conversions are explicitly not required unless multiple units are used in the scientific field concerned.

Strong national ties[edit]

As with spelling and dates, non-scientific articles with strong national ties to a particular English-speaking country should take the primary quantity as being the quantity in the units preferred in that country. For non-English speaking countries, refer to the advice above.

United States[edit]

The primary quantity is generally expressed in US customary units (97 pounds (44 kg)).

United Kingdom[edit]

As a rule, the UK does not rigidly use either the metric or the imperial system, but rather uses different units in different contexts. Because the primary reason for Wikipedia's existence is to provide an online encyclopedia that is readily accessible, the following approximation to local usage (broadly based on the style guide of The Times) is applied.

The primary quantity is generally expressed in an SI unit or a non-SI unit officially accepted for use with the SI (44 kilograms (97 lb)). However, in some contexts, the primary quantity is expressed in imperial units. These include:

  • miles for length and distance, miles per hour for speed, and miles per imperial gallon for fuel consumption;
  • feet and inches for personal height and stones and pounds for personal weight;
  • imperial pints for draught beer and cider, as well as for bottled milk;
  • (Draft alternative to consider: Where a commodity is priced in a given unit, use that unit for that commodity. For example, use imperial pints for draught beer but litres for cooking oil.)
  • hands for the height of horses, and most other equines

In articles specifically related to UK engineering, including all UK bridges and tunnels, the primary quantity is generally expressed in the units that the project was designed in, whether metric or imperial. However, the primary quantity for road distances and speeds should still be expressed in miles and miles per hour as above.

Some editors hold strong views for or against metrication in the UK. If there is disagreement about the units used for main quantities in an article, discuss the matter on the article talk page, and consult relevant WikiProjects, and/or MOSNUM. Exceptions to the above – in either direction – may be appropriate if there is a good reason; however, personal preference or alignment with the specific source cited to justify the measure are not considered good reasons.

Other countries[edit]

For all other English-speaking countries, the primary quantity is generally expressed in an SI unit or a non-SI unit officially accepted for use with the SI.

Unit conversions[edit]

Where English-speaking countries use different units for the same quantity, follow the main quantity with a conversion in parentheses. This enables more readers to understand the quantity. Examples: the Mississippi River is 2,320 miles (3,734 km) long; the Murray River is 2,375 kilometres (1,476 mi) long.

  • With imperial units which are not also US customary units, double conversions can be useful: The song's second verse reveals that Rosie weighs 19 stone (266 lb; 121 kg).
  • Generally, conversions to and from metric units and US or imperial units should be provided, except:
    • When inserting a conversion would make a common or linked expression awkward (The four-minute mile).
    • When units are part of the subject of a topic—nautical miles in articles about the history of nautical law, SI units in scientific articles, yards in articles about American football—it can be excessive to provide conversions every time a unit occurs. It could be best to note that this topic will use the units (possibly giving the conversion factor to another familiar unit in a parenthetical note or a footnote), and link the first occurrence of each unit but not give a conversion every time it occurs.
  • Converted quantity values should use a level of precision similar to that of the source quantity value, so the Moon is 380,000 kilometres (240,000 mi) from Earth, not (236,121 mi). However, small numbers may need to be converted to a greater level of precision where rounding would cause a significant distortion, so one mile (1.6 km), not one mile (2 km).
  • Category:Conversion templates can be used to convert and format many common units, including {{convert}}, which includes non-breaking spaces.
  • In a direct quotation:
    • Conversions required for quantities cited within direct quotations should appear within square brackets in the quote.
    • Alternatively, you can annotate an obscure use of units (e.g. five million board feet of lumber) with a footnote that provides conversion in standard modern units, rather than changing the text of the quotation. See the style guide for citation, footnoting and citing sources.

Conversion errors[edit]

Conversion errors may occur in general reports, so use the primary sources or the most authoritative sources available. This can help avoid rounding errors, like this: a general report stated that the Eurostar is designed for speeds of "186 mph (299 km/h)". However, the actual design speed was 300 km/h. (The error crept in because the original speed had been converted to 186 mph and then back to km/h.) When common conversion factors are given as quantities, this is a clue that there may be conversion problems. For example, if a number of moons are given estimated diameters in increments of 16 km or 6 miles (implied precision ±0.5 km or mi), it is likely that the estimates in the primary source were in increments of a less-precise 10 miles or 10 km (implied precision ±5 miles or km).

See uncertainty in data above.

Straightforward and accurate conversion may not be possible for loose estimates. For example, if the diameter of a moon is estimated to be 10 miles to within an order of magnitude, any simple conversion to kilometers would introduce a significant loss of accuracy or a gross change in precision. That is because an order-of-magnitude estimate of 10 miles implies a possible range of ≈ 3–30 miles, which would be ≈ 5–50 km. A secondary source will commonly convert such an estimate to a specious 16 km.