Wikipedia talk:Manual of Style/Dates and numbers/Units

Which units to use
In general, quantities should be specified in systems of measurement prevalent in English-speaking countries. In most cases, this means the SI, US Customary and imperial systems of measurement. In practice, the US customary and imperial quantities are normally 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 expressed in the unit that is used most frequently across the world. Generally this will be either an SI unit or a non-SI unit officially accepted for use with the SI. Other values should be provided as conversions (see MOS:CONVERSIONS). Exceptions are detailed below.

The choice of which unit should be used for the primary quantity should be guided with these goals in mind:
 * Use the same unit for primary 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 are accompanied by a citation using a method described at the style guide for citation.

General exceptions

 * Nominal and defined quantities should take the source quantity as the primary quantity, even if this makes an 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 mph to 50 km/h. (The focus is on the change of units, not on the 3.6% increase.)
 * In a quotation, always maintain the quoted quantity as the primary quantity.

Science-related articles
In science-related articles, the primary quantity is expressed in the unit that is used most frequently across the world within the context of the article. This will usually be either an SI unit or a non-SI unit 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
As with spelling and dates, non-scientific articles with strong national ties to a particular English-speaking country should use the unit most prevalent for a particular quantity as the primary quantity. For non-English-speaking countries, refer to the general advice above.

United States
The primary quantity is generally expressed in a US customary unit (97 pounds (44 kg)).

United Kingdom
There have been many disputes on Wikipedia over unit use on UK articles and most have arisen between editors advocating the superiority of one system over another. The current consensus position favours local usage, since the primary reason for wikipedia's existence is to provide an online encyclopedia that is readily accessible. Some editors hold strong opinions in favour or against metrication in the United Kingdom. To avoid disputes, do not edit articles to reverse the primary unit from one system to another and if there is disagreement about which unit should be used as primary in a UK-related article, discuss the matter on the article's talk page, and consult relevant WikiProjects. If consensus cannot be reached, refer to historically stable versions of the article in question, and retain the unit used in these as the primary unit. For further guidance consult the style guides of British publications such as the Times.

Although, the UK adopted the metric system in the 1970s, there remains the use of imperial measures in certain contexts. Hence, whilst the primary quantity should generally expressed in a metric unit (44 kilograms (97 lb)), an imperial unit is used as the primary unit in some contexts, including:
 * miles for 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;
 * hands for the height of horses, and most other equines

In engineering-related articles connected to the UK, including all bridges and tunnels, generally use the system of units that the topic was drawn-up in, whether metric or imperial. Provide conversions where appropriate. Road distances and speeds are an exception to this: use imperial units with a metric conversion.

Other countries
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
Where English-speaking countries use different units for the same quantity, follow the primary quantity with a conversion in parentheses. This enables a wider readership to understand the quantity. Examples: the Mississippi River is 2320 mi long; the Murray River is 2375 km 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 st.
 * 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, which includes non-breaking spaces.
 * In a direct quotation:
 * Conversions required for quantities cited within direct quotations should appear within square brackets in the quotation.
 * 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
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. This error is the result of the fact that the original speed, drawn-up in metric, had been converted into miles per hour and then back into kilometers per hour. 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.