User:Kahastok/Units

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
In general, quantities should be specified in all 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.

The general rule is that the quantity that should be placed first (the "main 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.

In case of dispute, the choice of which quantity should the main 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

 * Nominal and defined quantities should take the original quantity as main 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 mph to 50 km/h. (The focus is on the change of units, not on the 3.6% increase.)
 * In a quotation, always keep the original quantity as main quantity.

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

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

United Kingdom
The main quantity is generally expressed in metric units (44 kilograms (97 lb)), but imperial units are still used as the main units in some contexts, including:
 * miles, miles per hour, and fuel consumption in miles per imperial gallon;
 * feet/inches and stones/pounds for personal height and weight;
 * imperial pints for draught beer/cider and bottled milk.
 * hands for horses and most other equines

In UK engineering-related articles, 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 main 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 main quantity with a conversion in parentheses. This enables more readers 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 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
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.