Al leer etiquetas de productos o especificaciones en inglés muchas veces encontramos inconsistencias en la manera de escribir o abreviar las unidades de medida del sistema métrico. En parte se debe a que no es el sistema tradicional que se viene enseñando en las escuelas de EE.UU., entonces hay mucho desconocimiento sobre sus convenciones.

Dado que la industria, la ciencia y el comercio internacional han demostrado que es el sistema de unidades más práctico, se comenzó a adoptar el sistema métrico decimal en numerosos ámbitos y se está comenzando a enseñar en las escuelas de EE.UU. Por lo tanto, con el tiempo la difusión de este sistema evitará la proliferación de errores de escritura y confusión relacionada. Veamos a continuación las formas correctas de uso en inglés.

Correct SI/metric usage

SI (pronounced “ess-EYE”) is the initialism for the Système international d’unités, the modernized version of the metric system that the US and other nations have agreed to use. (Do not abbreviate it as S.I.)

This list is provided to point out the correct way to use the metric system and to show many of the incorrect examples of its usage that may be given on package labels and in other printed matter. These correct ways to use SI are set by the international standards that define the SI.

Important points

1. The short forms for SI units (such as mm for millimetre) are symbols, not abbreviations.
2. SI symbols never end with a period unless they are the last word in a sentence.
   • Correct: 20 mm, 10 kg
   • Incorrect: 20 mm., 10 kg.
3. SI symbols should be preceded by digits and a space must separate the digits from the symbol.
   • Correct: It was 300 mm wide. The millimetre width was given.
   • Incorrect: It was 300mm wide. The mm width was given.
4. Symbols always are written in the singular form (even when more than one is meant).
   • Correct: 1 mm, 500 mm, 1 kg, 36 kg
   • Incorrect: 500 mms, 36 kgs
   • But, it is correct to pluralize unit names: 25 kilograms, 250 millilitres
5. The symbol for a compound unit that is a product of two units is indicated by a multiplication dot (dot operator) or by a space. See How can I type unit symbols such as m², °C, N⋅m, and µm? in the FAQ for guidance on typing symbols.
   • Correct: kW⋅h or kW h (for kilowatt-hours)
   • Incorrect: kWh (do not write symbols next to each other)
6. The symbol for a compound unit that is a quotient of two units is indicated by a forward slash (solidus) or by a negative exponent.
   • Correct: km/h or km⋅h^-1 (for kilometres per hour)
   • Incorrect: kmph or kph (do not use p as a symbol for “per”)
   • But, it is correct to say or write “kilometres per hour”.
7. Symbols are case-sensitive, so the meaning of an SI symbol can be changed if you substitute an uppercase letter for a lowercase letter.
   • Correct: mm (for millimetre, which means 1/1000 of a metre)
   • Incorrect: MM or Mm (M is the prefix for mega, which means one million; a megametre is a million metres)
8. The points above are not subject to a house writing style nor limited to technical contexts, but are the only way to properly use the SI in any writing context.

Examples of incorrect SI/metric usage

CORRECT USAGE	INCORRECT USAGE	FOR
km	Km, km., KM, kms, K, k	kilometre
km/h	KPH, kph, kmph, km/hr	kilometre per hour
°C	C, deg C, ° C, C°	degree Celsius
m	M, m., mtr, mtrs	metre
L, l	L., l., ltr, ltrs	litre
mL, ml	ML, Ml, mL., ml., mls	millilitre
kg	KG, KG., Kg, Kg., kgr, kgs, kilo	kilogram
h	hr, hrs, HR, h., HR., HRS.	hour
s	sec, S, SEC, sec., s., S.	second
mm	Mm, mm., MM	millimetre
m2	sq m, sqm, sq. m., sq. mtr.	square metre
kW⋅h, kW h	kWh, kwh, Kwh, KWH, kWH, kw.h, kW.h, kw/h, KW/H	kilowatt-hour
g	G, G., g., gr, gm, gms, GR, GM, GRM, grms	gram
µg	mcg*, ug*	microgram
cm3	cc, cu cm	cubic centimetre
kHz	KHz, KHZ, Khz	kilohertz
MHz	MHZ, Mhz	megahertz
hPa	HPa, HPA, Hpa, mb	hectopascal
kPa	KPa, KPA, Kpa	kilopascal
K	°K, deg K	kelvin

Notes

^* Because the symbol “µg”, when handwritten, looks similar to “Mg” (megagram), which is often contextually interpreted as “mg” (milligram), and is therefore a frequent cause of overdoses, the abbreviation “mcg” is preferred in the medical field in the US (see the Joint Commission recommendations). For a similar reason and also the reason of being difficult to type, the symbol “ug” is sometimes used, especially in other countries, due to its similarity in appearance to “µg”. Despite their prevalence, these abbreviations are incorrect nevertheless.

Explanations

The spelling of metre and litre is the official one, and that of meter and liter is non-standard. In the US, the meter and liter spellings are commonly used despite their unofficial and non-standard status, even though metre and litre are just as valid and legal; the English spelling used by all other nations and international organizations (including the BIPM itself and ISO) is exclusively metre and litre.

In a strict sense, spelling and pronunciation are matters of language and are not set by the international standards that define SI. However, the SI is officially published in the English and French languages, and the spelling of SI units in these languages follows the official specification. Official specification notwithstanding, rules and patterns nevertheless exist within languages which guide spelling and pronunciation. As such, in keeping with the pronunciation of the other SI units involving prefixes, which all accent the first syllable, the logically consistent and proper pronunciation of the word kilometre is KILL-oh-meet-ur, not kill-AHM-it-ur. This is similar to the word nanometre, which is pronounced NAN-oh-meet-ur, not nan-AHM-it-ur.

The symbol for litre may be either a uppercase “ell” (L) or a lowercase “ell” (l); both are correct. In the US, Canada, and Australia, the uppercase “ell” (L) is preferred since it is easily distinguished with the digit one (1) and the uppercase “eye” (I), but most other nations use the lowercase “ell” (l) to adhere strictly to the standard of uppercase symbols being reserved for units derived from names of people.

A list of the SI/metric units and symbols, plus more details on their use is given in USMA’s Guide to the Use of the Metric System.

Fuente: https://usma.org/correct-si-metric-usage#