Pronouncing metric units and prefixes

The preferred pronunciation of metric units reflects the way in which the words are made up - that is, a prefix (such as "kilo") and a unit (such as "watt").

A unit name such as watt in a given language should be pronounced exactly the same regardless of any prefixes with which it is combined. Thus ‘watt' is pronounced the same in say

‘milli- watt '
‘kilo- watt '
‘mega- watt ' (hyphens and italics added for emphasis)

Similarly a prefix name such as ‘kilo-‘ should be pronounced and stressed exactly the same regardless of the unit it is combined with. For example in

‘kilo- watt'
‘kilo -gram'
‘kilo- metre'

With regard to this last unit (the kilometre), there is some controversy over the "correct" pronunciation. Consistency suggests that the word ‘kilometre' should be pronounced with stress on the first syllable (KIL-o-metre). However, it is recognised that some people stress the word on the second syllable - thus, "kil–OM–etre". It is likely that this pronunciation is based on the analogy with "barometer" and "speedometer". However, the latter are obviously measuring instruments - not units of measurement. It may be that the confusion first arose from the American spelling of "metre" as "meter".

Whatever its origins, the practice of stressing the second syllable is widespread, especially in the USA , Australia and Ireland . Languages are not necessarily logical, and it is more important that people should use kilometres than that they should pronounce the word in a particular way.

Having said this, UKMA believes in being consistent and recommends following the simple rules outlined above - that is, the stress should be on the first syllable. Indeed, it would be appropriate if this rule were to be taught in schools as part of the national curriculum.

The following pictures illustrate the point more graphically.