I have multiple doubts related to the usage of singular or plural SI base units when written in both symbol as well as name.

I have framed this question under two parts, namely, Part (a) and Part (b). Each part has three sentences which I have written on the basis of my understanding.

Please answer these 6 sentences/questions.

Part (a):

This page says that while using prefix for e.g., centi as in centimeter, it is

l = 75 cm long.(correct)
l = 75 cms long. (wrong)
  1. Does this rule is used for all SI prefixes (having powers of 10)?
  2. In this regard, we should be saying, or writing that, "how many cm are there in one metre?" (while saying we should say centimter or centimetres?)
  3. Please strike-through the wrong SI unit in the following sentences.

    My weight is 70 kg / kgs,

    or My weight is 70 kilogram / kilograms.

Part (b):

and the page next to above web link says, we should write:

2.6 m/s, or 2.6 metres per second.

In this regard, we should say, or write:

  1. Its speed is 0.26 metres per second.
  2. This pipe is 0.75 metres long.
  3. How many cm are there in 2 metres?

closed as off-topic by David Z Mar 24 '15 at 6:49

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  • $\begingroup$ my question is on hold; should I edit my question here or ask a new properly framed question? $\endgroup$ – a.s. Mar 24 '15 at 7:01
  • 3
    $\begingroup$ This might be better suited for English Language & Usage, as it's not really about physics but the usage of words & abbreviations. $\endgroup$ – Kyle Kanos Mar 31 '15 at 17:25

cms and kgs are wrong. The SI units are abbreviations which are also used in the plural. You will write 2.6 m/s or 1 m/s, but say "2.6 meters per second" or "1 meter per second" respectively.

Keep in mind the SI units are also used in tons of other languages that do not form the plural by attaching an -s. The units look the same in those languages. (e.g. German: 1 Meter pro Sekunde, 2.6 Meter pro Sekunde)

The prefix doesn't change anything: kilometers -> km, micrometers -> µm.

Also, by adding an s, you will confuse it with seconds: As could be "Ampere seconds", which would be a charge compared to "Amperes", a current. You see the problem.


One never pluralizes unit abbreviations.

Your link goes to the BIPM, the body responsible for maintaining the definitions of the international system of units, and is authoritative. The folks at NIST agree and address most of your questions.

I would say

  1. The pipe is 0.75 m long.


  1. The pipe is 75 centimeters long.

or even

  1. The pipe is seventy-five centimeters long.

In your example #6 you use an abbreviated unit name without a number, which seems improper but isn't forbidden by either of the references I examined tonight. I would have said

  1. How many centimeters are there in 2 m?
  • $\begingroup$ ok; does it mean that we write "75 cm / 70 kg" and say "75 centimetres / 70 kilograms"? $\endgroup$ – a.s. Mar 24 '15 at 6:56
  • $\begingroup$ As another example you could say "a seventy five centimetre long pipe"... $\endgroup$ – danimal Mar 24 '15 at 9:42
  • $\begingroup$ @a.s. Yes, one says "centimeters" and not "see em" $\endgroup$ – rob Mar 31 '15 at 16:48

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