Proper Abbreviation of Measurement Units

Date: 03/04/2004 at 09:25:51
From: Norma
Subject: Customary measurement abbreviations

Is a period used after the abbreviations for the customary units of 
measurement?  In the past, I understood that all abbreviations used a
period.  Now, I typically see inch as in., but ofter see foot (ft),
yard (yd), and others with no period.  I think maybe in. is used so as
not to be confused with the word "in".

Date: 03/04/2004 at 12:21:40
From: Doctor Peterson
Subject: Re: Customary measurement abbreviations

Hi, Norma.

As the following page points out, there is no "official" rule for 
writing customary units, as there is for SI (which omits all periods 
in unit symbols):

  Using Abbreviations or Symbols
    http://www.unc.edu/~rowlett/units/symbol.html 

That site chooses to follow SI rules for all units, so that no periods 
are used, even for "in".  I found various writing style guides that 
recommend the rule you have observed, omitting the period except after 
"in.", presumably because of potential confusion.  Here is one:

  ASAE Authors' Guide
    http://www.asae.org/pubs/style.html 

  Omit periods after abbreviated units (except "in." for inch):

    5 m, 30 cm, 40 ft

This presumably results from familiarity with the metric system and 
its rules, which then spread to other units for the sake of 
consistency.  Whether "in." will lose its period in common usage
before the inch is fully replaced by the metric system is a good 
question!

If you have any further questions, feel free to write back.

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/