How to spell out measurements correctly is a hot topic for technical editors and I suspect will remain so for eternity.
Ultimately, there is no absolutely-correct and absolutely-wrong way of writing anything. I’ve seen so many different writing guidelines and style books recommending so many different principles that I cannot in all good conscience be an absolutist in this matter.
Yet I also have to admit that there does exist “time honored” ways of spelling and punctuation widely used by most technical communicators. Here is my take on some of them:
No period after abbreviations, except one
Only “in” for inch takes a period afterward, in order to differentiate it from the preposition “in.” Others don’t.
Non-breaking space in between a number and the measurement unit
Why? If your number ends up at the end of a line, the measurement will not appear all alone in the beginning of the next line. It makes sense to keep a number and its unit together, at all times. Thus, the non-breaking space.
Leave one space between a number and its unit and do not italicize the unit abbreviation.
A bit (b or bit) versus a byte (B)
A bit is the abbreviation of “binary digit” and its value is either one (1) or zero (0).
A byte, on the other hand, means “a collection of bits” and is used to code alphanumeric characters. That’s why it does not make sense to talk about “bit size” (it’s always 1 digit) but makes perfect sense to refer to “byte size” (since the number of bits in a byte differs from one application to another). If in doubt, you can safely assume that 1 Byte is 8 bits.
Recommended practice: write out bits openly as in “123 kbits” to differentiate it from “bytes.”
1 B (byte)
1 bit (bit)
1 MB (megabyte)
Physical Dimensions and Screen Resolution
Write the physical dimensions of a page or an object by using the word “by” and not multiplication sign or the letter “x”.
Use the multiplication sign “x” (× in HTML) (but not the letter “x”) for expressing screen resolution.
1280 by 1024
8.5″-by-11″ or 8.5-by-11-inches
1280 x 1024
Different notation for dates
God knows how many mistakes have been committed in the past due to the differences in American and non-American way of writing the dates. 5/11/2010 is May 11, 2010 in the USA but November 5, 2010 in many other countries around the world. That’s why I have to classify the American way of writing dates as “incorrect” since it can lead to errors, depending on the cultural background of the reader.
May 11, 2010
December 1, 2010
Singular and plural for units
If the measurement is less than one use singular unit — except for zero :-). For units of measure larger than one use plural units.
No comma for addresses, decimal fractions, and page numbers
67,890 Main Street
67890 Main Street
Different notations for decimal point
Also take note that again USA and some other countries use period and comma differently when marking a decimal point.
$ 1,200,00 (One million and two hundred thousand dollars)
3,400.25 (Three thousand four hundred and a quarter)
Some other countries
$ 1.200.000 (One million and two hundred thousand dollars)
3.400,25 (Three thousand four hundred and a quarter)