Double-spacing is a concept traditionally used in word processing. In professional page layout, as you would use it in InDesign, the space between lines is called "leading" and refers to finer increments than word processing (Word's Single, 1.5 lines, Double, etc.) It is measured in units called "points." There are 12 points in an inch.
In most professionally laid out publications, body copy of average column width is set with 1 to 4 points of space beyond the point size of the type. If the type size were 11 point, the leading might be set as between 12 and 15 points, depending on how much space you desired between lines.
Why don't you tell us what is you're trying to accomplish, and perhaps attach a screen capture of what you're trying to do?
Steve Werner wrote:
There are 12 points in an inch.
There are 12 points in a Pica, 6 picas to the inch (using postscript picas), so there are 72 points per inch.
Word, for example, and ID, by default set "automatic" leading to 120% of the type size. This makes for comforatble spacing for typical text sizes in most fonts, but isn't uninversally correct. Assuming 120% single space leading, double space would be 240%. If using absolute leading, you would double the leading value, which most likely will be somewhat larger than the type size itself to put a bit of space betwwen the descnders on one line and the caps and ascenders onthe next.
Thanks, Peter, for correcting my "duh" moment. It's hard coming back from vacation.