From the ECMA Spec (4th Edition part 1) Page setup: “Left Page Margin in inches” p. 1647
Docs from http://startbigthinksmall.wordpress.com/2010/01/04/points-inches-and-emus-measuring-units-in-office-open-xml/
See also http://msdn.microsoft.com/en-us/library/dd560821(v=office.12).aspx
dxa: The main unit in OOXML is a twentieth of a point. Also called twips. pt: point. In Excel there are 72 points to an inch hp: half-points are used to specify font sizes. A font-size of 12pt equals 24 half points pct: Half-points are used to specify font sizes. A font-size of 12pt equals 24 half points
EMU: English Metric Unit, EMUs are used for coordinates in vector-based drawings and embedded pictures. One inch equates to 914400 EMUs and a centimeter is 360000. For bitmaps the default resolution is 96 dpi (known as PixelsPerInch in Excel). Spec p. 1122
For radial geometry Excel uses integer units of 1/60000th of a degree.