There are industry standards that dictate the nominal inside and outside daimeters of pipes. In general the size quoted is approximately the inside diameter of the pipe, although it varies depending on wall thickness. The outside diameter is therefore larger than the quoted size. A 3/4" pipe for example has OD of 1.05 inches, and an inside diameter that could be between 0.74 to 0.88 inches. See the attached link for a chart of the different standard sizes.
By the way - there is no need to use exclamation points on every sentence.
http://i65.photobucket.com/albums/h2...eDiameters.jpg