Absolute lengths
Since picas and points are typographical units of measurement, using them to design your Web pages might seem logical. Print designers use picas to measure the dimensions of pages and page elements (like column widths and page margins) and points to measure type size. A pica is 1/6 of an inch, and a point 1/12 of a pica.
However, avoid picas, points, and other absolute length units—inches (in), centimeters (cm), and millimeters (mm)—for Web design.
The size of elements displayed on the browser is affected by the size and resolution (expressed as the number of pixels, or dots, per inch) of the user's monitor, about which the Web browser knows nothing. An operating system can only guess (badly) how long or wide a pica, a point, or any other absolute length unit will be on a monitor. Thus, a Web page header set to a width of 48 picas, or 8 inches, will probably not actually be 48 picas. Thus, relative units are much more useful for sizing and positioning Web page elements. On the other hand, if you are creating a print style sheet or a print-friendly version of a Web page, then picas and points are the best choice.