Beginning plumbers often spend more time running back and forth to their supplier than they spend doing the actual work because it takes practice and experience to be able to figure out everything you need ahead of time. The first step in becoming an efficient plumber is to learn to correctly identify the pipes and fittings a job requires.
Plumbing dimensions aren't always what they appear to be. A plastic pipe with a 7/8-inch outside diameter, for instance, is actually called a 1/2-inch pipe because it has a 1/2-inch inside diameter and pipes are usually sized according to their inside diameter (ID). This dimension is also referred to as the nominal size, the size you ask for at a plumbing supplier.
If you are at all unsure about getting the right material, make things perfectly clear by specifying ID for most pipes. In a minority of cases -- flexible copper lines, for example -- pipe is ordered by using the outside diameter (OD).
If you can measure the inside dimension, you're home free. However, often you won't have a way of measuring the inside of the pipe. Holding a ruler against a pipe will give you only a rough idea of the outside diameter. Instead, use a string or a set of calipers for a more exact measurement. Once you find the outside dimension, use the chart on the last page to find the nominal size.
Fittings can be just as confusing. Their inside diameters must be large enough to fit over the pipe's outside diameter so that a half-inch plastic elbow, for example, has an outside diameter of about 1-1/4 inches.
As a rule of thumb, the OD of copper is 1/8 inch greater than its ID, the nominal size. For plastic pipe, measure the OD and subtract 3/8 inch. For threaded and cast-iron, subtract 1/4 inch.
Another mathematical pitfall for a beginning plumber is measuring the length of a pipe running from one fitting to the next. Pipes must fully extend into fixture and fitting sockets, or the joint could leak. Socket depths vary from one pipe size and material to another, so you must account for the depth of each fitting's socket in the total length of pipe needed between fittings.
The only times you don't have to take socket depth into account are when you are using no-hub cast-iron pipes (see Tapping Into Cast-Iron Drain Lines, Related Projects) or slip couplings with copper or plastic pipe (see Winterizing a House, Related Projects).