I'm not sure what you're asking. Inside or outside. The size is the approx. inside diameter. The outside diameter depends on the pipe you get. Look at this chart.http://www.engineeringtoolbox.com/pvc-cpvc-pipes-dimensions-d_795.htmlThat should get you your information.
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To help you use Re-design's chart, your pipe is probably "Schedule 40" - the first table shown. It wouldn't be the "Schedule 80" pipe in the second table unless thick-walled pipe is needed for some heavy-duty application.
Do you mean using the circumference (distance around the pipe) to calculate diameter? If so, then here is a conversion table.
All pvc piping is measured on the inside diameter (ID). That is copper, pvc, abs, cast iron, etc. Anything used for plumbing. If you're looking for something that uses the outside diameter (OD), look to refrigeration tubing, that is measured by the OD