"Pipe" and "tubing" get used interchangeably all the time — but if you're sourcing materials for a construction, fencing, or fabrication project, knowing the difference can save you money and prevent a costly mismatch. The Core Difference: How They're Measured Pipe is traditionally measured by nominal pipe size (NPS), which refers to a rough inside diameter, not the actual outer dimension. A "2-inch pipe" from a standard plumbing supplier doesn't actually measure 2 inches any