Inside Diameter vs Outside Diameter: What 1 1/2 Inch Pipe Actually Measures | Pipe Size Truth

NPS 1 1/2 pipe measures 1.900 inches outside diameter; the “1 1/2” is a nominal label, and the inside diameter changes with wall thickness.

A tape measure is a terrible judge of pipe. Put it across the end of a 1 1/2 inch pipe and you get 1.900 inches — not 1.500 — and nothing is wrong with the pipe. For pipe sizes below NPS 14, the nominal number functions as a label rather than a dimension. The outside diameter is locked to that label, while the wall thickness shifts by schedule, which moves the inside diameter. Get this straight once and fittings, clamps, and thread specs all start making sense.

The Nominal Size Rule

The number stamped on pipe refers to a nominal pipe size, or NPS. Wikipedia’s Nominal Pipe Size reference and the Engineering Toolbox NPS chart both list NPS 1 1/2 at 1.900 in (48.3 mm) outside diameter, which also corresponds to DN 40 in metric designations. That outside figure stays fixed no matter which schedule you pick.

Wall thickness is the variable. Schedule 40 pipe at this nominal size runs 0.145 in thick, leaving a 1.610 in inside diameter. Bump up to Schedule 80 at 0.200 in, Schedule 160 at 0.281 in, or down to Schedule 10 at 0.109 in or Schedule 5 at 0.065 in — the outside never moves, and the inside shrinks or grows with each step. The engineersedge pipe schedule tables confirm the same pattern across the standard schedules.

Inside Diameter vs Outside Diameter: Which Number You Actually Need

Match the outside diameter when the pipe has to fit into something. Fittings, clamps, couplings, and thread specs built for “1 1/2 inch pipe” are built for 1.900 in OD — that is the number your caliper needs to agree with. A fitting or clamp labeled for this nominal pipe size will not seat correctly on a literal 1.500 in tube, because the two are different products.

Match the inside diameter when the pipe has to carry something. Flow capacity, pressure drop, and anything sliding through the bore depend on the ID, and the ID is a schedule decision, not a size label. If you are shopping for a specific application, know your wall thickness before you buy — our tested 1 1 2 inch gas pipe roundup sorts options by matching specs instead of nominal labels.

Pipe vs Tubing: Why the Numbers Behave Differently

Pipe sizes are nominal; tubing sizes are usually actual. That single distinction explains most of the confusion. Order tubing and the stated number is the real outside dimension. Order pipe and the stated number is a designation that maps to a standard OD through published tables maintained by standards bodies and charted by sources like Engineering Toolbox.

Tubing also tends to be sold by OD with wall listed separately, while pipe gets sold by nominal size with schedule doing the work. Same material, same diameter on paper, two different systems. The Engineers Edge pipe schedule data covers the full range, but for a single pipe at a single nominal size, the pattern above is all you need: fixed OD, schedule-driven ID.