Pipe Size Table

It is a fossil, like so much else in trade sizing. Early American pipe was specified by its approximate inner bore in inches: 1 inch pipe carried roughly an inch of opening. As manufacturing improved, walls could be made thinner for the same pressure, and the industry froze the outer diameters instead - because fittings, threads and dies depend on the outside, not the inside. The result: nominal names kept their historical bore labels while actual bores drifted, and today NPS 1 means a pipe family whose OD is exactly 1.315 inches and whose opening depends on which schedule you buy. Two boundary notes make the system look even stranger: for very small pipes (1/8 to 3/8), the nominal number is larger than the actual OD, and from NPS 14 upward the nominal number equals the OD exactly (NPS 14 is truly 14 inches across). So the honest rule: below 14, treat NPS as a pure name and read the chart; at 14 and above, the nominal and the OD finally agree. The metric world ran the same fossilization in parallel - DN is the ISO nominal name with the same non-dimensional character - which is why the DN column here matters more than any straight millimeter conversion of inches.

Schedule is the wall thickness code - the second half of a pipe spec after the nominal size. The two numbers you meet constantly are Schedule 40 (the standard wall, what stores sell by default) and Schedule 80 (extra strong, roughly 1.3 to 1.8 times the wall depending on size). They exist because the same outer diameter must serve different pressure duties: a 1 inch Schedule 40 pipe has a 0.133 inch wall for ordinary water and air service, while Schedule 80 at 0.179 inch buys higher pressure capacity, deeper NPT thread engagement, and survival against mechanical abuse. Three selection habits. First, match the system: replacing pipe in an existing line means matching the schedule that is there, because the ID - and therefore flow, velocity and fitting behavior - changes with the wall. Second, match the medium: compressed air, steam and process lines commonly spec 80 where condensate, vibration or code requirements push that way; residential plumbing almost never does. Third, remember the algebra: Schedule 80 trades opening for strength, so the same nominal pipe flows measurably less - if a job computes its capacity from nominal size instead of actual ID, the error is the schedule. Higher schedules (120, 160) exist for high-pressure work, and the chart carries the two everyday ones because they cover nearly everything a generalist touches.

Three different abbreviations, three different jobs. NPS (Nominal Pipe Size) is the American name for the pipe family; DN (Diametre Nominal) is the ISO name for the same concept internationally, with a fixed mapping - NPS 1 is DN 25, NPS 2 is DN 50, and the DN number is roughly 25 times the NPS for sizes of 14 inches and below (DN follows a preferred-number series above that, so the doubling is exact only in the small sizes). Neither NPS nor DN is a measurement; both are order language. NPT (National Pipe Taper) is something else entirely: it is a thread standard, not a size system - the tapered threads that seal pipe joints. You order NPT fittings by nominal size, and an NPT 1/2 fitting threads onto NPS 1/2 pipe, but the letters describe the thread geometry (taper angle, threads per inch) rather than the pipe dimensions. The mix-up that matters in practice: NPT threads seal by thread interference plus sealant, and their depth of engagement depends on wall thickness - which is one more reason Schedule 80 exists for threaded work. When a drawing mixes the three (NPS pipe, DN valves, NPT ports), this chart is the translator for the dimensions, and the thread spec travels alongside unchanged.