Pipe sizes explained: NPS, schedule, and why 1/2 inch pipe isn't half an inch

Why isn't 1/2 inch pipe actually half an inch?

Because pipe sizes are names, not measurements. A 1/2 pipe measures 0.840″ across the outside, and the hole through a schedule 40 wall is 0.622″. The naming system is NPS, nominal pipe size, and nominal means exactly what it says: in name only. Every size up to 12 NPS works this way; the name matches neither the outside nor the inside.

The why goes back to the 1800s. Wrought iron pipe was named for its bore, and a 1/2 pipe really did carry about a half-inch hole inside the thick walls of the day. As steelmaking improved, mills thinned the walls, but every fitting and threading die already fit the outside. So the outside diameter froze, the spare metal came off the inside, and the bore grew while the name stayed put. Today you buy the name, the OD is guaranteed, and the schedule decides what's left in the middle.

How does the pipe schedule system work?

Schedule is the wall thickness series. For any one pipe size the outside diameter never moves, and the schedule number sets the wall: schedule 40 is the everyday standard, schedule 80 is the heavy wall, and the extra metal always comes out of the bore. A 1/2 NPS pipe is 0.840″ outside whether it's schedule 10, 40, or 80. Only the hole changes.

That one rule is why the system survives. Because the outside never changes, one fitting fits every schedule of its size, and one die threads them all. It also keeps upgrades cheap. As of June 2026, 12″ of 1/2 black schedule 40 runs $6.32 and the same pipe in schedule 80 runs $7.61. That's $1.29 for roughly 35% more wall.

What are the real dimensions for each pipe size?

Here's the chart we'd hand you at the counter, with the ASME B36.10 figures that define steel pipe. OD is the true outside diameter behind the name, wall is the metal, and ID is the hole left over. Everything is in decimal inches, with the OD in millimetres for anyone converting.

Two notes for reading it. At 14 NPS and up the system finally tells the truth: OD equals the name, so a 14 pipe really is 14.000″ outside. And metric drawings label the same pipe in DN, so DN 15 is 1/2 NPS and DN 50 is 2 NPS. Our carbon steel rack covers the working middle of this chart: black A53 from 1/4 to 6 NPS and galvanized from 1/2 to 4 NPS, cut to your length up to 96″ online.

What's the difference between pipe and tube?

Pipe is sized by name and schedule because its job is carrying something: water, air, gas. The bore is the point and the OD exists for fittings. Tube is sized by its real outside diameter and wall because the metal itself is the part: axles, sleeves, rollers, pivots, frames. Order 3/4″ mechanical tube and it measures 0.750″ outside. Order 3/4 NPS pipe and it measures 1.050″. Mix them up and nothing fits.

Tube is also the precision side of the family. The grade machinists ask for is DOM, drawn over mandrel: a welded tube cold-drawn over a mandrel until the wall is uniform, the bore is true, and the surface is smooth. Our rack plays that role with 1026 cold rolled mechanical tube, labelled the honest way by OD, bore, and wall. A 12″ piece of 3/4″ OD with a 0.120″ wall runs $9.42 as of June 2026, while the physically bigger 3/4 NPS schedule 40 pipe runs $6.76. You're paying for accuracy, not diameter.

Can you build with pipe, or is it only for plumbing?

People build with pipe constantly, and the spec holds up: A53 is usually supplied as grade B, which carries a minimum yield of 35,000 psi, real structural-grade steel; the mill test report with your order shows the grade. Schedule 40 makes solid railings, fence posts, gate frames, bollards, and awning frames, and off-the-shelf railing clamps are sized to pipe names, mostly 1-1/4 and 1-1/2 NPS.

The limits are practical ones. Pipe is round, so it copes and bolts less cleanly than square tube, and for benches, gates, and most frames square or rectangle tube is the better buy; our angle vs channel vs tube guide walks that choice. And a column holding up a building or a deck is an engineering job, not a chart lookup. We'll cut whatever the drawing calls for, but the sizing should come from your engineer.

Should you buy black or galvanized pipe?

Buy black A53 for anything indoors, anything you'll weld, and anything getting paint anyway. Buy galvanized when the pipe lives outside and you don't want to babysit a coating: the zinc goes on at the factory and outlasts anything brushed on later. One caution before you weld galvanized, grind the zinc back around the joint and ventilate, because zinc fume causes metal fume fever. The full playbook is in our guide to keeping carbon steel from rusting.

Lengths price by the inch as you type, so 36″ of that 1/2 black pipe runs $11.51, and volume discounts of 5-15% apply automatically. If your drawing says A106 instead, that's the cousin spec made without a weld seam, written for heat and pressure work like boiler and steam lines; we stock it alongside A53 in schedule 40, 80, and heavier walls. Pipe sizing isn't a steel-only system either: our 304, 316, and 6061 pipe wear the same NPS names, and June 2026 numbers for those sit in our Canadian metal prices guide.

What about threaded pipe and NPT?

NPT, national pipe taper, is the standard thread on North American pipe, defined by ASME B1.20.1. The threads sit on a shallow cone that narrows 3/4″ per foot, so the joint wedges tighter metal-to-metal as you wrench it, with PTFE tape or pipe dope sealing the spiral path. The die is cut to the pipe's true OD, one more reason that OD never changes, and thread names follow pipe names: 1/2 NPT fits 1/2 NPS. Threading removes wall, which is why high-pressure threaded systems spec schedule 80; there's more metal left under the thread.

Here's where we're straight with you about our part. We sell A53 and A106 pipe cut to your length, free cuts to ±1/8″, and an order placed by 1pm Atlantic on a business day usually ships the next. We don't thread online orders. Need threaded ends, a groove, or full lengths up to 21 ft? Send a quote request with the size, schedule, and count, and we'll price it; sourcing requests run 2-21 days.