A 1-inch PEX pipe measures 1.125 inches outside and roughly 0.875 inches inside — “1-inch” is a nominal CTS label, not a literal measurement.
Buy the wrong coil and you find out at the crimp ring. Copper Tube Size (CTS) sizing runs on outside diameter, so a 1-inch PEX pipe is never 1 inch across — the number on the label describes a nominal standard rather than the physical tube. This matters before checkout and again before the first fitting goes on.
Those two numbers differ because PEX tubing is sized by outside diameter first. Wall thickness fills the rest, which leaves the inside diameter smaller than the name suggests. Fittings key off the outside dimension and the CTS standard, so a mismatch in one of those breaks the joint.
What Is 1-Inch PEX Pipe?
1-inch PEX pipe is flexible cross-linked polyethylene tubing sold by nominal Copper Tube Size, not by measured diameter. ASTM F876 covers PEX tubing used for hot- and cold-water distribution, hydronic heating, and related pressure applications.
The standard sets an outside diameter rule the trade relies on: OD equals the nominal size plus 1/8 inch. Apply that to a 1-inch nominal tube and you get 1.125 inches outside — the number printed on every credible spec sheet.
PEX also isn’t a threaded pipe, so its sizing does not follow NPT or BSP thread naming. A “1-inch” copper pipe and a “1-inch” PEX pipe describe different physical tubes. Confusing the two is a common reason parts come home wrong.
Inside Diameter vs. Outside Diameter
The outside diameter of 1-inch PEX is 1.125 inches, and its inside diameter is about 0.875 inches under ASTM F876.
Three terms do all the work when reading a spec sheet:
- Nominal size (CTS): the trade label — the “1 inch” on the coil.
- Outside diameter: the measurement fittings must match, 1.125 inches here.
- Inside diameter: the flow path, roughly 0.875 inches here.
Water moves through the inside diameter, not the nominal size.
| Dimension | 1-Inch PEX Spec | Why It Matters |
|---|---|---|
| Nominal size | 1 in (CTS) | Label and ordering reference |
| Average outside diameter | 1.125 in | What fittings clamp to |
| Minimum wall thickness | 0.125 in | Sets the inner bore |
| Average inside diameter | 0.875 in | Flow path for water |
| Governing standard | ASTM F876 | OD-based sizing framework |
| Typical role | Mains / long runs | High flow, fewer joints |
Does 1-Inch PEX Use Copper Tube Size Sizing?
Yes — 1-inch PEX uses CTS sizing, the same nominal framework copper tubing follows, which is why the name looks familiar. The naming convention describes nominal size, not the measured outside or inside dimension.
Because sizing is OD-based, dimensional tolerances are governed by outside diameter and wall thickness. Fittings are then selected to match that outside measurement and standard, not the printed name alone. Naming the size is not enough on its own.
When you’re comparing brands before ordering, our roundup of the best 1 inch pex pipe options lines up what to check on each listing.
How Do You Verify You Have The Right Size?
Confirm the outside diameter and the standard, not the label.
Checklist for any coil or straight length:
- Nominal size reads 1 inch (CTS).
- Outside diameter is stated as 1.125 inches.
- Minimum wall thickness is 0.125 inches.
- ASTM F876 or F877 is named on the label or datasheet.
- The fitting family matches the outside diameter and standard.
Skip the size check on an unmarked coil and you risk a mismatch that shows up mid-install. The nominal name is a starting point, and the tolerance and fitting spec confirm the rest.
FAQs
Is 1-inch PEX actually 1 inch in diameter?
No. The “1 inch” is a nominal CTS size, not a physical measurement. Common 1-inch PEX tubing has an average outside diameter of 1.125 inches and an inside diameter near 0.875 inches under the ASTM F876 framework, so the label describes a sizing standard rather than the tube itself.
Can I swap 1-inch PEX for 1-inch copper?
Not by matching the name. PEX sizing runs on an outside diameter framework and its own fitting families, while copper and threaded pipe follow different conventions. Match the outside diameter and confirm the tubing standard and fitting system before substituting one material for the other.
What size fittings does 1-inch PEX need?
Fittings are selected to match the tubing’s actual outside diameter and standard, not the nominal label. For 1-inch PEX that means fittings built for 1.125-inch outside diameter tubing rated to PEX tubing standards. Using nominal size alone is a common cause of mismatched connections.
References & Sources
- ASTM International. “Standard Specification for Crosslinked Polyethylene (PEX) Tubing (F876-24).” Sets the OD-based sizing framework and scope for PEX tubing.
- The Engineering Toolbox. “ASTM F876 PEX Tube — Sizes.” Lists nominal, outside, wall, and inside dimensions for 1-inch PEX.
