One-inch copper water tube has an actual outside diameter of 1.125 inches; the type letter — K, L, or M — sets wall thickness and inside diameter.
For the full breakdown, see our best 1-Inch Copper Tubing guide.
Buy copper by the size on the price tag and you can come home with the wrong thing: “1 inch” copper tubing actually measures 1.125 inches across the outside. The letter that follows the size — K, L, or M — sets the wall thickness, inside diameter, pressure limits, and price, and ASTM B88 governs all of it.
You’ll find the full range of 1 inch copper tubing types and sizes under ASTM B88, the standard that defines seamless copper water tube in US plumbing. In practice, home projects reach for just three — Type K, Type L, and Type M — and all three share the same outside diameter, so standard fittings swap freely between them.
Why Is “1 Inch” Copper Tubing Actually 1.125 Inches?
“1 inch” is a nominal label, not a measured promise. Copper water tube is manufactured to exact outside diameters, so one OD serves every wall thickness: 1-inch ASTM B88 tube is always 1.125 in across, whether it’s stamped K, L, or M. The type letter changes the wall, and the wall changes the inside diameter.
Keeping the OD constant lets one set of fittings serve every type — elbows, tees, and solder joints are sized to the 1.125-in outside diameter, so K, L, and M interchange at the connection. That also explains a classic leak: copper tube fittings follow copper-tube sizing, never the pipe-thread sizing used for steel pipe. The two systems stay separate.
1-Inch Copper Tubing Types: K, L, and M at a Glance
At 1 inch, the three types differ in wall thickness and inside diameter only. The figures below come from the Copper Tube Handbook dimension tables, which list the standard ASTM B88 water-tube sizes.
| Tube Type | Wall Thickness | Inside Diameter |
|---|---|---|
| Type K (green marking) | 0.065 in | 0.995 in |
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