Copper Pipe Sizing & Types (ASTM B88 Types K, L, M)
Direct Answer
Seamless copper water tubing is standardized under ASTM B88 into three primary wall thickness grades: Type K (Green / Heavy Wall for underground mains), Type L (Blue / Medium Wall for standard interior residential plumbing), and Type M (Red / Thin Wall for low-pressure hydronic heating). All three types share identical outer diameters for standard solder, flare, and press fittings.
Source: ASTM B88-20 (Standard Specification for Seamless Copper Water Tube)
Specs
| Governing Standard | ASTM B88 / NSF 61 |
| Type K (Green Stripe) | Heavy wall: 1/2" = 0.049" (1.24 mm) | 3/4" = 0.065" (1.65 mm) wall |
| Type L (Blue Stripe) | Medium wall: 1/2" = 0.040" (1.02 mm) | 3/4" = 0.045" (1.14 mm) wall |
| Type M (Red Stripe) | Thin wall: 1/2" = 0.028" (0.71 mm) | 3/4" = 0.032" (0.81 mm) wall |
| Outer Diameter Formula (CTS) | Actual OD = Nominal Size + 1/8 inch (0.125") |
| Joining Methods | Capillary solder (sweat), ProPress copper crimp, flare, compression |
| Operating Temperature | Up to 400°F (204°C) continuous in hydronic heating |
CTS Sizing and Shared Outer Diameters
Copper water tube adheres to Copper Tube Size (CTS) standards, where the actual outer diameter is always $1/8\text{ inch}$ ($0.125\text{ in}$) larger than the nominal size:
- $1/2\text{-inch Nominal}$: Actual $OD = 5/8\text{ in}$ ($0.625\text{ in} / 15.88\text{ mm}$)
- $3/4\text{-inch Nominal}$: Actual $OD = 7/8\text{ in}$ ($0.875\text{ in} / 22.23\text{ mm}$)
- $1\text{-inch Nominal}$: Actual $OD = 1\text{-}1/8\text{ in}$ ($1.125\text{ in} / 28.58\text{ mm}$)
Because Types K, L, and M maintain identical outer diameters, all standard copper fittings (elbows, couplings, tees, valves) are fully interchangeable across all three wall thickness grades.
Type K vs Type L vs Type M Applications
The colored longitudinal stripe identifies the wall thickness grade and code compliance:
- Type K (Green Stripe): The heaviest wall thickness. Required by plumbing codes for underground water service lines, trenchless moleing, commercial municipal supply, and severe vibration environments. Available in rigid straight lengths or annealed soft rolls.
- Type L (Blue Stripe): The standard interior residential potable water line grade. Balances structural strength, corrosion resistance, and material cost for domestic hot and cold branches.
- Type M (Red Stripe): The thinnest wall. Used predominantly for closed-loop residential hydronic heating systems and low-pressure domestic water where local plumbing codes permit. Rigid straight lengths only.