Type L and Type M are the same ASTM B88 copper tube in two wall thicknesses, with the same outside diameter and the same fittings; blue is L, red is M. Type L is roughly 40% thicker and is the default for indoor plumbing. Type M is the thin one, fine on accessible above-ground runs where local code allows it.
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Here is the honest comparison: what the letters mean, what the standard actually specifies, where Type M is perfectly fine, and the two places I would never put it.
The short answer
Type L and Type M have the same outside diameter. Only the wall thickness differs, so they use the identical fittings — the same solder fittings, the same press fittings, the same cutter. Nothing about your tooling changes.
Type L is roughly 40% thicker in the wall at common residential sizes. That is the whole product difference.
Use Type L for anything buried, anything inside a wall or slab, and any repipe you want to forget about. Type M is defensible for accessible above-ground runs in a house with gentle water — and it is what a lot of tract housing is plumbed with.
Check your local code before you buy. Some jurisdictions restrict Type M for water distribution outright, and buried water service outside the building is typically Type K, not L or M.
What the letters actually mean
K, L and M are not grades of quality or different alloys. They are wall-thickness classes defined in ASTM B88, Standard Specification for Seamless Copper Water Tube. The copper is the same copper. Type K has the thickest walls, Type L is medium, Type M is the lightest.
Type K Copper Pipe
Type K has the heaviest wall of the three B88 classes and is printed green. At 3/4″ it carries 0.065″ of wall against Type L’s 0.045″, on the identical 0.875″ outside diameter. It takes the same fittings as Type L and Type M, because a fitting grabs the outside of the tube. Type K is the usual choice for a buried water service — a soft coil with no joints underground — while Type L is the inside-the-building default.
The head-to-head on that heavier class is in Type K vs Type L copper pipe.
Type L Copper Pipe
Type L is the medium-wall class of ASTM B88 seamless copper water tube, printed in blue. At 1/2″ its wall is 0.040″; at 3/4″, 0.045″. It is the usual choice for hot and cold water distribution inside a building, and it is the base minimum the Uniform Plumbing Code sets for water piping, with exceptions for Type M (UPC 604.3, below).
Type M Copper Pipe
Type M is the lightest-wall class of ASTM B88 copper water tube, printed in red. Its wall measures 0.028″ at 1/2″ and 0.032″ at 3/4″ — the same outside diameter as Type L, with roughly 30 percent less metal in the wall. It is a recognized, code-listed ASTM B88 material that many jurisdictions permit for above-ground water distribution, and a great many American houses are plumbed with it.
Copper DWV Pipe
There is a fourth copper tube you will meet in the same aisle, DWV, and it is not a B88 tube at all. The Copper Development Association’s Table 14.1 gives DWV its own standard, ASTM B306, its own yellow color code, and a size range that starts at 1¼″ — it is not made in the 1/2″ and 3/4″ sizes you buy for water.
Its wall is thinner than Type M at every size the two share (Table 14.2d). It is a drain, waste and vent material, so it is not a potable water tube. CDA does list HVAC and solar among its uses, so DWV is out of this comparison because it is not a water-supply tube.
Blue vs red copper pipe: which is which?
Blue is Type L, red is Type M. The Copper Development Association’s Table 14.1 gives the print-line color code as green for Type K, blue for Type L, red for Type M and yellow for DWV. The marking runs along the tube, so you can identify a pipe after it is installed.
If you remember one thing from this article, remember that red is the thin one.
The actual numbers
This is the part most articles skip, and it is the only part that matters. These are the ASTM B88 dimensions for the three sizes you will actually buy, as published in the Copper Tube Handbook (Table 14.2a, 14.2b and 14.2c):
| Nominal size | Outside diameter | Type K wall | Type L wall | Type M wall |
|---|---|---|---|---|
| 1/2″ | 0.625″ | 0.049″ | 0.040″ | 0.028″ |
| 3/4″ | 0.875″ | 0.065″ | 0.045″ | 0.032″ |
| 1″ | 1.125″ | 0.065″ | 0.050″ | 0.035″ |
Two things fall out of that table.
First: the outside diameter is identical across all three types. A 3/4″ Type M and a 3/4″ Type L are both 0.875″ on the outside. That is why the fittings are interchangeable — a fitting grabs the outside of the tube, and the outside has not changed. You can solder or press Type M and Type L with exactly the same parts, and you can join a length of one to a length of the other with an ordinary coupling.
Second: at 1/2″, Type L has 0.040″ of wall against Type M’s 0.028″. That is about 43% more copper standing between the water and your drywall. At 3/4″ it is 0.045″ against 0.032″ — about 41% more. Those are not rounding errors. That is the entire argument.
Where code decides for you
Before this becomes a judgment call, find out whether it is one. Copper type is one of the most commonly amended items in local plumbing code, and the rules for residential plumbing systems are not uniform:
- Buried water service outside the building is Type K in most of the country by practice — a soft coil with no joints underground — and many utilities and local amendments require it. The model codes are looser than the field: the IPC lists Types K, L and M for water service (Table 605.3), and the UPC’s Type L minimum for water piping makes an exception for Type M above ground in or on a building and underground outside the structure (UPC 604.3). I would still not bury Type M: it is the thinnest wall, and a buried leak is the one you find last.
- Water distribution inside the building is where jurisdictions diverge. Plenty allow Type M above ground. A meaningful number of local amendments restrict it or prohibit it outright for potable distribution.
- Under a slab inside the building is a Type L or K question under the UPC (minimum Type L for water piping) and, in practice, nearly everywhere else — and honestly it should be. See below.
One phone call to your building department settles this, and it settles it for free. If you are permitting the work, the inspector’s answer is the only answer that counts. If an inspector fails a Type M repipe after the drywall is up, the material savings evaporate several times over.
I send an occasional note when a material or code question like this is worth the write-up. Email only.
What is Type M copper pipe used for?
Type M is used for domestic water service and distribution, and the Copper Development Association (Table 14.1) also lists fire protection, solar, fuel oil, HVAC, snow melting and vacuum among its applications. In a house it shows up on above-ground, accessible water lines — basement ceilings, garages, utility rooms — where local code permits it.
I am not going to tell you Type M is junk, because it is not, and pretending otherwise is the kind of advice that makes people distrust plumbers. Type M is a listed, code-recognized material that meets ASTM B88, and enormous numbers of American houses are plumbed with it and have been for decades without incident.
Type M is a reasonable choice when all of these are true:
- The run is above ground and accessible — a basement ceiling, an unfinished utility space, an exposed run in a garage.
- Your water is not aggressive — not markedly acidic, not high-velocity, no history of pinhole leaks in the neighborhood.
- Local code permits it for the application.
- You are cost-sensitive and the run is long enough for the difference to be real money.
The classic legitimate use is a long horizontal feed across an open basement where you can see every inch of it. If it ever weeps, you will know within a day, and you can cut out a section without opening a wall.
Where I would never use Type M
Anywhere I cannot see it. That is the rule, and it is not really about metallurgy — it is about consequence. A pinhole in an exposed basement run is a bucket and an afternoon. The identical pinhole inside a finished wall, above a ceiling, or in a slab is drywall, insulation, flooring, and possibly a mold remediation invoice. The failure rate difference between L and M might be modest; the cost difference when one does fail is enormous. You are not really buying copper thickness, you are buying margin against a bad day.
Anywhere the water is aggressive. Copper pinhole corrosion attacks from the inside out, and it does not care what type it is eating — but on Type M it has about 30% less wall to get through. If your area has a known history of pinhole leaks in copper pipe, that is your water telling you something, and thin wall is the wrong response to it.
Anywhere with high velocity. Undersized lines and high pressure produce erosion-corrosion, which thins the wall at elbows and tees where the water changes direction. Again — same mechanism, less material to lose.
On a whole-house repipe. If you are already paying for the labor of opening walls, the material delta on the pipe itself is a small fraction of the job. Spending it to get Type L is the simplest insurance in the entire project. Our repipe cost breakdown puts that difference in context against the labor, and it is not close.
What the thinner wall actually saves
Type M puts less copper in the wall than Type L on the same outside diameter — about 30 percent less wall at 1/2, 3/4 and 1 inch on the CDA figures in the table above (our arithmetic). That is the whole saving: a stick of M is less metal than a stick of L. On a small repair the difference between an L and an M stick is minor. On a few hundred feet of repipe it adds up.
The honest way to frame it: the difference is real, and it is also small next to what the pipe protects. Weigh it against what sits under the pipe.
Does the type change how you join it?
No — and this surprises people. Because the outside diameter of copper tubing is fixed by ASTM B88 regardless of type, every joining method works the same on both:
- Soldering — identical fittings, identical process. Type M’s thinner wall does heat faster, so it is slightly easier to overheat and burn the flux. Back off the torch a little sooner than you would on L.
- Press — ASME B16.51 scopes press-connect fittings to hard drawn seamless copper water tube conforming to ASTM B88, and the same fitting presses onto K, L or M. Viega’s ProPress tube compatibility sheet puts it plainly: “When pressing onto B88 copper tube, types K, L, and M may be used.” Our guide to the best copper press fittings and the press tools that drive them applies identically to either type.
- Cutting — same tubing cutter either way. Type M deforms a little more easily under an over-tightened cutter wheel, so take an extra turn rather than crushing it. Any of the cutters we recommend handle both.
If you are weighing the joining method itself rather than the tube, we compared them directly in ProPress vs soldering.
Which type of copper pipe should you use for water lines?
For water lines inside a building, Type L. For a buried water service outside the building, Type K. Type M is a legitimate choice only where the run is above ground, accessible, the water is not aggressive, and the local code permits it — which is the case in plenty of jurisdictions.
Buried outside the building: Type K. Not a judgment call.
Inside a wall, ceiling, or slab: Type L. The extra cost is trivial next to what a concealed leak destroys.
Exposed, accessible, gentle water, code permits it: Type M is a legitimate choice and you are not cutting a corner.
Whole-house repipe: Type L. You are paying for labor anyway.
Not sure about your water: Type L. It is the default for a reason, and nobody has ever regretted the thicker wall.
The short version I would give a neighbor: if you can see the pipe, Type M is fine. If you cannot, buy Type L and stop thinking about it.
Pressing any of these types takes a tool and a fitting brand: NIBCO Press vs Viega ProPress covers the fittings, and RIDGID vs Milwaukee press tools covers the tools.
Working with either type
The cutter and reamer below are Amazon affiliate links; for the coupling and the fitting brush, the notes give each listing’s seller or stock on the day we checked.
- Viega ProPress copper couplings (not linked: third-party seller only, low stock, on 10/4/2026) — press fittings join Type L and Type M identically; this is the system our copper guides recommend.
- RIDGID 101 close quarters tubing cutter (40617) — RIDGID designs its close-quarters cutters for restricted spaces on hard and soft copper tubing. RIDGID lists a 1/4 to 1 1/8 inch capacity, and the 1/2-, 3/4- and 1-inch tubes in the table above measure 0.625, 0.875 and 1.125 inch outside diameter. On Type M, feed the wheel gently, as above.
- RIDGID 227S inner-outer reamer (29993) — RIDGID lists it for copper and stainless steel tubing from 1/2 to 2 inch, with 45 cutting edges on the inner cone for reaming the inside and deburring the outside of a cut end.
Cutting and joining: our pipe cutter guide and copper press fittings guide.
Whichever wall thickness you run: joints fail from poor prep, not from the pipe. Clean the tube and the fitting socket every time. 4-in-1 copper fitting brush (not linked: third-party seller only on 10/3/2026).
Frequently asked questions
Can you connect Type L and Type M copper together?
Yes. They share the same outside diameter, so an ordinary coupling joins them with no adapter and no special technique. Repairing a Type M line with a length of Type L is completely normal and is a small upgrade to that section.
Is Type M copper against code?
Not universally — Type M is a recognized ASTM B88 material and many jurisdictions permit it for above-ground water distribution. But local amendments vary a great deal, and some areas restrict or prohibit it. Buried water service outside the building is a separate question and is typically Type K. Confirm with your local building department before buying, especially on permitted work.
How much thicker is Type L than Type M?
About 40% at common residential sizes. At 1/2 inch, Type L is 0.040 inch of wall against Type M’s 0.028 inch. At 3/4 inch it is 0.045 inch against 0.032 inch. At 1 inch it is 0.050 inch against 0.035 inch. The outside diameter is identical in every case.
Does Type M fail sooner than Type L?
Under identical conditions it has less wall to lose, so a corrosion mechanism that would eventually breach either one breaches Type M first. In water that is not aggressive, both can last for decades. The practical issue is less about average lifespan and more about where the pipe sits — a thin wall concealed in a finished wall is a much worse bet than the same wall in an open basement.
What color is Type L copper pipe?
Blue. The print-line color code is green for Type K, blue for Type L, red for Type M, and yellow for DWV — but only K, L and M are ASTM B88 tube. DWV is a separate specification, ASTM B306, and its yellow marking belongs to that standard rather than to B88. The marking is printed along the tube, so you can identify it after installation as well as in the rack.
Can you use press fittings on Type M copper?
Yes. ASME B16.51 scopes press-connect fittings to hard drawn seamless copper water tube conforming to ASTM B88, and because the outside diameter does not change between types, the same fitting presses on Type K, L or M. No different jaws, no different fitting.
Sources
- ASTM B88-22 — Standard Specification for Seamless Copper Water Tube (dimensions and type classes; current edition, Oct 2022)
- ASME B16.51 — Copper and Copper Alloy Press-Connect Pressure Fittings
- Copper Development Association — Copper Tube Handbook, Table 14.1 (tube types, color codes, governing ASTM standards and listed applications)
- Copper Tube Handbook wall thicknesses: Table 14.2a (Type K), 14.2b (Type L), 14.2c (Type M) and 14.2d (DWV)
- International Plumbing Code (Georgia 2024, in force since January 1, 2026) — Table 605.3 Water Service Pipe (lists copper Types K, WK, L, WL, M, WM)
- Uniform Plumbing Code §604.3 Copper or Copper Alloy Tube (as adopted in the Hawaii Plumbing Code 2021, the edition UpCodes serves on this page: “Copper or copper alloy tube for water piping shall have a weight of not less than Type L. Exception: Type M … where piping is aboveground in, or on, a building or underground outside of structures.”)
- Viega — ProPress Copper Tube Compatibility, TD-PP 0824 (“When pressing onto B88 copper tube, types K, L, and M may be used”)
For the plumbers reading this
Most of this site is written for homeowners — this page clearly isn’t. If you’re in the trade and want the occasional straight-shooting note when I publish something like this (fittings comparisons, tool guides, code changes), drop your email. No spam, no course funnel, written by a licensed GA journeyman.