Stand in the copper aisle and you will see two stickers on nearly identical pipe: a blue one and a red one. Same diameter, same fittings, and the red one is meaningfully cheaper. That is Type L and Type M, and the difference between them is one number — wall thickness — that decides how long the pipe lasts and, in a lot of jurisdictions, whether you are allowed to use it at all.
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 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 is the heaviest wall, Type L is medium, Type M is the lightest. There is a fourth, Type DWV, that is thinner still and is for drain, waste and vent only — never for pressurized water.
They are color-coded on the print line so you can tell them apart on a truck: Type K green, Type L blue, Type M red, DWV yellow. 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:
| 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 are not uniform:
- Buried water service outside the building is typically Type K. Soft copper in a continuous coil with no joints underground is the norm, and Type M is generally not accepted there. Do not bury Type M.
- 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 is effectively a Type L or K question everywhere, 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.
Where Type M is genuinely fine
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 cheapest insurance in the entire project. Our repipe cost breakdown puts that difference in context against the labor, and it is not close.
What it actually costs
Type M runs meaningfully cheaper than Type L — you are buying less copper, and copper is priced as a commodity, so the gap tracks the metal market rather than staying at a fixed percentage. On a small repair, the difference between an L and an M stick is pocket change. On a few hundred feet of repipe it becomes a real line item, which is exactly why builders specify M in volume housing.
The honest way to frame it: the saving is real, and it is also small relative to what it 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 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 press fittings are specified for use on ASTM B88 tube, and the same fitting presses onto K, L or M. 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.
So which should you buy?
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
Heads up: the product links below are Amazon affiliate links — you pay no more, and as an Amazon Associate we earn from qualifying purchases.
- Viega ProPress copper couplings — press fittings join Type L and Type M identically; this is the system our copper guides recommend.
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.
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 ASTM B88 print-line color code is green for Type K, blue for Type L, red for Type M, and yellow for DWV. 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. Press fittings built to ASME B16.51 are specified for ASTM B88 copper tube, 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 B88-20 — Standard Specification for Seamless Copper Water Tube (dimensions and type classes)
- ASME B16.51 — Copper and Copper Alloy Press-Connect Pressure Fittings
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 tests, code changes), drop your email. No spam, no course funnel, written by a licensed GA journeyman.
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