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What Size Copper Pipe Do You Need For Water Lines?

A plumbing system, which mainly consists of water supply lines and DWV (Drain, Waste, Vent) lines, is a system of pipes, fittings, and faucets that are used to deliver potable water to your home and get water-based waste out.

Water lines are part of your property’s plumbing system that supplies water throughout your building. The water might come from your city’s water main or a private well. The piping system carrying the water into your building from the city’s water line is called a service-main supply line. These supply pipes are usually buried underground and made of galvanized iron pipes, copper pipes, or plastic pipes.

Water pipes are then divided into two directions. It might go straight to your faucets as cold water, or be redirected into your water heater to generate hot water.

If you’re planning to replace or restore your pipes, copper pipes and cross-linked polyethylene (PEX) pipes are the most likely options for your water systems.

In a plumbing system, copper tubing is mainly used for water lines, which uses water pressure to operate. But what size copper pipes do you need for your water lines? Keep reading to find out!

The Copper Pipe Types to Know

So, why copper pipes? 

Since the 1960s, copper piping has been used to deliver water to buildings in the US. Copper pipes are highly durable, naturally non-corrosive, and non-toxic compared to traditional piping systems. These characteristics prevent internal buildup that can cause your pipes to burst.

However, to prolong the lifespan of your copper pipes, you still need to monitor the water content and pressure running through your pipes. Water chemistry, leaks, and an unsuitable water pressure are some of the common factors that cause your pipes to wear down much quicker than they’re supposed to. 

You also need to monitor your pipes for leaks, cracks, and corrosion, as they’re telltale signs of failure in your piping system.

Copper pipes come in two forms: rigid pipes and soft tubes. 

Based on their thickness, there are three main types of copper pipes to know: Type K, Type L, and Type M.


  • Type K

Out of the three types often used for water distribution, Type K copper pipes have the thickest walls. This thickness makes it more durable and more expensive than the other two. For this reason, Type K pipes are often used underground, for industrial use, or in commercial buildings.

The wall thickness of a Type K pipe varies between 0.035 inches to 0.271 inches, depending on the nominal pipe size (NPS).


  • Type L

Type L copper pipes are the most common type used to distribute water in residential buildings, considering the balance between its durability and price. It’s not as expensive as Type K copper pipes, but it’s still durable enough for underground and outdoor use. 

Just like Type K pipes, the wall thickness of a Type L pipe varies according to the NPS you’re using and range from 0.03 inches for a ¼-inch pipe to 0.2 inches for an 8-inch pipe.


  • Type M

Type M copper pipes are the cheapest type of the three. The cost is a tempting factor for many homeowners; however, keep in mind that they’re only suitable for indoor plumbing. Make sure to check your local building codes before using Type M copper tubes, as it’s not allowed in some areas due to high pressure in your local water main.

Type M pipes are only available starting from ⅜ inches with a wall thickness of 0.025 inches to an 8-inch pipe with a wall thickness of 0.17 inches.


How to Size Your Water Supply Lines with Copper Pipes

When you’re replacing or adding new pipes, there are a couple things to consider before deciding what copper type and size of pipe to use. 

Start with considering the function of the pipes and which appliances it needs to connect with. Figure out the fixture unit of your appliances in your local building code. The fixture unit is a rating that determines how much water you need to supply, which will determine the size of the pipe needed. 

Then, map out the distribution of your water lines, including the sizes for your supply branch and main water line. Note that the pressure of your water lines decrease as you get higher above your water supply.

If you’re feeling overwhelmed, consider consulting professional plumbers to help you decide what type of pipe you need and in what size.

The size of your pipes is something a little confusing to measure, as the numbers marked on the pipes doesn’t really correlate with any measurement you can find on the pipe.

Initially, the size of a pipe is just the inner diameter. However, as technology has advanced, the pipe walls became stronger and didn’t need to be as thick. The inside diameter (ID) also changed to accommodate the various types of thickness available in the market.

The size of your pipes don’t really match the number engraved to them anymore. This brings us to the current pipe size standard used in North America: NPS, or the nominal pipe size.

While the inner diameter (ID) and wall thickness varies based on the type of the pipe, you can determine the nominal size of a copper pipe based on its outer diameter (OD). Fortunately, it’s easy to decide the NPS of a copper pipe if you know its outside diameter. 

Use this rule: The nominal pipe size of a copper pipe is always ⅛ inch less than the actual outer diameter. If, for some reason, the size of the pipes aren’t written on the pipes, you can use the string method to find out.

You only need four ingredients to do this: The pipe you’d like to measure, a piece of string, a ruler, and a marker.

  1. Find out the circumference of the pipe by wrapping the string around the pipe once. Mark the section of the string where it starts to overlap.
  2. Lay the string out on a flat surface. Measure the length of the string that was wrapped over the pipe with a ruler, using the mark on the string as reference.
  3. Divide the length by 3.143, and this is the outer diameter of your pipe.
  4. To find the NPS, you just need to subtract ⅛ inches (or 0.125 inches) from the outer diameter of the pipe.


Need to quickly check the size of your pipes? Here’s a table from Petersen Products to help you match the nominal pipe size to the diameter of your copper pipes.

Pipe size chart.

Note that this is the general method used for copper pipes. The size of a small diameter tubing, like ones you see with units such as a refrigerator or an ice maker, is measured based on its measured outer pipe diameter.


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