Pipe Sizing Explained

Days Gone By

Determining pipe sizes can be somewhat confusing, primarily because of the historical method of sizing and the fact that some current sizing still refers to those legacy systems. Take this example:

Years ago, a half‐inch pipe had an inner diameter of 1/2 inches. It also had thick walls, as that was the manufacturing standard at the time.

Over the years, improved technology has enabled the walls to be thinner while retaining the same capabilities. However, to match up with existing (larger‐walled) pipe, the inside diameter of the new pipe had to be larger.

Which means the pipe was not 1/2 inches, no matter which way you measured.

When mass production of pipe began, there was a need for standardization. In 1927, the American Standards Association – which has since evolved into the American National Standards Institute (ANSI) – convened a committee to standardize the dimensions of wrought steel and wrought iron pipe and tubing. Back then, only a few wall thicknesses were used: standard weight (STD), extra‐strong (XS), and double extra‐strong (XXS), based on the iron pipe size (IPS) system of the day.

By 1939, schedule numbers were starting to come into use, but the original terms stuck and are often still used today, though XS and XXS were revised slightly to extra‐heavy (XH) and double extra‐heavy (XXH), respectively.

By the 1950s, stainless steel was coming into use more frequently, allowing the use of thinner pipes (e.g., 5S and 10S), which were based on pressure requirements. (It is worth noting that because of their thin walls, the smaller "S" sizes cannot be threaded but must be fusion welded.)

Alphabet Soup of Pipe Metrics: IPS, DIPS, NPS, CTS

If the legacy of sizing isn't confusing enough, consider that pipes have been sized differently over the years depending on the specific sizing system:

• Iron Pipe Size (IPS) — sized by reference to the inside diameters; the standard from early 19th century through just after World War II; still used in PVC manufacturing and steel gas and water piping
• Ductile Iron Pipe Size (DIPS) — similar to NPS but used for larger pipes
• Copper Tube Sizing (CTS) — in the 1920s, this was combined with the IPS standard; the inside diameter is measured in the "types" (M, L, and K for thinnest, thicker, and thickest, respectively)
• Nominal Pipe Size (NPS) — outside diameter is fixed for a given pipe size and inside diameter varies depending on the wall thickness (referred to as "schedules")
• Plastic Irrigation Pipe (PIP) — used in agricultural applications; comes in pressure ratings (psi) and is available in diameters from 6" to 24"

NPS is the North American standard today. The current practice is to determine pipe size through two numbers: 1) the pipe bore (or diameter) and 2) the pipe schedule (or wall thickness) – though these two numbers can be configured in slightly different ways depending on the specific pipe used.

NPS sizes are documented by a number of standards, including API (American Petroleum Institute) and ANSI/ASME (American Society of Mechanical Engineers).

The most common pipes used today are:

• Cast iron — mostly in use before 1960; used for drain/waste/vent (DWV) lines
• Steel (galvanized pipe) — common in older homes; lasts only about 50 years
• Plastic — used since mid‐1970s; two types:
• ABS (acrylonitrile‐butadiene‐styrene) — black color; first to be used in residential homes, though some areas restrict their use in new construction
• PVC (polyvinyl‐chloride) — white or cream color; rating and diameter are stamped on the pipe; a few notes on PVC:
• Schedule 40 PVC is strong enough for drain lines and cold‐water lines, but local code will determine applicability. When used for cold‐water lines, it is generally not allowed for use inside a building
• Schedule 80 PVC is often used for cold water lines but isn't allowed for use inside a building in some areas because it isn't suitable for hot water
• CPVC (chlorinated polyvinyl chloride) is as strong as PVC but is heat‐resistant, which makes it acceptable in most areas for interior supply lines; it is most commonly measured with CTS standards (which is important when considering fittings for existing pipe; for example, a 2" fitting will not always fit on a 2" CTS pipe, but it will always fit on a 2" nominal size PVC pipe). Schedule 40 and 80 CPVC pipe and schedule 80 CPVC fittings are available and generally used in industrial applications
• Copper — commonly used in water lines and some drain lines; resists corrosion, lasts a long time
• Rigid Distribution Pipe — comes in three thicknesses: type M (thinnest), type L (thicker), type K (thickest)
• Rigid Drain Pipe — comes in one thickness Marked DWV and is thinner walled than type M
• Flexible (soft) — often used with appliances lines (e.g., dishwasher, refrigerator, icemaker) and rolled out for under slab installations
• PEX (cross‐linked polyethylene) — newest pipe for residential use; easy to install (cuts easily, is flexible); it can use compression fittings or push on fittings, more permanent connections require crimp style fittings and a crimping tool

Calculating Pipe Size

Knowing the background on how and why the sizing is determined is critical, though you will not necessarily need to resort to advanced math to figure it out. Fortunately, there are tables for every type of piping that is approved for water service and distribution. These charts are used to determine pipe size that will deliver the amount of water and at the pressure needed to operate a plumbing system that uses a variety of common household fixtures and appliances.