To measure cast iron pipe, wrap a flexible tape measure or string around the outside of the pipe, divide that number by 3.1415, and you’ll have the outside diameter (OD). From there, you can match the OD to a nominal pipe size, which is what you’ll use when buying fittings or replacement sections. The tricky part with cast iron is that two pipes labeled the same nominal size can have different outside diameters depending on the type and weight class.
Measure the Outside Diameter
The most reliable measurement you can take on an installed cast iron pipe is the outside diameter. If the pipe is accessible and you can reach around it, wrap a flexible tape measure or a piece of string around the pipe at a point along the straight barrel, away from any hub or joint. If you used string, lay it flat against a ruler to get the circumference.
Divide the circumference by 3.1415 (pi) to get the outside diameter. For example, a circumference just under 6 inches divided by 3.1415 gives you about 1.9 inches, which corresponds to a 1-1/2 inch nominal pipe. A circumference of about 13.75 inches works out to roughly 4.38 inches OD, which is a 4-inch hubless pipe.
If you can get a caliper or even a rigid ruler across the pipe’s cross-section, you can measure the OD directly. Just make sure you’re measuring the barrel (the straight section), not the hub, which flares out and will give you a misleadingly large number.
Identify Your Pipe Type First
Cast iron drain pipe comes in two basic styles, and you need to know which one you have before your measurement means anything.
Hub and spigot (also called bell and spigot): One end of each pipe section has a wider bell-shaped opening (the hub), and the plain end of the next section slides into it. These joints were traditionally sealed with molten lead and oakum, or with a rubber compression gasket. If you see that flared bell at every joint, you have hub and spigot pipe.
Hubless (no-hub): Every end is the same diameter, with no flare. Sections connect using rubber couplings held tight with stainless steel band clamps. This style became common from the 1960s onward and is simpler to work with for repairs.
This distinction matters because hub and spigot pipe comes in two weight classes, Service (SV) and Extra Heavy (XH), and those classes have different outside diameters for the same nominal size. Extra Heavy pipe has thicker walls, and that extra thickness is added to the outside, making the pipe physically larger. Service weight and Extra Heavy are not interchangeable without transition fittings. Hubless pipe is only made in one thickness class.
Match Your OD to a Nominal Size
Cast iron pipe is sold by nominal size, which roughly corresponds to the inside diameter but doesn’t match any single measurement exactly. What you need is the outside diameter to figure out which nominal size you’re dealing with. Here are the most common sizes you’ll encounter in residential drain, waste, and vent systems:
Hubless Pipe
- 1-1/2 inch nominal: 1.96 inches OD
- 2 inch nominal: 2.35 inches OD
- 3 inch nominal: 3.35 inches OD
- 4 inch nominal: 4.38 inches OD
- 5 inch nominal: 5.30 inches OD
- 6 inch nominal: 6.30 inches OD
Service Weight Hub and Spigot
- 2 inch nominal: 2.30 inches OD
- 3 inch nominal: 3.30 inches OD
- 4 inch nominal: 4.30 inches OD
- 5 inch nominal: 5.30 inches OD
- 6 inch nominal: 6.30 inches OD
Extra Heavy Hub and Spigot
- 2 inch nominal: 2.38 inches OD
- 3 inch nominal: 3.50 inches OD
- 4 inch nominal: 4.50 inches OD
- 5 inch nominal: 5.50 inches OD
- 6 inch nominal: 6.50 inches OD
Notice how the differences between types can be small. A 4-inch hubless pipe measures 4.38 inches OD, while a 4-inch Service weight pipe measures 4.30 inches and a 4-inch Extra Heavy measures 4.50 inches. That’s a spread of only 0.20 inches across all three types. Getting a sloppy measurement here can easily send you home with the wrong coupling.
Why Inside Diameter Is Less Useful
You might think measuring the opening you can see at the end of the pipe is the simplest approach, but the inside diameter is harder to measure accurately on installed pipe and less useful for selecting fittings. Fittings and couplings slip over or into the outside of the pipe, so the OD is what determines compatibility.
That said, the inside diameter does correspond more closely to the nominal size. A 4-inch Service weight pipe has an inside diameter of 3.94 inches, and a 4-inch Extra Heavy pipe has an inside diameter of exactly 4.00 inches. If you can access a clean, open end of the pipe, a rough inside measurement can help confirm your nominal size. Just don’t use it to buy fittings.
Measuring for a Replacement Section
When you’re cutting out a damaged section and replacing it, you need two measurements: the outside diameter (to select the right pipe and couplings) and the length of the section you’re removing.
For length, measure the distance of the section you plan to cut out, then subtract the space needed for your couplings on each end. No-hub rubber couplings typically need about an inch of pipe inserted into each side, so your replacement piece should be cut slightly shorter than the gap to allow room. Check the specific coupling you purchased, as insertion depths vary by manufacturer and size.
When measuring length on a run that includes fittings like elbows or tees, measure to the center of each fitting. This is called a center-to-center measurement and is the standard way plumbers communicate pipe run lengths.
Tips for Measuring Installed Pipe
Old cast iron in basements and crawl spaces is often coated in decades of grime, paint, or corrosion that can add thickness to your measurement. Scrape a small section of the barrel clean before wrapping your tape. Even a thin layer of paint can throw off your reading enough to confuse a 4-inch Service weight pipe (4.30 inches OD) with a 4-inch hubless pipe (4.38 inches OD).
Cast iron pipe is also allowed a small amount of manufacturing variation. Hubless pipe in sizes 1-1/2 through 5 inches can be off by up to 0.09 inches from the listed OD, and can be out of round by 0.04 inches. Larger sizes (6 inches and up) can be out of round by 0.07 inches. If your measurement doesn’t land exactly on a number from the charts above, round to the nearest listed OD.
If the pipe is completely enclosed in a wall or ceiling and you can only access one end, measure the inside diameter at the opening and compare it to known values. A 2-inch Service weight pipe has a 1.96-inch bore, while a 3-inch has a 2.96-inch bore. Those are different enough to identify with a tape measure. Then confirm whether it’s hubless or hub and spigot based on the joint style you can see, and you’ll have enough information to buy the right parts.