Determining what size PVC pipe you use in your project or that you picked up from the home center can be frustrating and challenging.  FORMUFIT products are available in six (6) PVC sizes that work with FORMUFIT PVC pipe, but also work with off-the-shelf plumbing grade PVC pipe, available from your home center or hardware store.  

If you have not worked with PVC pipe before or are not familiar with how PVC pipes are sized, we highly recommend you review the following three important details about PVC Pipe sizing:

1. The Outside Diameter is not the PVC Pipe Size.
2. PVC Schedule is Important
3. Some Pipe isn't Compatible

The Outside Diameter of the pipe is not the PVC Pipe Size.

PVC Pipe sizes are so named by measuring the inside diameter (also called the bore) of the PVC pipe and not the outside diameter.  If you measure the outside diameter, it will give you a larger reading than the actual PVC Pipe Size.  The pipe will always be larger on the outside than the size name, as the thickness of the ‘walls’ of the pipe make it larger.

Correct way to Measure

FORMUFIT products follow the Nominal Pipe Size (NPS) naming convention or PVC specifications, which identifies the diameter of the hole of a PVC pipe with a closely-related dimensional number for its size. Therefore the Pipe Size for all of our products is based on the inside hole or ‘bore’ of the PVC pipe that it fits. This is the correct way to measure what PVC Pipe Size you need.

Incorrect way to Measure

You should never use the outside diameter of the pipe to determine your PVC pipe size unless going off of a reference chart. If you measure the outside diameter of your PVC pipe and order based on that measurement, you will almost always receive the next PVC size up. Never order PVC products based off the outside diameter, unless referencing our PVC pipe dimensions page.

PVC Schedule (abbrev. SCH or Sch.) is the thickness of the PVC pipe wall. The most common Schedule is 40, but Schedule 80 PVC is available from home centers or hardware stores.  As the schedule gets larger, the pipe wall gets thicker from the outside in, so the inside hole (bore) of the pipe gets smaller.

FORMUFIT internal-fit products, such as Internal Dome Caps, Adjustable Elbows, and Internal Couplings, are only compatible with Schedule 40 PVC pipe.   Our internal-fit products will not work with PVC Schedule 80, as the inner diameter of 80 Schedule PVC is much smaller than PVC Schedule 40, and they won't work with Thinwall as the inner diameter of the pipe is too large.

All external-fit products (Tees, Elbows, 3-Ways, etc.) will fit almost all schedules, including Thinwall, Schedule 40, Schedule 80, and Schedule 120 PVC, as the outside diameter is the same.

All off-the-shelf pressurized plumbing-grade PVC pipe is marked with the pipe schedule. Please see our PVC Pipe Markings Page to learn how to read them.

Not all types of pipe will work with FORMUFIT fittings and accessories:

CTS/CPVC Pipe

The most commonly confused non-compatible pipe is Copper Tube Size (CTS) plastic pipe, made from CPVC and uses an entirely different sizing system.  If you use a CTS pipe with FORMUFIT fittings or accessories, they will be much too large for the pipe. CTS and CPVC can be identified by their yellowish or tan color and are marked in the manufacturer printings as CTS or CPVC.  For more information on CPVC, please see our CPVC Incompatibilities page here.

ABS Pipe

While ABS pipe is size-compatible with most FORMUFIT products, it cannot be cemented together using standard PVC cement.  A special transitional cement must be used, or the pipe-to-fitting connection must be screwed together to connect them securely.  ABS pipe is also very brittle and is not recommended for structural use.

Steel Conduit or Pipe

Steel pipe and conduit have the same outside diameters as the corresponding PVC pipe sizes and are size-compatible, but are not recommended for use with PVC joints & fittings, as there is no way to connect the steel pipe to the plastic PVC fitting. In most cases, the steel pipe will easily slide out of the fitting, resulting in loose or possibly collapsing structures.