A tee fitting’s size is read in a specific order: run first, then branch. The run is the straight-through pipe, and the branch is the perpendicular outlet. For a standard straight tee where all three openings are the same size, you’ll see a single number, like “1 inch tee.” For a reducing tee, where the openings differ, the size is written as three numbers separated by “x” marks, and knowing which number refers to which opening is the whole trick.
The Three-Number Rule for Reducing Tees
Every reducing tee is described with three dimensions, always in the same order. The first number is the largest run opening (the inlet side). The second number is the other run opening (the outlet side, continuing straight through). The third number is always the branch, the perpendicular outlet that forms the “T” shape. So a fitting labeled “1-1/2″ x 1/2″ x 1-1/4″” tells you the inlet run is 1-1/2 inches, the outlet run is 1/2 inch, and the branch coming off the side is 1-1/4 inches.
This order matters when you’re ordering parts. If you flip the run and branch numbers, you’ll get the wrong fitting. A 2″ x 2″ x 1″ tee has a full-size run with a smaller branch, while a 2″ x 1″ x 2″ tee has a reduced outlet on the run and a full-size branch. Those are physically different fittings.
Straight Tees vs. Reducing Tees
A straight tee (also called an equal tee) has all three openings at the same nominal size. It’s listed with just one dimension because there’s nothing to differentiate. A “2-inch straight tee” means all three ports are 2 inches.
A reducing tee has at least one opening that differs from the others. Catalogs group reducing tees by their largest run size, then list the available combinations of smaller outlets. If you’re browsing a supplier’s website, you’ll typically find reducing tees organized under headings like “NPS 1/2 to 4” or “NPS 5 to 16,” with each listing showing the full three-number designation.
What a Bullhead Tee Is
A bullhead tee is a special case where the branch opening is larger than the run. Flow enters through two smaller run connections and exits through the larger branch, which is the opposite of how most tee fittings work. The naming convention stays the same: run, run, branch. So a bullhead tee might read “1” x 1″ x 1-1/2″,” telling you the branch is the biggest port. Bullhead tees are less common and can cause pressure issues if used incorrectly, so they tend to show up only in specific applications where flow needs to merge.
Figuring Out the Size Without a Label
If the markings have worn off or you’re working with an unlabeled fitting, you’ll need to measure. For standard pipe fittings, measure the inside diameter of the opening. That measurement corresponds to the nominal pipe size (NPS). A fitting with an inside diameter of roughly 1.049 inches, for example, is a 3/4-inch NPS fitting. Pipe sizing is notoriously unintuitive because nominal sizes don’t match actual measurements, especially at smaller diameters.
For tubing fittings, the convention is different. You measure the outside diameter instead, since tubing sizes are based on external dimensions. Getting this distinction wrong is one of the most common mistakes when identifying unlabeled fittings. If you’re unsure whether you’re dealing with pipe or tubing connections, check whether the fitting has threads (usually pipe) or compression-style ends (often tubing).
Fittings manufactured to ASME B16.9, the dominant standard for butt-weld fittings from 1/2 inch to 48 inches, are required to be permanently marked with the manufacturer’s name, material grade, wall thickness, and nominal pipe size. If any marking remains, even partial, you can often cross-reference it against a manufacturer’s catalog to identify the full size.
Sanitary Tees in Drain Systems
Sanitary tees follow the same sizing convention (run, run, branch) but have a physically different shape. The branch curves gently into the run instead of meeting it at a sharp 90-degree angle. That sweep directs wastewater downward and prevents backups. You’ll find sanitary tees wherever a horizontal drain line connects to a vertical drain stack, or where venting ties into the drainage system.
The size is read the same way, but the fitting itself is not interchangeable with a standard tee. Sanitary tees are designed specifically for vertical-to-horizontal transitions. Using one to join two horizontal drain lines creates a sharp turn that slows flow and invites clogs. For horizontal-to-horizontal connections in drainage, a combination tee or wye fitting is the correct choice. When reading a sanitary tee’s label, you may also see “DWV” (drain, waste, vent) printed alongside the dimensions to distinguish it from a pressure-rated fitting of the same size.
Quick Reference for Reading Any Tee
- First number: the larger or inlet end of the run (the straight-through path)
- Second number: the opposite end of the run
- Third number: always the branch (the perpendicular outlet)
- Single number: all three openings are the same size (straight tee)
When the first two numbers match and the third is smaller, you have a standard reducing tee with a smaller branch. When the third number is the largest, you have a bullhead tee. And when all three differ, you simply read them in order: inlet run, outlet run, branch.