shower-supply-line-size-guide
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The shower supply line size is the pipe diameter that carries water from the distribution system to the shower valve. Many readers ask this question before a bathroom remodel, a valve change, or a move of a shower to a new wall. This guide sets out the pipe size the plumbing codes require for a shower supply, how the fixture supply pipe must run to the valve, and which facts a reader still has to check on site. It does not recommend a product or a brand.
Quick answer
The 2021 International Plumbing Code sets a minimum fixture supply pipe size in its table for fixture supply. For a shower with a single head, the table lists a 1/2 inch minimum. The code also says the fixture supply pipe must terminate not more than 30 inches from the point of connection to the fixture. The Massachusetts state rules give a sizing table for shower valves, with a 1/2 inch pipe for a single head and a 3/4 inch pipe for multiple heads. The two sources agree on the single-head size, so a 1/2 inch supply is the baseline. A larger pipe may be needed for a multi-head shower, and the sources do not give a number for every layout.
How we chose
We opened two government code texts on 29 October 2026. The first is the International Plumbing Code 2021 as published by UpCodes in its Utah adoption, chapter 6, which covers water supply and distribution, including the minimum sizes of fixture supply pipes in section 604.5. The second is the Massachusetts state plumbing rule 248 CMR 10.14, which includes a sizing table for water supply fixtures. These two come from different code families, so we treat them as two independent sources. The Utah text is a state adoption of the model code, and local rules can differ.
We did not use any retailer listing, contractor page, blog, or forum for a statement in this guide. We did not name any product.
The fixture supply pipe rule
Section 604.5 of the 2021 IPC is the main rule. It says that “The minimum size of a fixture supply pipe shall be as shown in Table 604.5.” The table lists the minimum size for each fixture type, and it has a row for a shower with a single head, with a minimum of 1/2 inch. The table also lists the minimum for a bathtub of 1/2 inch, for a dishwasher of 1/2 inch, and for a flushometer water closet of 1 inch, so the shower line is in line with the other common small fixtures.
The same section sets the length rule that matters for a shower valve. It says that “The fixture supply pipe shall terminate not more than 30 inches (762 mm) from the point of connection to the fixture.” This means the pipe that serves the valve should reach its connection point within that distance. Where the supply is longer than 30 inches from the connection, the code is not met by the table size alone.
The section also allows a flexible connector in one case. It says that “A reduced-size flexible water connector installed between the supply pipe and the fixture shall be of an approved type.” This sentence allows a short, approved, reduced-size flexible piece at the fixture. It does not allow the main supply line to shrink below the table size. A reader should check that any flexible piece is an approved type and that the supply line itself keeps the table size.
The Massachusetts sizing table
The Massachusetts rule in 248 CMR 10.14 includes a table of fixture factor values and pipe sizes. The table lists “Shower Valve (single head)” with a nominal pipe size of 1/2 inch and a factor value of 2. It lists “Shower Valve (Multiple heads)” with a nominal pipe size of 3/4 inch and a factor value of 6. The factor value is a sizing weight used in the state method, so a shower with more heads carries a larger weight in the sizing calculation and needs a larger pipe.
This table agrees with the model code on the single-head size. A 1/2 inch shower supply is the minimum in both texts for one head. The table adds a clear step up for a multi-head shower, which the model code table summary we read does not state in the same form. The 3/4 inch figure for multiple heads comes from the state table only, so it should be read as a Massachusetts requirement, not a model code rule.
Why the pipe size is not the whole story
Pipe size is one part of a shower supply. The code also sets flow limits at the fixture supply outlet. Table 604.3 in the same chapter of the IPC lists the required capacity at a shower outlet with a balanced-pressure, thermostatic, or combination valve as 2.5 gallons per minute at 20 pounds per square inch. The table is a design minimum at the outlet, not a pipe size. A pipe that is the right size can still fall short of that flow if the rest of the system is undersized or the pressure is low.
The code also limits pressure. Section 604.8 says that “Where water pressure within a building exceeds 80 psi (552 kPa) static, an approved water pressure-reducing valve conforming to ASSE 1003 or CSA B356 with strainer shall be installed.” Pressure above that level changes the operating conditions of every fixture on the line, so a reader who plans a larger shower should check the building pressure before cutting a new branch.
Steps before you cut or replace a shower supply
- Measure the existing shower supply pipe. Do not assume the size from the valve connection, because a reduced fitting can hide the true pipe size.
- Compare the pipe size with the 1/2 inch single-head minimum in the IPC table. If the existing line is smaller, the code table is not met.
- Count the shower heads planned for the shower, including body sprays. The Massachusetts table treats multiple heads differently from one head, and a multi-head layout should be sized by the state table.
- Measure the length from the point of connection to the shower valve. The fixture supply pipe must reach the fixture within 30 inches of the connection point.
- Check any flexible connector for approval. The code allows a reduced-size flexible connector only where it is an approved type.
- Check the building static pressure. Above 80 psi, the code requires a pressure-reducing valve.
- Have a licensed plumber confirm the layout before the pipe is cut, because the local permit office decides the final size.
What we could not verify
We could not verify the full Table 604.5 in the form the official code prints it, because the online copy we read did not show every row cleanly. We therefore state only the single-head shower row and the bathtub and dishwasher rows that the text read clearly. We did not verify the flow rate from the Massachusetts table, because the table lists a factor value and not a flow rate. We did not find a source that gives the pipe size for a shower with a rain head or a custom body spray system, so none is given. We did not find a manufacturer installation sheet that we could open and read for a shower valve, so we do not quote a valve-level size. The code texts we read are state adoptions, and local amendments may apply. We did not check any city or county rule for a given address.
How the sources agree and differ
The two sources agree that a single-head shower supply starts at 1/2 inch. They differ in how they treat a multi-head shower. The Massachusetts table gives a 3/4 inch figure for multiple heads and a factor value that rises with the number of heads. The model code text we read does not state a multi-head size in the same table row, so we do not claim the model code requires 3/4 inch for multiple heads. This difference is one reason to check the local rule before buying materials.
FAQ
Is a 1/2 inch supply enough for a shower?
For one head, the code table and the Massachusetts table both start at 1/2 inch. Whether a particular supply meets the flow requirement depends on the pressure and the rest of the system, and the sources do not give a flow figure for a given pipe.
What size is needed for two shower heads?
The Massachusetts table lists 3/4 inch for multiple heads, with a higher factor value. The model code text we read does not state the same figure in the same row, so the state rule should be checked for a multi-head layout.
How far can the fixture supply run?
The code says the fixture supply pipe shall terminate not more than 30 inches from the point of connection to the fixture. Longer runs need a different layout, which the sources do not describe.
Can a flexible connector be smaller?
The code allows a reduced-size flexible water connector installed between the supply pipe and the fixture, if it is an approved type. It does not allow the supply line itself to drop below the table size.
