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I have building with 16 showers , 6 toilets, 8 sinks and 6 urinals what size of water pipe should be for the supply?

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    Commercial or residential? How many people may be expected to be using these facilities simultaneously? – FreeMan Apr 28 at 16:37
  • If all fixtures can be turned on at the same time then you need a 4 inch supply pipe. This assumes a half inch supply to each individual fixture. This would let you turn on all hot and cold lines at the same time. – MonkeyZeus Apr 28 at 16:37
  • If you anticipate that only half of the fixtures will be in use at any given time then a 2.75 inch supply would be appropriate. – MonkeyZeus Apr 28 at 16:38
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    @MonkeyZeus, you should be posting that as an answer and you should support it with reasoning or resources. Comments aren't the place for it. – isherwood Apr 28 at 18:13
  • @isherwood Done, hopefully the math is understandable :) – MonkeyZeus Apr 28 at 19:46
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Usually a fixture will receive a 1/2 inch supply line; one for hot and one for cold.

Based on "16 showers, 6 toilets, 8 sinks, and 6 urinals" we would want 2 lines to each shower and sink:

(16 * 2) + 6 + (8 * 2) + 6 = 60

Using the pie * radius squared formula, we know that a half inch diameter circle has an area of 0.2 sq. inches (0.19635 if you want to be exact).

Now multiply by the number of half inch lines you need to run 0.2 * 60 = 12

Now that we know our supply pipe needs to have an area of 12 sq. inches we can work backwards: ( ( square root of ( 12 divided by pie ) ) and double the result ) to get a diameter of 4 inches.


If you think that only half of the fixtures will be used at any given time then we cut our square inches (12) in half to 6 and work backwards to give us a 2.75 inch pipe.

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  • Math is usually a good way to figure things out. However, in this case I suspect there are some code (IRC) based rules that would provide a more definitive answer. – manassehkatz-Moving 2 Codidact Apr 28 at 19:51
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    @manassehkatz-Moving2Codidact I have no doubt about that! I think one would also have to factor in the water pressure coming into the building, run distance, elbows, aerators which reduce water flow, low-flow shower heads, and even the fact that toilets usually reduce to 3/8 inch (for residential). I don't think the question belongs on DIY in the first place but the math was fun :-) It really makes you wonder what kind of "fake it till you make it" situation OP got themselves into which requires assistance from Internet strangers. – MonkeyZeus Apr 28 at 20:07

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