I am researching plumbing for moving a laundry sink. It seems the all the supply pipes (bathroom sink, kitchen sink, toilet) from the wall are 1/2" pipes. Looking at homedepot's site, the most common valve is 1/2" to 3/8". And the most common faucet supply line is the opposite 3/8" to 1/2".

Wouldn't the 3/8" size becomes the bottleneck? Why not 1/2" all the way?

  • The supply line size is not the "bottleneck" for any of the applications you listed. Internal restrictions in the valve assemblies are much smaller than any tubing or line that would be commonly used to connect those fixtures. Commented Feb 18, 2017 at 16:44
  • But pipes also have wetted area, which creates drag, and 3/8 has a higher proportion of wetted area, being 3/4 of the wetted area on 9/16 of the volume. Wetted area is a term in aerodynamics and hydrodynamics. Commented Feb 18, 2017 at 19:55

2 Answers 2


The reduction is extremely important! So when someone else turns the water on it will have enough pressure to feed both.

Notice toilet flush reduces water pressure in the shower. If it were 1/2" all the way to every access, and you turned on multiple valves at multiple elevations, you'd lose pressure and run the risk of creating a syphon. That would suck air in through the open valve.

Imagine taking a shower up stairs, dish washer going, Wife starts to water the garden, hot water & air start spraying out, shower head becomes a vacuum. Also do the math limited diameter pipe. Can only send a limited a limited amount of water. It's all about supply & demand. It basically holds pressure for when you need it.

If it was all the same size diameter, when you turned on several things at once, the city or "your supplier" would need to turn up water pressure feeding your structure (somehow 🤷) every time you turned on everything.

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If you are using a kitchen style faucet it is a flow restricted type and will not need piping larger than the 3/8" tubing normally recommended for this application.Smaller sinks like kitchen sinks, toilets, and bathroom hand washing sinks work okay with the supply lines being 3/8" OD, (1/4" ID nominal size). Consequently, this size pipe (tubing) is the standard in the industry today, since all of these devices use some type of flow restrictors mandated by government regulations (big brother). Laundry tubs however are exempt from these regulations today, and can be piped with standard 1/2" copper pipe (tubing) or what ever size you choose to use and reduced or increased at the faucet. Fitting size at the faucet is dictated by the manufacturer. Most single supply home piping is 1/2" copper at the wall penetration, or this new and better (cheaper) supply tubing. My laundry tub faucets are piped with 1/2" copper which allows the sinks to be filled quickly. If the faucets you are using are new look at the instructions supplied with the faucets for their recommendations for connection size and type.

  • Thank you. It makes sense. However, I am still curious why is going 1/2" all the way to the faucet being the norm? So the homeowner has the maximum flexibility of flow? Using a 1/2" valve for the toilet would allow the toilet to be refilled quicker? BTW, what is the GPM can a 3/8" pipe allow?
    – some user
    Commented Feb 18, 2017 at 15:40
  • 2
    Pipes are referred to only by their trade size, which is their nominal inside diameter (ID). If someone says 3/8 pipe when they want 3/8 OD, they will not get what they want. There is no correspondence anymore between pipe size and actual ID or OD. You have to look it up in tables if you want to know actual sizes. Commented Feb 18, 2017 at 20:00
  • Plumbers use inside diameter for their pipe sizes and refrigeration and A/C people use outside diameter for their pipe sizes. Hence, normal 1/2" copper pipe (tubing) for a plumber is 1/2" copper, but for refrigeration that same pipe (tubing) would be 5/8" copper. I am typing " pipe (tubing)" since pipe can be threaded and tubing can not.
    – d.george
    Commented Feb 19, 2017 at 11:21
  • For your reference to pipe size to a toilet ,the fill valve for the toilet as well as all other devices will only flow a certain amount of water through them so there is no need or gain to use valves or tubing larger than those normally used for that application.
    – d.george
    Commented Feb 19, 2017 at 11:26

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