It looks like incorporating a wider toilet pipe "system" (everything from the u-shaped tube of the bathroom toilet down to the home pipe that finally connects to the greater "public" sewer system) should make for an overall "less cloggable" toilet experience.

But, since homebuilders can't alter the general public sewer pipes dimensions, is there a legal limit as to how maximally wide the pipes of a home sewer system (just for the house itself!) can be? It seems like there would be some limit here to prevent the general/public sewer system itself from getting clogged -- by keeping everyone's "home" sewer plumbing widths to a minimum!

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    Standard toilet plumbing is a 3" drain pipe, which often expands to a 4" before leaving the home. That's almost always going to be larger than the trap in the toilet itself. There's obviously no benefit to going larger.
    – isherwood
    Oct 31, 2017 at 18:07
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    Two large of a pipe will actually plug faster because the water leaves the solids behind.
    – Ed Beal
    Oct 31, 2017 at 19:04
  • This is location-dependent (and you haven't specified yours). In the US, 3" seems to be the standard, but where I'm originally from it's 100mm.
    – brhans
    Oct 31, 2017 at 19:13
  • @EdBeal only if the slope is incorrect for the pipe size. Too many would-be plumbers think that if a little slope is good, more is better, and it's not (for the reason that you attribute to larger pipe size.)
    – Ecnerwal
    Nov 1, 2017 at 23:45
  • Just to add the reference, I live in the city of San Francisco, California...
    – ManRow
    Nov 2, 2017 at 4:31

1 Answer 1


Regulations (if any - they are mostly concerned with minimum sizes) will vary with the LAHJ (local area having jurisdiction) but the simple and practical consideration (if connecting to a sewer) is that your pipe size from the house should be equal to or smaller than the pipe size from the street.

A reduction in size will be more prone to clogging, and it's your house that will have the clog, not the general public.

There is no real benefit to the public system in limiting the size of pipe in houses - the size of pipe does not materially affect the amount of flow, which is based on number of occupants and fixtures, not the size of the pipe.

Where there is no sewer, the input connection on your septic tank will set the maximum practical pipe size (typically 4" in the US unless special-ordering a larger intake, which is really not called for on any normal-scale single-dwelling septic system.)

  • Ahh, I see -- so there's usually no maximum width to a toilet's and sewer pipes' width since the amount of material flow shouldn't really vary (at least not above a certain width!), and there's no need to "regulate" maximum home toilet and sewer pipes width since it's highly unlikely or difficult to clog the public system anyway, right?
    – ManRow
    Nov 2, 2017 at 4:31

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