The datum problem
A fixture rough-in sheet gives heights above the finished floor. On the day you are drilling, the finished floor is a drawing. What exists is a subfloor, and between the two there may be nothing, or there may be a layer of backer board, a bed of thinset and a tile, which together are commonly around three quarters of an inch and can be more.
Mark the sheet heights directly on the studs and every one of them ends up low by the thickness of the floor you have not built yet. On a shower valve that is survivable. On a drain it can be the difference between a trap arm that falls the right way and one that does not. On a washing machine box it can put the standpipe outside the range the machine needs. The whole purpose of this page is that one correction, applied consistently to every fixture, before anything is bored.
The same problem repeats horizontally with the wall. A stub-out dimensioned to project a certain distance past the finished face has to clear the drywall or the backer and tile first, and a valve body has a depth setting that references the finished surface rather than the framing.
What the numbers in the list are, and are not
The defaults loaded into the fixture list are ordinary values of the kind found on common fixtures. They are there so the page does something useful on first load and so you have a shape to edit. They are not requirements, they are not code, and they are not the sheet for the fixture in the box.
| Where a dimension comes from | Who decides it | How it can differ from a general figure |
|---|---|---|
| Supply and drain heights for a fixture | The fixture manufacturer, on its rough-in sheet | Wall-hung, vessel and console basins differ by many inches from a standard pedestal |
| Toilet rough-in from the wall | The model, and the wall it is going against | Several standard spacings exist and the wrong one will not fit at all |
| Shower valve depth | The valve manufacturer, against the finished surface | A drywall setting behind tile leaves the trim unable to seat |
| Trap arm length, slope and venting | The adopted code and the inspector | Limits vary by jurisdiction, code edition and pipe size |
| Notching and boring in framing | Structural rules and the inspector | A joist that permits a hole may not permit it where the drain wants to go |
| Accessible or barrier-free layouts | A separate body of requirements entirely | Heights, clearances and approach spaces are all different |
Read the table as a list of the places to go and check, in the order you will need them.
Marking a wall so it can be built from
The output above is deliberately shaped as marks rather than as a specification. Pick one corner of the room and measure everything along the wall from it, so no dimension depends on another dimension being right. Strike a level line at each supply height across the whole bay before marking individual stubs, because a line is easier to sight along than a series of dots and errors show up immediately.
Where two supply stubs straddle a centreline, the offsets are half the spacing each side, and the check worth doing is against the studs: at sixteen inch centres a stub that lands within an inch or so of a stud centre either goes through it or forces the fixture to shift. That is a decision to make with a pencil, not with a hole saw. Blocking goes in for anything that gets fastened rather than clamped, and it goes in before the marks are covered, not after somebody notices the valve has nothing behind it.
Everything gets photographed before the drywall goes on. That photograph is the only record of where the pipe is, and it is worth more later than any of the measurements on this page.
Where this stops and other work starts
This page organises vertical heights and stub offsets. It says nothing about whether the fixtures fit in the room, which is a plan-view question and belongs on the bathroom fixture clearance planner, and nothing about the drainage below, which is the drain slope calculator for the fall and the fixture unit calculator for what the supply has to carry. Where the branch pipe is going to move as it heats, the pipe expansion calculator covers how much room to leave it.
Nor does it decide anything. Rough-in heights, drain positions, venting arrangements, structural notching and accessible clearances are decided by the adopted code, the fixture manufacturer and the authority having jurisdiction, and where those three disagree it is the inspector who settles it. Anything tying into the water service, a gas line or a backflow point is licensed work regardless of who marked the wall. The right sequence is to lay it out here, check every figure against the real sheet, and have it looked at before it is covered.
Questions people ask
What height should a lavatory supply and drain be roughed in at?
At whatever the rough-in sheet for that specific basin says, measured from the finished floor and then corrected for the floor you have not laid yet. There is no single correct figure: a pedestal basin, a wall-hung basin, a vanity with a vessel bowl on top and a console all put the trap and the supplies in different places, sometimes by six inches or more. What this page is for is applying your figures consistently and converting them to marks on the studs. Where a general starting figure is genuinely useful is in sizing the problem — knowing roughly what band you are in before the fixture arrives — and for that the defaults in the list are ordinary values worth checking rather than following.
Do I measure rough-in heights from the subfloor or the finished floor?
Rough-in sheets are written from the finished floor, and you are standing on the subfloor, which is the mismatch this page exists to fix. Add the total floor build-up — backer board, thinset, tile, or underlayment and flooring — to every height on the sheet before you mark anything, and mark all of them from the same datum so that an error in one does not propagate. If the floor finish has not been chosen yet, that is a decision to force before the rough-in rather than after, because it moves every height in the room and it is much cheaper to argue about tile than to move a drain.
Can I move a toilet drain a few inches?
Sometimes, and it is not the kind of change to make from a dimension on a screen. A closet flange position is fixed by the model of toilet and how far it sits from the finished wall, and the standard spacings are genuinely different sizes rather than a tolerance. Moving the drain means moving pipe below the floor, which raises questions about the joists it passes, the fall it needs, and whether the vent arrangement still works once the fixture has moved. All three are code questions with jurisdiction-specific answers. Get the toilet first, work to its sheet, and take any change in the drain position to whoever is inspecting the work.
How much fall does a trap arm need?
Enough that it drains, and not so much that the water outruns the solids or siphons the trap, and the actual permitted range is set by the adopted code for the pipe size you are using and confirmed by your inspector. The slope field on this page takes whatever figure you are working to and shows you the consequence of it: how far the pipe drops over the run, and therefore how high the tie-in has to be. The number worth getting from that is the tie-in height, because it is what determines whether the connection lands where there is actually a branch to connect to. The maximum length of a trap arm before it has to be vented is a separate limit and is also a code question.
Does this calculator know my local plumbing code?
No, and that is deliberate rather than an omission. Adopted codes differ by jurisdiction and by edition, fixture manufacturers publish their own rough-in dimensions that a general figure cannot substitute for, accessible layouts are governed by a different set of requirements again, and the inspector is the one who decides when those disagree. A page that stated dimensions as fact would be wrong somewhere for someone, and confidently so. What this page does is arithmetic on numbers you supply — the datum correction, the stub offsets, the fall over a run — which is exactly the part that is universal and exactly the part that gets miscalculated at eight in the morning with a hole saw in hand.