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The length of the floor that falls to the drain, between its two end walls.
Measure the whole area that falls to this drain — the room, or the shower zone if only that part falls and the rest of the floor stays flat. With a linear drain, measure from the wall the drain runs along to the opposite wall: that is the direction the whole floor falls.
Tools needed: Tape measure
The width of the same floor, between its two side walls.
With a linear drain this is the length of the drain channel, wall to wall.
Tools needed: Tape measure
Where the water leaves the floor, which decides the shape of the fall.
A point drain (a gully, or a square grate) makes the floor four sloping faces meeting at the drain, like a shallow upside-down pyramid, so every wall starts its tiles on a level line. A linear drain along a wall makes the whole floor one plane falling towards it, which large tiles follow easily and a level-access entrance suits — but the fall runs the full length of the floor, so it needs more depth at the far wall.
How far the centre of the drain is from one of the end walls, measured along the length.
A drain in the middle of the floor gives the gentlest faces. Moving it towards a wall or a corner lengthens the run from the farthest corner, which deepens the whole floor, and shortens the run to the near wall, which steepens that face.
How far the centre of the drain is from one of the side walls, measured across the width.
Measure from the same corner as the distance from the end wall, so the two together place the drain.
The flattest the floor may be anywhere: 1 unit of fall in N units of run — enter N.
48 means 1:48, which is 1/4 in per foot (about 21 mm per metre) — the least slope US plumbing codes allow a shower floor, and a sound fall anywhere. A flatter floor holds a film of water; much steeper is uncomfortable to stand on and hard to tile. Where a preformed tray or former is used, its own fall governs, so enter that. The page holds this fall on the flattest line of the floor and reports how much steeper the rest is.
The thinnest the screed gets — at the drain's flange.
The greater of two figures: the least thickness the screed or mortar bed may be laid to, which depends on whether it is bonded to the deck, unbonded or floating, and the height the drain's flange stands above the deck. A bonded sand-cement screed is commonly laid no thinner than about 25 mm (1 in), and an unbonded or floating one much thicker; a thin proprietary fall mortar can go lower, to its maker's figure. It is carried across the whole floor under the fall.
How far the waterproofing turns up every wall around the floor.
Outside the showering area the membrane commonly turns up the walls about 150 mm (6 in); inside it, the walls are tanked far higher, to the height the membrane maker or your code sets. The area here is the floor plus this height all round — add the showering walls on top, or enter a full height to tank the whole room to it.
Screed depth at the walls
2.25 in
The walls stay level, so every wall starts its tiles on one line. The floor falls in four faces to the drain; water crossing a face runs into the valleys from the corners, so the fall is held on the longest valley and the faces are steeper. The steepest face falls at 1:29, against 1:48 along the valley from the farthest corner.
- Screed depth at the drain
- 1 in
- Drop from the walls to the drain
- 1.25 in
- Longest run, from the farthest corner to the drain
- 5 ft
- Fall along that valley — the flattest line on the floor
- 2.08 %
- Fall on the steepest face, towards the nearest wall
- 3.47 %
- Screed volume, with the depth at the drain under all of it
- 54.86 gal
- Floor area
- 48 ft²
- Area to tank — the floor and its upstand
- 62 ft²
- Floor-to-wall junction to reinforce
- 28 ft
They open the calculator with your figures already in it
Wet Room and Curbless Shower Floor Fall Calculator: 2.25 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- The International Plumbing Code sets a shower floor's slope to the drain at not less than 1/4 in and not more than 1/2 in per foot (about 21 and 42 mm per metre): 1:48 to 1:24. ICC A117.1 limits the floor of an accessible shower to no steeper than 1:48 in any direction
- With the perimeter level, a floor falling to a point drain is four planes meeting at the drain, and water on each plane runs across it into the valleys that run from the corners to the drain. The valleys are the flattest lines on the floor, so the minimum fall is held along the longest of them: drop = distance from the drain to the farthest corner × the minimum fall. Each face then falls that drop over its own distance from the drain to its wall, which is steeper
- Screed volume = floor area × (depth at the drain + two-thirds of the drop) for a point drain — a pyramid holds a third of its box, wherever its apex sits — and floor area × (depth at the drain + half the drop) for a one-way fall to a linear drain. BS 8204-1 governs the screed, including the least thickness it may be laid to
Inputs used
- Floor Length, End Wall to End Wall
- 8 ft
- Floor Width, Side Wall to Side Wall
- 6 ft
- Drain
- A point drain — the floor falls to it from every wall, which stay level
- Drain Distance From One End Wall
- 4 ft
- Drain Distance From One Side Wall
- 3 ft
- Minimum Fall Ratio, 1:N
- 48
- Screed Depth at the Drain
- 1 in
- Tanking Height Up the Walls
- 6 in
Intermediate steps
- Screed depth at the drain
- 1 in
- Drop from the walls to the drain
- 1.25 in
- Longest run, from the farthest corner to the drain
- 5 ft
- Fall along that valley — the flattest line on the floor
- 2.08 %
- Fall on the steepest face, towards the nearest wall
- 3.47 %
- Screed volume, with the depth at the drain under all of it
- 54.86 gal
- Floor area
- 48 ft²
- Area to tank — the floor and its upstand
- 62 ft²
- Floor-to-wall junction to reinforce
- 28 ft
Confidence note: The walls stay level, so every wall starts its tiles on one line. The floor falls in four faces to the drain; water crossing a face runs into the valleys from the corners, so the fall is held on the longest valley and the faces are steeper. The steepest face falls at 1:29, against 1:48 along the valley from the farthest corner.
What this calculation does not cover
- One rectangular floor and one drain. An L-shaped room, two drains, or a shower zone that falls while the rest of the floor stays flat is set out zone by zone, and the flat part is a levelling job, not a fall.
- The depth at the drain is the reader's: the screed's own least thickness for how it is laid, or the drain flange's height above the deck, whichever is greater. On a timber floor the joists usually have to be dropped or the deck lowered to find this depth, which is structural work this page does not check.
- The tanking area is the floor and an upstand around it. The walls inside the showering area are tanked much higher and are not included unless entered as the height; primer, membrane and tape are then bought from these areas and lengths at the product's own coverage.
- It reports the falls and does not judge them. An accessible shower has its own limit on how steep its floor may be, and a point drain's faces are always steeper than its valleys — which a floor held to its minimum along the valleys cannot avoid.
Add the equipment this sizes
This result is a specification — 2.25 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-21 · v1.0.0
Regulatory standards & verification citations3
- The International Plumbing Code sets a shower floor's slope to the drain at not less than 1/4 in and not more than 1/2 in per foot (about 21 and 42 mm per metre): 1:48 to 1:24. ICC A117.1 limits the floor of an accessible shower to no steeper than 1:48 in any direction
- With the perimeter level, a floor falling to a point drain is four planes meeting at the drain, and water on each plane runs across it into the valleys that run from the corners to the drain. The valleys are the flattest lines on the floor, so the minimum fall is held along the longest of them: drop = distance from the drain to the farthest corner × the minimum fall. Each face then falls that drop over its own distance from the drain to its wall, which is steeper
- Screed volume = floor area × (depth at the drain + two-thirds of the drop) for a point drain — a pyramid holds a third of its box, wherever its apex sits — and floor area × (depth at the drain + half the drop) for a one-way fall to a linear drain. BS 8204-1 governs the screed, including the least thickness it may be laid to
Which documents these citations point at
- International Plumbing Code (United States)Plumbing systems — fixtures, water supply, sanitary drainage, venting and storm drainage.
Standards referenced: ICC A117.1 (International Code Council, United States).
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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