SettingsSettings for this calculationUS
How the floor falls, which is what sets the average depth.
This is the whole difference between the two answers and it is a 33% one. A one-way fall is a plane: depth runs from the maximum at the high edge to zero at the channel, and the average across it is exactly half. A fall to a central point is not a plane but a set of triangular facets meeting at the drain, and the deep material at the perimeter covers far more plan area than the shallow material near the middle — the average works out at two thirds of the maximum. Sizing a point-drain floor at half under-orders the screed by a quarter.
The floor area that slopes down toward the drain.
Measure the full area draining to this point, such as a shower pan, wet room floor, or mechanical room floor with a central drain.
The screed thickness at the perimeter, where it is deepest, tapering to zero at the drain.
This is set by the required slope (commonly around 2%, i.e. 1:50) times the distance from the drain to the farthest perimeter point.
Screed volume needed
13.4 gal
Assumes a simple linear slope from the perimeter down to a single central drain — a floor with multiple drains or a non-uniform slope will need a more detailed volume takeoff.
They open the calculator with your figures already in it
Floor Drain Slope Screed Volume Calculator: 13.4 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- Screed volume = area × maximum depth × the average-depth factor for the fall geometry. A ONE-WAY fall, from a high edge down to a linear channel or a threshold, averages HALF the maximum depth. A fall to a CENTRAL POINT drain averages TWO THIRDS of it: the shallow ground near the drain occupies far less plan area than the deep ground out at the perimeter, so the mean is pulled up. Integrating a linear fall over a plane triangle from an edge to a central apex gives exactly 2/3, whatever the room's plan shape
Inputs used
- Fall Geometry
- Falls to a central point drain (average depth = 2/3 of maximum)
- Floor Area Sloping to Drain
- 43 sq ft
- Maximum Screed Depth at Perimeter
- 0.75 in
Confidence note: Assumes a simple linear slope from the perimeter down to a single central drain — a floor with multiple drains or a non-uniform slope will need a more detailed volume takeoff.
What this calculation does not cover
- The wedge tapers to nothing at the drain, and no screed can. A bonded sand-cement screed wants roughly 25 mm (1 in) at its thinnest and an unbonded or floating one 50 mm (2 in) or more, so the real pour is this wedge PLUS that minimum carried across the whole area — which on a small wet room is usually more material than the wedge itself. Feather it out below that instead and the thin ring around the drain debonds and cracks under the tile.
- Depth here is measured from a substrate assumed flat and level, which a deck rarely is. If the slab already falls the wrong way, or the drain body and its clamping flange sit proud of it, the maximum depth entered above stops describing the pour. Take levels off the drain flange and off the highest point of the slab before ordering — on a dished floor the make-up can be more material than the fall itself.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
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 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Screed volume = area × maximum depth × the average-depth factor for the fall geometry. A ONE-WAY fall, from a high edge down to a linear channel or a threshold, averages HALF the maximum depth. A fall to a CENTRAL POINT drain averages TWO THIRDS of it: the shallow ground near the drain occupies far less plan area than the deep ground out at the perimeter, so the mean is pulled up. Integrating a linear fall over a plane triangle from an edge to a central apex gives exactly 2/3, whatever the room's plan shape
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To lay a floor screed. Generic types, no brands, no prices.
Protection this work requires
- Cutting or grinding concrete, masonry, screed, tile or fibre-cement board releases respirable silica: cut wet or extract at the tool, and wear a P2/N95 respirator at minimum — a nuisance dust mask does not filter it.
- Wet cement and lime burn skin and eyes painlessly until the damage is done: waterproof gloves to EN 374, safety glasses whenever the mix can splash, and never kneel in wet mix in permeable trousers.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
Show the 6 tools this job needsHide tools
Essential
Mixing bucket or tub
Screed bar or rail
Shovel and spade
Spirit level
Recommended
Moisture meter
Paddle mixer
what each flooring tool is for, and the spec that decides which to buy where one does.