Level entry is a framing decision, not a tiling one
The whole idea of a curbless shower is that the tile inside finishes flush with the floor outside. Work backwards from that and the arithmetic is forced. The tile, the setting bed, the membrane and the slope layer all sit on the subfloor inside, and together they are thicker than the floor covering outside, so the subfloor inside has to start lower by the difference. Then the drain body, its trap and the fall on the branch to the stack need their own depth below that.
Add those together and the total is usually more than a subfloor thickness, which means it is into the joists. That is the moment the project stops being about tile. Whether a joist can give up any depth, where in its span, and what has to be built instead — a dropped and framed box, sistered members, a header, or simply putting the shower over a bay where the plumbing already runs — is structural work with a permit and an engineer or designer on it. By the time anyone is choosing tile the answer has already been decided by whoever framed the floor.
The thickest point sets the recess, and the entry decides which point that is
The slope layer is thin at the drain and thick at the far end, and the recess is set by whichever end lands at the entry. That gives you a lever most people do not know they have. A linear drain at the entry puts the thin end of the slope where the two floors meet; a drain at the back wall or in the middle puts the thick end there. On a run of three feet the difference is the whole fall, and it comes free, because it is a decision about where a pipe goes rather than about how anything is built.
The calculator reports the build-up at the thickest point and at the entry separately for exactly this reason, and it tells you what moving the drain to the entry would save on your own dimensions. That saving comes out of the depth the framing has to give up, which is the expensive number on the page.
Working out the mortar honestly
A sloped floor is a wedge, and the average thickness is not halfway between the thin end and the thick end unless the slope is a single plane. A floor sloping in from all sides to a drain near the middle is closer to a shallow pyramid, and its average thickness is nearer a third of the way up than half. Rather than pick a rule, the calculator walks the floor point by point, works out the thickness at each point from the fall you entered, and averages those. It reports the average it used so you can see it.
Volume in cubic feet is the honest output. Bags are only as good as the coverage figure on the bag you actually buy, which is why the coverage field is yours to fill in and the count only appears once you do. A pre-sloped tray or panel system skips this section entirely — it arrives with the fall built in — and it is worth pricing against a mortar bed before committing, because the two answers differ by more than the material cost.
Where the fall figure comes from
The fall field ships empty, and this page will not put a number in it. Two separate authorities have a view: the manufacturer of the waterproofing system, who tested their membrane, bonding flange, drain and corners together and publishes what fall their system requires, and the plumbing code your jurisdiction has adopted. They do not always agree, and where they do not, the one that governs your installation is a question for your building department rather than for a calculator.
The related rule is that waterproofing is a system and not a product. The membrane, the drain flange, the corner pieces, the bonding and the sealant are tested as one assembly by one manufacturer, and substituting a piece because the merchant had a different brand in stock is how the assembly fails. Follow their instructions. This page counts material; it does not tell you how to build anything.
What happens when it goes wrong
A curb exists to stop water. Take it away and the only things left between the shower and the rest of the floor are the fall and the waterproofing. Where either is wrong the water leaves the shower, and on a recessed floor it leaves into the structure, where nobody sees it until the ceiling below shows a stain and the framing has been wet for a season. That is a bigger repair than the shower was.
The other failure is immediate: a wet floor at the entry to a shower, in a room where falls are already the thing that happens. Getting the fall right is not only about the framing below. And none of the arithmetic on this page says a shower is accessible, adequate or compliant — it counts material from dimensions you supplied. What applies to your building, and to a rental or a building open to the public, comes from your building department, an accessibility specialist and where relevant a lawyer, and what a particular person needs comes from them and their occupational therapist.
Questions people ask
How deep does the floor have to drop for a curbless shower?
By the whole build-up inside, less whatever the floor covering outside is, and then the drain body needs its own depth below that. The build-up is the slope layer at its thickest point, the membrane, the setting bed and the tile. On the figures this page starts with, the tile and mortar side comes to around two inches, and the drain body underneath is often more than everything above it put together, which is why the total nearly always reaches into the joists.
Does a linear drain reduce the recess?
Only if it goes at the entry. A linear drain is often put along the back wall for looks, which puts the thick end of the slope at the entry and buys nothing. Put it at the entry and the thin end of the slope is where the two floors meet, so the recess loses the entire fall over the run. The calculator shows what that swap is worth on your own dimensions. It is a decision about where a pipe goes and it has to be made before the plumbing is in.
How much mortar does a sloped shower floor take?
Less than the thickest point suggests and more than the thinnest. Because the floor is a wedge, the average thickness is what governs, and for a floor sloping in from all sides that average is nearer a third of the way between the thin and thick ends than halfway. This page walks the floor point by point and reports the average it used rather than applying a rule of thumb. Turning that volume into bags needs the coverage figure off the bag you are buying, which is why the field is yours to fill in.
Can I cut the joists to get the recess?
Not a question this page answers, and the answer is not a general one. Whether a joist can give up depth at all, in which part of its span and by how much, is structural and depends on the member, the span, the load and what else is happening in that bay. The alternatives — a dropped and framed section, sistered members, a header, or putting the shower where the plumbing already runs — are all decided by an engineer or a designer and by the building department, on a permit. Do not take a saw to a joist on the strength of a subtraction.
What fall does a curbless shower floor need?
Not published here, and the field is empty on purpose. Two authorities have a say: the manufacturer of the waterproofing system, who tested the membrane, drain and corners together and specifies the fall their system requires, and the plumbing code your jurisdiction has adopted. Where the two differ, your building department decides which governs your installation. Take the figure from them, put it in the field, and every slope number on the page fills in from it.