Shower pan calculator

How much dry-pack mortar a tiled shower pan takes comes down to the pan size and the slope to the drain — and to whether it is a traditional liner pan, which needs two sloped beds, not one. Enter the pan and get bags to order.

01 How the math works

A shower-pan mortar bed is a wedge, not a flat slab. It is thinnest at the drain and thickens toward the walls at the slope the floor needs to drain — 1/4 inch of fall for every foot of run, the code minimum. So the volume follows from three things: the pan area, the slope, and the minimum thickness you hold at the drain.

The run is the horizontal distance from the drain to the wall that governs the fall. For a centered drain that is half the longer pan dimension; for a corner drain it is the diagonal to the far corner. A bed over a 1-1/4 in (31.75 mm) drain minimum, sloped 1/4 in per ft across a 2.5 ft run, stands 1.875 in (47.63 mm) thick at the wall and averages 1.5625 in (39.69 mm) — that average, not a number off the top of your head, is what sets the volume.

The part most calculators drop is the second bed. A traditional pan built on a sheet liner is poured twice: a pre-slope bed on the subfloor first, so the liner itself drains to the weep holes, then the top setting bed over the liner. Both carry the same slope, so a liner pan takes close to twice the mud of a single bonded bed. A pan over a pre-formed foam tray, or a bed bonded straight to a slab under a topical membrane, is the single-bed case.

02 Worked example

Take a 3 ft × 5 ft alcove shower (0.914 m × 1.524 m), 15 ft² (1.39 m²), center drain, sloped the code 1/4 in per ft. The run is half the long side, 2.5 ft (0.762 m), so the slope adds 0.625 in (15.88 mm) of fall from the drain to the walls.

The top setting bed holds a 1-1/4 in (31.75 mm) minimum at the drain, so it averages 1.5625 in (39.69 mm) and works out to 15 × 1.5625 ÷ 12 = 1.95 ft³ (0.0553 m³). The pre-slope under the liner feathers from 1/4 in (6.35 mm) at the drain and adds 0.70 ft³ (0.0199 m³). Together the liner pan needs 2.66 ft³ (0.0752 m³).

Add a 10% waste factor — 2.92 ft³ (0.0827 m³) — and divide by the 0.66 ft³ an 80 lb bag of Quikrete Sand/Topping Mix yields: 5 bags. Build the same pan as a single bonded bed and it is 4 bags. The pre-slope is the whole difference, and it is not optional on a liner pan.

03 When this calculator is wrong

The bed volume is only as honest as the thickness profile behind it, and there are a few places the simple wedge model and the material itself part ways from the number above.

• The pre-slope is a real second bed, not a rounding error. On the 3 × 5 pan it adds 0.70 ft³ (0.0199 m³) — about a bag and a half of material. A calculator that asks for one "thickness" and returns one bed under-orders a liner pan by roughly a third. If your drain is a two-piece clamping type on a sheet liner, you are pouring two beds. Only a foam tray or a bed bonded to a slab with a topical membrane is a single pour.
• The average is a straight ramp; a point drain needs a touch less. This calculator averages the bed as a linear ramp from the drain to the wall. A real center-drain pan slopes as four hip planes meeting at the drain, so its true average is slightly lower, and the corners sit higher than the wall midpoints because they are farther from the drain. The gap is well inside a 10% waste factor, and it errs toward ordering one more bag — the safe direction for a material you cannot top up mid-pour.
• Mind the minimum and the slope window. ANSI A108.1A puts the minimum cured thickness of an unbonded bed at 1-1/4 in (31.75 mm); many setters float the top bed at 1-1/2 in (38.1 mm). The slope has a ceiling as well as a floor: code allows 1/4 in to 1/2 in per foot, and a floor steeper than 1/2 in per ft feels like standing on a hill. Pans larger than 65 ft² (6.04 m²) want galvanized lath in the bed, which the bag count doesn't include.
• It sizes the bed, and only the bed. The number here is dry-pack mortar. The liner or topical membrane, the drain assembly, the thinset that bonds the tile, the tile itself, and any curb or bench are all separate take-offs. A curb alone can swallow most of a bag.

04 What to do with the result

Order the rounded-up bag count, and lean to the high side of the bed minimum rather than the low. The right rounding direction on every material estimate is up: compute the number, add the waste, round up to the whole bag, and stop. Dry pack mixed stiff has a workable life measured in tens of minutes, and a bed finished in one continuous pour sets as one plane — run short at the far wall and the batch you mix to finish lands as a ridge or a soft seam where the two pours met. The exception is the usual one for high-cost materials in matched lots, and mortar is not that; a spare bag of sand-topping on the shelf costs less than a second trip.

Buy a sand-topping or floor-mud mix, not a standard concrete mix — deck mud is sand and portland cement with no coarse aggregate, which is why it packs to a slope and screeds flat. If your bag quotes a different yield, enter it: the field defaults to the 0.66 ft³ of an 80 lb Quikrete Sand/Topping Mix bag, but a 60 lb bag yields 0.50 ft³ and other brands differ. For a pan over 65 ft², add lath to the order and plan on reinforcing the bed.

05 Common questions

How much mortar do I need for a shower pan?

A 3 × 5 ft (15 ft² / 1.39 m²) center-drain pan sloped 1/4 in per ft takes 2.66 ft³ (0.0752 m³) of dry pack as a traditional two-bed liner pan — about 5 80 lb bags of sand-topping mix with 10% waste — or 1.95 ft³ (roughly 4 bags) as a single bonded bed. Scale with the pan area and the slope.

Do I need a pre-slope under the shower liner?

On a traditional sheet-liner pan, yes. Water passes through tile and grout over time and rides the liner to the drain's weep holes — but only if the liner is sloped. A flat liner holds that water against the pan. The pre-slope bed is what gives the liner its 1/4 in per ft fall, which is why a liner pan is two beds and needs close to twice the mud of a single bed.

How thick should a shower mortar bed be?

At least 1-1/4 in (31.75 mm) at the thinnest point per ANSI A108.1A for an unbonded bed; many setters float the top bed at 1-1/2 in (38.1 mm). It is thickest at the walls, where the slope has added its full rise — 1.875 in (47.63 mm) on a 2.5 ft run at the minimum. Pans over 65 ft² (6.04 m²) should be reinforced with lath.

What is deck mud, and what mix do I use?

Deck mud — also called dry pack or floor mud — is a stiff sand-and-cement mortar with no coarse aggregate, roughly 4 to 5 parts sand to 1 part portland cement by volume. A bagged sand-topping mix (such as Quikrete Sand/Topping Mix) or a purpose-labeled floor mud is the ready-made version. A standard concrete mix with gravel in it will not screed to a clean slope.

What slope does a shower floor need?

A minimum of 1/4 in per foot (a 2% grade) to the drain, and no more than 1/2 in per foot, per the TCNA Handbook and the plumbing code. The pre-slope under the liner carries the same 1/4 in per ft so the liner drains. Flatter than 1/4 in per ft and water stands; steeper than 1/2 in per ft and the floor feels like a ramp.