ToolwickHow to Use This Bathtub & Shower Pan Mortar Bed Calculator
Choose whether you are building a sloped shower pan or a level bathtub setting bed, then enter the length and width in inches. For a shower, pick where the drain sits (center, corner, or a linear drain along one edge), set the minimum mortar thickness at the drain, and slide the slope between ¼ and ½ inch per foot. Pick your bag size and a waste factor, then press Calculate.
You get the number of bags to buy (always rounded up), net and with-waste volume in cubic feet, the thickest point of the bed, a full breakdown table, and a side-by-side slope comparison. The diagram above redraws with your numbers each time you calculate.
Why This Matters
A shower pan is the one place in a bathroom where mortar quantity really matters. Sand topping mix has a working life of roughly one to two hours, so you want to mix, place and screed the whole bed in a single session. Run short halfway and you get a cold joint, a seam that can crack and telegraph through the tile. Over-buy and you're left with 60 lb bags of cement-based mix that harden in a damp garage.
The numbers are smaller than most people expect. A 36 × 36 in shower with a center drain, ¼ in/ft slope and ¾ in at the drain needs about 0.86 ft³ with waste, which is 2 bags of 60 lb mix. A 60 × 36 in walk-in with the same settings needs about 1.57 ft³, or 4 bags (240 lb to carry upstairs). A 60 × 32 in tub bed at a uniform 1 in needs roughly 1.1 ft³ net, or 3 bags with 10% waste. Tile setters use the count to quote materials, remodelers to plan one mixing session, and homeowners to avoid a second trip to the store.
How It's Calculated
A shower bed is a tapered slab: thinnest at the drain, thickest at the point farthest from it. Thickness at any spot equals the minimum thickness plus the slope times its distance from the drain. The calculator averages that distance over the whole floor, which gives the average thickness and therefore the volume.
Max thickness = min thickness + slope × farthest distance (ft)
Volume (ft³) = length × width × average thickness ÷ 1,728
With waste = volume × (1 + waste %)
Bags = round up( with waste ÷ (bag lb × 0.0075 ft³ per lb) )
For a center or corner point drain, the average distance is the exact mean straight-line distance from the drain to every point of the rectangle (a closed-form result), not the "half the farthest distance" shortcut, which underestimates on square pans. For a linear drain the slope runs across the width, so the average distance is half the width and the farthest is the full width. A bathtub setting bed is level, so thickness is uniform.
Both assumptions lean toward more mortar: the slope is measured straight from the drain to every point, and bags are always rounded up. The ¼ in/ft minimum slope (¼ to ½ in/ft range) reflects IPC Section 417 and UPC Section 408, and the ¾ in minimum at the drain reflects common practice in TCNA Handbook shower receptor methods. Bag yield uses a rule of thumb of 0.0075 ft³ per pound of dry mix, so always check the yield printed on your bag. Codes and manufacturer requirements vary by location and product.
Worked Example
A 36 × 36 in shower, center drain, ¼ in/ft slope, ¾ in minimum thickness, 10% waste, 60 lb bags:
- Distances: farthest point (corner) = √(18² + 18²) = 25.46 in = 2.12 ft. Average distance to the drain = 0.7652 × 18 in = 13.77 in = 1.15 ft.
- Thickness: max = 0.75 + 0.25 × 2.12 = 1.28 in. Average = 0.75 + 0.25 × 1.15 = 1.04 in.
- Volume: 36 × 36 × 1.037 ÷ 1,728 = 0.78 ft³ net.
- Waste: 0.78 × 1.10 = 0.86 ft³.
- Bags: bag yield = 60 × 0.0075 = 0.45 ft³, so 0.86 ÷ 0.45 = 1.90, rounded up to 2 bags (120 lb).
Because 1.90 is so close to 2, the tool suggests an extra bag. Steepening the slope to ⅜ in/ft raises the need to 2.16 bags, so you'd buy 3.
Tips & Common Mistakes
- Don't go thinner than about ¾ in at the drain. Thin edges crack and can fail under tile. If your drain assembly needs more depth, enter that number.
- Use sand topping mix or deck mud, not gravel concrete. Mix it damp and packable, like wet sand. A soupy mix shrinks and is weaker.
- Calculate each layer of a two-layer system. A pre-slope under a membrane plus a second bed above needs two runs of this tool. Add the bag counts and check each layer's required thickness with your membrane maker.
- Measure the real floor. Use the finished-bed footprint after framing, not nominal sizes. Curbs, benches and niches add volume this tool doesn't include.
- Buy the extra bag when you're close. If the exact number sits within about 0.15 of a whole bag, a spare costs far less than a cold joint.
- Check code before you start. Most codes want roughly 30 × 30 in (about 900–1,024 in²) of finished shower interior and a ¼–½ in/ft floor slope. Your inspector's word overrides any calculator.
Frequently Asked Questions
What slope should a shower pan have?
The usual requirement is ¼ inch per foot (about 2%) toward the drain, with a ½ inch per foot maximum under IPC and UPC rules. Steeper slopes drain faster but use more mortar and can make large-format tile harder to lay flat.
How thick should a shower pan mortar bed be?
Common practice is at least ¾ inch at the drain, growing thicker toward the walls as the slope rises. Your membrane or drain manufacturer may require more, so check their instructions and the current TCNA Handbook.
How many bags of mortar do I need for a 36 x 36 shower?
With a center drain, ¼ in/ft slope, ¾ in minimum thickness and 10% waste, you need about 0.86 ft³, which is 2 bags of 60 lb mix. Because the exact figure is 1.90 bags, many installers buy a third bag as insurance.
Can I use regular concrete mix instead of sand topping mix?
It isn't recommended. Concrete contains gravel that is hard to pack into a thin, sloped bed and leaves voids. Sand topping mix or a site-mixed deck mud (roughly 4 to 5 parts sand to 1 part Portland cement by volume) packs tighter and screeds smoother.
How long should a mortar bed cure before tiling?
Many installers wait 24 to 48 hours before setting tile, though some products allow tiling sooner with the right thin-set. Follow the mortar and thin-set manufacturers' instructions, and flood-test any waterproofing layer as they specify.
Do I need a mortar bed under a bathtub?
Many tub manufacturers call for a bed of mortar or similar support under the base so it doesn't flex, but requirements differ by model and material. Follow your tub's installation sheet for thickness, then enter that number as the bed thickness here.