A shower pan mortar order starts with five numbers on the measurement record and ends with a bag count that can be checked step by step. This page runs one complete record through the slope math: a 42 by 36 inch pan with an offset drain, built as a two-layer liner bed with 80 lb bags of preblended floor mud. If the pan is not measured yet, measure the pan first: every step below assumes the record table exists.
How many bags of mortar does this pan need?
Four bags: 2 per sloped layer, recorded on two separate rows. The record below shows the arithmetic behind that count.
What does the measurement record carry?
Six numbers define this build:
| Record field | Value | How it was set |
|---|---|---|
| Pan footprint, inside finished walls | 42 in x 36 in | Tape measure |
| Drain position | 12 in from one end, 12 in from one side | Measured from the finished boundaries |
| Longest drain-to-boundary run | 38.42 in (3.20 ft) | Diagonal to the farthest corner |
| Minimum bed thickness at the drain | 0.75 in | Project record; the common minimum over a liner |
| Slope rate | 0.25 in per ft | Code minimum; inside the IRC 1/4 to 1/2 in per ft range |
| Sloped layers | 2 (pre-slope + bed) | Liner build |
How does the record turn into volume?
The slope rate turns the longest run into a rise. The rise plus the minimum thickness gives the maximum thickness at the farthest corner, and the average of the two times the footprint gives the volume.
- Longest run: the farthest corner sits 30 in from the drain along the 42-in side and 24 in along the 36-in side. The diagonal is the square root of (30 squared + 24 squared), which is 38.42 in, or 3.2016 ft.
- Rise: 3.2016 ft x 0.25 in per ft = 0.8004 in.
- Maximum thickness at the farthest corner: 0.75 + 0.8004 = 1.5504 in.
- Average thickness: (0.75 + 1.5504) / 2 = 1.1502 in.
- Footprint area: 42 x 36 = 1512 in2 = 10.5 ft2.
- Volume: 10.5 ft2 x (1.1502 / 12) ft = 1.0064 ft3 per sloped layer.
Record 38.42 in, 1.1502 in average, 1.0064 ft3, and 1.5249 bags per layer. Rounding 1.1502 to 1.15 changes nothing here, but on a larger pan the same rounding can move the bag count.
How many bags does one layer need?
Divide the layer volume by the manufacturer's yield and round up. The bag is Quikrete Floor Mud, 80 lb, yielding about 0.66 ft3 per bag.
- 1.0064 / 0.66 = 1.5249 bags.
- Round up to whole bags: 2 bags per layer.
Result: 2 bags for the pre-slope and 2 bags for the bed, 4 bags total. Keep the unrounded 1.5249 on the record so a different product's yield can be substituted without remeasuring.
The 0.48-bag overage covers mixing and handling loss. As the shower pan ordering guide explains, whole-bag rounding already holds the margin that installer guides describe as extra purchase for waste, spillage, and mixing errors.
What does the two-layer record look like?
| Layer | Measured volume | Unrounded bags | Order quantity |
|---|---|---|---|
| Pre-slope (under the liner) | 1.0064 ft3 | 1.5249 | 2 bags |
| Bed (over the liner) | 1.0064 ft3 | 1.5249 | 2 bags |
| Total | 2.0128 ft3 | 3.0497 | 4 bags |
A single combined row would hide which layer a shortfall belongs to. Round each layer up before summing, so a mixing error in one layer does not stall the other.
How do you check the arithmetic?
Run one independent cross-check before ordering. Convert the layer volume to cubic inches, and separately multiply the footprint area by the average thickness in inches. Both paths must agree.
- 1.0064 ft3 x 1728 = 1739.10 in3.
- 1512 in2 x 1.1502 in = 1739.10 in3.
The two paths agree to the displayed precision, so the unit conversions and the average are consistent. A second useful check: the rise of 0.8004 in over 3.2016 ft is exactly the 0.25 in per ft slope the record states, so no step drifted.
Which mistakes change the bag count?
- Using the 30-in straight wall distance instead of the 38.42-in diagonal. The understated rise of 0.625 in cuts the volume to 0.9297 ft3. The bag count holds at 2 here, but on a larger pan the same error crosses the rounding boundary.
- Calling 1512 x 1.1502 = 1739 cubic feet. The product is cubic inches; dividing by 1728 gives 1.0064 ft3. A missed unit conversion is the most expensive arithmetic error on this page.
- Rounding 1.5249 bags down to 1. Bag counts round up only; a partial bag cannot be bought.
- Counting only one layer on a liner build. The pre-slope under the liner is a full sloped volume; omitting it halves the order to 2 bags.
- Dividing by the wrong bag's yield. The 60 lb bag yields about 0.50 ft3, so the same layer needs 1.0064 / 0.50 = 2.0128, rounded up to 3 bags. The product name stays on the record row.
Safety and professional scope
A quantity record does not select the mortar mix, the waterproofing method, the drain assembly, or the slope beyond the code range. Those follow the project specification, the product data, and the applicable code.
Mixing and placing mortar follow the product data and the governing workplace rules. This fixture is a calculation example, not an installation guide. The shower pan mistakes article covers the build errors that change the job.
Sources and scope
- International Residential Code, P2709.1: finished shower floors slope uniformly toward the drain not less than 1/4 unit vertical in 12 units horizontal (2 percent) nor more than 1/2 unit vertical per 12 units horizontal (4 percent). Retrieved 2026-10-23.
- Quikrete, Floor Mud #1548-81, -82 product data: preblended floor mud nominally proportioned per ANSI A108.1A.2.3.1; an 80 lb bag yields approximately 0.66 cubic feet; a 60 lb bag yields approximately 0.50 cubic feet. Confirmed live 2026-10-23.
- NIST Handbook 44, 2026, Appendix C: exact published length, area, and volume unit relationships: 12 in = 1 ft, 144 in2 = 1 ft2, 1728 in3 = 1 ft3.
- How to Measure for Shower Pan Mortar: the five-number measurement record and the installer average-thickness convention this fixture uses.
Scope: the fixture demonstrates the calculation on a stated measurement record; it is a calculation fixture, not a product recommendation. The minimum thickness, mix, membrane method, and drain design come from the project specification, the product data, and the applicable code.
Review responsibility: Saleem Sial owns the published calculation, source, and editorial checks. Waseem Sial, External Reviewer and Engineer, remains listed for ongoing external review; no completed external-review date is claimed.
Next, turn the record into a supplier order with the shower pan ordering guide, or return to the Tile Quantity Planning Tools and Guides hub.
