Mortar for a shower pan is ordered by volume, and a shower pan is a sloped volume. Record the footprint, the drain position, the minimum thickness at the drain, and the slope rate, and the volume follows from those numbers.
What does a shower pan mortar measurement record?
Five numbers define the bed: pan length and width inside the finished walls, the longest horizontal run from the drain to the pan boundary, the minimum bed thickness at the drain, and the slope rate in inches of drop per foot of run.
Record length and width in inches. Measure the longest run from the drain flange to the farthest point of the finished boundary. Note the minimum thickness planned at the drain; 3/4 inch is the common minimum for a bed laid over a liner. Confirm the slope rate against the job: the plumbing code keeps the finished shower floor between 1/4 and 1/2 inch of drop per foot toward the drain.
How do you measure the longest drain-to-wall run?
On a center-drain rectangle, the longest run is the diagonal from the drain to the farthest corner, not the straight distance to the nearest wall. A 36 by 48 inch shower with a centered drain splits into an 18 by 24 inch half-rectangle, so the diagonal is 30 inches: the square root of (18 squared + 24 squared).
On a linear drain set along one wall, the slope runs in one direction only, so the longest run is the distance from the drain channel to the opposite wall. On an offset drain, measure from the drain to the farthest boundary point; that is the controlling run.
How does the record turn into mortar volume?
The slope rate turns the longest run into a rise: rise in inches equals the run in feet times the slope rate. Add that rise to the minimum thickness at the drain to get the maximum thickness at the farthest boundary.
Installers estimate the bed with the average thickness: the minimum at the drain plus the maximum at the boundary, divided by 2. Multiply the footprint area by that average thickness to get the volume in cubic feet.
A liner build carries two sloped mortar volumes: the pre-slope under the liner and the bed over it. Measure and calculate each one; a single-layer number leaves the pre-slope unaccounted.
Worked example: a 36 by 48 inch shower
Pan: 36 by 48 inches, center drain, minimum thickness 3/4 inch at the drain, slope 1/4 inch per foot.
Longest run: 30 inches, which is 2.5 feet. Rise: 2.5 times 0.25 = 0.625 inch. Maximum thickness at the farthest corner: 0.75 + 0.625 = 1.375 inches. Average thickness: (0.75 + 1.375) / 2 = 1.0625 inches.
Volume: 3 ft times 4 ft times (1.0625 / 12) ft = 1.0625 cubic feet. With a preblended floor mud sold in 80 lb bags yielding about 0.66 cubic feet per bag: 1.0625 / 0.66 = 1.61 bags, so 2 bags for this one layer, before any waste allowance.
How do you check the record before the calculator?
Check five things. The footprint is measured inside the finished walls and curb, not the rough framing. The longest run reaches the farthest boundary point for the drain type on the job. The minimum thickness and the bag product are named on the record. The slope rate sits inside the 1/4 to 1/2 inch per foot code range. The layer count matches the build: one bed for a bonded membrane, two volumes for a liner build.
Sources and scope
- International Residential Code, P2709.1: finished shower floors slope 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).
- Quikrete, Floor Mud #1548-81 product data: preblended floor mud nominally proportioned per ANSI A108.1A.2.3.1; an 80 lb bag yields approximately 0.66 cubic feet.
- NIST Handbook 44, 2026, Appendix C: inch-to-foot and volume-unit relationships.
Scope: the worked fixture demonstrates the measurement record and the installer average-thickness convention; 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. The product data and workplace rules control mixing and safety.
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, use the tile measuring guide for the finished surfaces that sit on the cured bed.
