Deck Mud Calculator for Shower Pans

How this calculator works

Slope. Plumbing codes require a shower floor to fall at least 1/4 in per foot to the drain and not more than 1/2 in per foot (IRC P2709.1, IPC 417.5.2). The calculator warns if your slope is outside that range.

Centre drain. The bed is four sloped planes meeting at the drain with a level edge at the walls. The rise at the walls is the slope times the longest run from the drain to a wall, so every plane gets at least the minimum fall. Thickness grows in a straight line from the drain outward, and over the whole floor the extra depth averages two thirds of the rise, so volume = area × (drain thickness + 2/3 × rise).

Linear drain. The floor is one plane falling toward the drain wall, so the extra depth averages half the rise: volume = area × (drain thickness + 1/2 × rise).

Worked example. A 3 × 5 ft pan with a centre drain, 1 in of mud at the drain and 1/4 in per foot: the longest run is 2-1/2 ft, so the walls are 1-5/8 in thick. The average is 1 + 2/3 × 5/8 = 1.42 in, the volume 15 × 1.42 ÷ 12 = 1.77 cu ft, which is 3 bags at 0.66 cu ft each (the yield QUIKRETE lists for its 80 lb Sand/Topping Mix).