Shower Pan Slope Calculator
Shower floor slope and mortar bed (dry pack) calculator: ¼″ per foot (2%) to the drain, thickness at the walls for a center or linear drain, mortar volume, bags and cement.
How it works
center drain: rise = slope × longest side ÷ 2, volume = area × t + area × rise ÷ 3 (pyramid) · linear drain: rise = slope × length, volume = area × (t + rise ÷ 2) · bags ≈ 12.5 L per 25 kg, 0.5 ft³ per 60 lb
A tiled shower floor must slope uniformly to the drain: the International Plumbing Code (section 417) and IRC P2709 call for at least ¼″ per foot (2%) and no more than ½″ per foot (4%). With a center drain and a level perimeter the mortar bed forms a shallow pyramid, so the rise is set by the distance to the farthest wall; with a linear drain it is a single plane. The TCNA Handbook asks for a mortar bed (dry pack, about 1 part cement to 4–6 parts sand) at least 1¼″ (3.2 cm) thick at the drain. The calculator gives the thickness at the walls, the volume of mortar and the bags of premixed sand mix or the cement for a site mix.
Worked example
A 1.2 × 0.9 m shower with a center drain at 2% slope rises 0.02 × 0.6 m = 1.20 cm to the farthest wall. With 3.20 cm of mortar at the drain the bed is 4.40 cm at the walls, and its volume is 1.08 × 0.032 + 1.08 × 0.012 ÷ 3 = 0.0389 m³ (38.9 L), or 42.8 L with 10% waste — 4 bags of 25 kg premixed mortar.
Sources: ICC — 2021 International Plumbing Code §417 Showers · Tile Council of North America — TCNA Handbook (mortar bed showers)
Frequently asked questions
What is the correct slope for a shower floor?
At least ¼″ per foot (2%) toward the drain and no more than ½″ per foot (4%). For a center drain in a 36 × 48″ shower that means about ½″ to ⅝″ of rise from the drain to the walls.
How thick should a shower mortar bed be?
At least 1¼″ (3.2 cm) at the drain, increasing toward the walls by the slope. Thinner beds crack; most installers use 1¼–2″ at the drain.
How much dry pack mortar do I need for a shower?
Area × average thickness. A 1.2 × 0.9 m shower with a center drain, 3.2 cm at the drain and a 2% slope needs about 39 litres, or about 43 L with waste — four 25 kg bags of premixed sand mix.
Is a linear drain easier to slope?
Yes. A linear drain at one wall lets the floor slope in one direction like a single ramp, so large-format tiles need no cuts; the far wall is higher by slope × length.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.