How it works
The calculator adds up the surfaces that get waterproofing, then turns the area into gallons of liquid membrane or rolls of KERDI sheet membrane and KERDI-BAND.
Area = Floor L × W + Wall run × Height + Curb length × (2 × Curb height + Curb width) + Niche (W × H + 2 × (W + H) × Depth)
Wall run: three walls L + 2 × W · two walls L + W · four walls 2 × (L + W)
Liquid: Gallons = Area × (1 + Extra ÷ 100) ÷ Coverage per gallon or Area × (1 + Extra ÷ 100) × Wet thickness × Coats
Pails = ⌈ Gallons ÷ Pail size ⌉
KERDI rolls = ⌈ Area × (1 + Extra ÷ 100) ÷ (Roll length × (1 m − 5 cm)) ⌉ with overlapped seams
KERDI-BAND = Floor/wall joint + Inside corners × Height + Curb length (+ butted seams)
⌈ ⌉ rounds up to a whole number. One US gallon is 231 cubic inches.
Worked example in words
A 5 × 3 ft alcove shower has a 15 sq ft floor. The back wall and both ends add up to 5 + 3 + 3 = 11 ft, and at 6 ft high that is 66 sq ft. A 5 ft curb 4 in high and 4 in wide has 4 + 4 + 4 = 12 in, or 1 ft, of surface across it, so 5 sq ft. In all 86 sq ft. RedGard as a waterproof membrane covers 192 sq ft per 3.5 gallons, about 54.9 sq ft per gallon, so the shower needs 86 ÷ 54.9 = 1.57 gallons – two 1 gallon pails.
Coverage from the data sheets
The liquid presets use each maker’s waterproofing figure: RedGard 55 sq ft per gallon and 192 sq ft per 3.5 gallons (the lower, 54.9 sq ft per gallon, is used); LATICRETE HYDRO BAN 50 sq ft per 1 gallon pail and 250 sq ft per 5 gallon pail; MAPEI Mapelastic AquaDefense 60 sq ft per gallon. These are approximate figures for the finished membrane; rough surfaces and extra coats at corners use more. If you know the wet film thickness instead, choose “By wet film thickness” – 1 gallon spread 0.02 in thick covers 231 ÷ 0.02 = 11,550 sq in, about 80 sq ft per coat.
KERDI sheet membrane
Schluter KERDI comes in rolls 1 m (3 ft 3 in) wide and 5 to 30 m long. Seams are overlapped 2 in (5 cm), or butted and covered with KERDI-BAND, which Schluter also uses at floor/wall connections. KERDI-BAND 5 in wide comes in 5, 10 and 30 m rolls. Inside and outside corners use preformed KERDI-KERECK pieces, which are not counted here.
Worked example
Inputs
- Shower length
- 5 ft
- Shower depth
- 3 ft
- Walls to waterproof
- Three walls (alcove – back and both ends)
- Waterproof the floor
- Yes
- Height on the walls
- 6 ft
- Curb length
- 5 ft
- Curb height
- 4 in
- Curb top width
- 4 in
- Membrane type
- Liquid (roll-on or trowel)
- Liquid membrane
- RedGard (192 sq ft per 3.5 gal)
- Pail size
- 1 gal
- Extra allowance
- 0 %
Result
- Floor area
- 15 sq ft
- Wall area
- 66 sq ft
- Curb area
- 5 sq ft
- Area to waterproof
- 86 sq ft
- Liquid membrane
- 1.57 gal (6.27 qt)
- Pails to buy
- 2
Common mistakes
- Using the crack-isolation coverage. RedGard lists up to 100 sq ft per gallon as a crack prevention membrane but only 55 as a waterproof membrane – a shower needs the waterproofing figure.
- Applying one thick coat instead of two. Liquid membranes reach their tested thickness in two coats, each allowed to dry; one coat is too thin in places.
- Forgetting the curb and the niche. Their tops, sides and backs add several square feet and many corners.
- Leaving gaps between KERDI strips. Seams are either overlapped 2 in or butted and covered with KERDI-BAND – never left open.
- Not flood-testing. Each maker gives a waiting time before a flood test; test before setting tile.
Assumptions and limits
- The shower floor is a rectangle; walls are counted as three walls (back and both ends), two walls or four walls of that floor.
- The curb is a box with two faces and a top; the niche is a recess with a back and four sides.
- Liquid coverage is the maker’s figure per gallon for the finished waterproof membrane; where several pail sizes are listed, the lowest area per gallon is used.
- KERDI rolls are 1 m wide; overlapped seams take 5 cm of that width. KERDI-BAND covers the floor/wall joint, the vertical inside corners and the joint between floor and curb.
- Thinset for setting KERDI, preformed corners, drain kits, pipe seals and reinforcing fabric are not included.
Frequently asked questions
How much RedGard do I need for a shower?
Custom Building Products lists RedGard as a waterproof membrane at 55 sq ft per gallon and 192 sq ft per 3.5 gallon pail. A 5 × 3 ft shower with walls to 6 ft and a curb is about 86 sq ft, which is about 1.6 gallons – two 1 gallon pails or one 3.5 gallon pail.
How many square feet does a gallon of HYDRO BAN cover?
LATICRETE lists about 50 sq ft for its 1 gallon pail and 250 sq ft for the 5 gallon pail, with two coats on every area.
How much KERDI membrane do I need for a shower?
Add up the floor, wall, curb and niche area and divide by the area of the roll, less the 2 in overlap at seams. The 86 sq ft shower above fits in one 1 × 10 m (108 sq ft) roll with overlapped seams.
How much KERDI-BAND do I need?
Add the floor/wall joint, the inside corners from the floor to the top of the membrane and the joint at the curb, plus every butted seam if you butt the strips. For a 5 × 3 ft alcove with walls to 6 ft and a 5 ft curb that is 28 ft – one 10 m (33 ft) roll.
How high should shower waterproofing go?
That depends on your membrane maker’s instructions and local code. The calculator uses the height you enter and does not give a recommendation.
Sources
- RedGard Family Sell Sheet (SS178) — Custom Building Products. RedGard as waterproof membrane: 1 gallon (3.78 L) 55 sq. ft. (5.1 m²), 3.5 gallon (13.25 L) 192 sq. ft. (17.8 m²); as IAPMO pan liner 40 sq. ft. per gallon; "Charts are for estimating purposes." Preset uses the 3.5-gallon figure, 192 ÷ 3.5 ≈ 54.9 sq ft per gallon.
- HYDRO BAN Data Sheet (DS-663-1026) — LATICRETE International. Commercial Unit 5 gal (18.9 L) pail: approximate coverage 250 ft² (23.2 m²); Mini Unit 4 × 1 gal (3.8 L): 50 ft² (4.6 m²) per pail, i.e. 50 ft² per gallon; "All areas must have two coats"; wet coat 0.015–0.022 in (0.4–0.6 mm) per coat; 0.020–0.030 in (0.5–0.8 mm) thick when cured.
- Mapelastic AquaDefense Installation Instructions (US) — MAPEI Corporation (hosted by BuildSite). Approximate coverage: "60 sq. ft. (5.57 m2) per 1 U.S. gal. (3.79 L)", "210 sq. ft. (19.5 m2) per 3.5 U.S. gals. (13.2 L)", "300 sq. ft. (27.9 m2) per 5 U.S. gals. (18.9 L)".
- Waterproofing Membranes data sheet – KERDI, KERDI-DS, KERDI-BAND (11/2025) — Schluter-Systems. KERDI roll width 1 m (3' 3"), lengths 5, 7, 10, 20, 30 m (54, 75, 108, 215, 323 ft²); "Seams can be constructed by overlapping the edges of the KERDI 2" (5 cm) … or by abutting the edges and covering the joint with KERDI-BAND"; "For floor/wall connections, use KERDI-BAND"; KERDI-BAND 5" (12.5 cm) wide in 16' 5" (5 m), 33' (10 m) and 98' 5" (30 m) rolls; preformed KERDI-KERECK corners.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 6, 2026