How this calculator works
The waterproofed area is the shower floor plus the walls, less openings. The walls are the run of wall times the height, and the run depends on how many walls are waterproofed: an alcove shower has the back wall and two side walls. Sheet membrane is bought in rolls, and every seam across the roll width needs a 2 inch overlap, which adds a little. Liquid membrane is the area divided by its coverage per gallon, and a band covers the floor-to-wall joint and the vertical corners.
Worked example: a 5 × 3 ft alcove shower with the walls waterproofed 6 ft high has 81 ft² to cover: 15 ft² of floor and 66 ft² of wall (a wall run of 11 ft, 6 ft high). With 10% waste that is 2 rolls of 54 ft² sheet membrane (including the 2 in overlap where the 6 ft wall needs two strips of the 3 ft 3 in roll), or 2 gallons of liquid at 55 ft² per gallon, plus 26 ft of band. A 6 × 3 ft shower has 90 ft². Taking the walls to 8 ft raises the 5 × 3 ft shower to 103 ft².
Assumptions and sources
- Assumptions used: 10% waste added; walls lapped 2 in where the sheet is joined across the 3 ft 3 in roll width; 55 ft² per gallon of liquid membrane; a band over the floor-to-wall joint and every inside corner.
- This counts a shower with a flat floor and walls. A curb, a bench, a niche, the drain and any pipe penetrations all need extra membrane and fittings: add them separately, and use the preformed corners and seals that the maker sells for them. The membrane under a shower floor must be part of a system that slopes the floor to the drain, so follow the maker's instructions and your local code. Choose either sheet or liquid, not both: the two columns are alternatives.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Waterproofing for common showers
Alcove with three walls, waterproofed 6 ft high, 10% waste, 54 ft² rolls and 55 ft² per gallon.
| Shower | Area | Rolls (54 ft²) | Liquid | Band |
|---|---|---|---|---|
| 3 × 3 ft | 63 ft² | 2 | 2 gal | 24 ft |
| 4 × 3 ft | 72 ft² | 2 | 2 gal | 25 ft |
| 5 × 3 ft | 81 ft² | 2 | 2 gal | 26 ft |
| 6 × 3 ft | 90 ft² | 2 | 2 gal | 27 ft |
| 5 × 4 ft | 98 ft² | 3 | 2 gal | 28 ft |
Tips before you buy
- Waterproof the whole shower, not just the floor. Water splashed on the walls goes through the grout, so the membrane behind the tile is what keeps the wall dry.
- Seal every inside corner, the floor-to-wall joint and the drain with the band or fabric and the fittings from the same system. Most leaks start at a corner or the drain.
- Flood-test a shower pan before you tile: plug the drain, fill it with an inch or two of water, mark the level and check it after 24 hours.
- Two coats of liquid membrane are applied to a stated wet thickness: roll or trowel them on evenly and let the first coat dry. A thin spot is a leak.
- Use the thinset the membrane maker names. Not every mortar bonds to every sheet.
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Frequently asked questions
How much waterproofing membrane do I need for a shower?
Add the floor and the walls to be covered. A 5 × 3 ft alcove shower waterproofed 6 ft high is 81 ft², which is 2 rolls of 54 ft² sheet membrane or 2 gallons of liquid, with 10% extra.
How high should shower waterproofing go?
The IRC asks for a nonabsorbent finish on shower walls to at least 6 ft above the floor. Many installers take the membrane to the ceiling in the shower area. The membrane maker's instructions say what they require.
How much does a gallon of liquid waterproofing cover?
About 55 ft² for RedGard in two coats. The label of your product states its own coverage, and it is lower on rough surfaces.
How much overlap does sheet membrane need?
Schluter KERDI is lapped 2 inches at seams, or the edges are butted and covered with a band. Follow the instructions for the membrane you buy.
Do I need to waterproof the walls as well as the floor?
Yes, in the shower itself. Water gets through the grout, so the tile alone is not waterproof. Outside the shower, a moisture-resistant backer is usual.
Add this calculator to your website
Running a blog, a contractor site or a class page? You can put this calculator on it for free. Copy the code below, and please leave the link underneath the calculator in place. The small script just resizes the frame to fit the results; if your site strips scripts, the calculator still works inside a scrolling frame.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.