Shower Waterproofing Calculator: How Much RedGard, Hydro Ban or Kerdi You Need

1. Pick the membrane. For RedGard, pick the purpose too, because its coverage changes with it. The coverage and pack size fill in.
2. Enter the floor size, the wall run and the wall height you will coat. Add the curb, bench or niche area, and subtract any opening you will not coat.
3. Type your own waste allowance. Read the pails (or rolls) to buy, then check the verdict band against the data sheet.

A shower waterproofing calculator estimates how much membrane a wet area needs. It adds up the floor, the walls and any curb or niche, then divides by the coverage the manufacturer publishes.

The step most people skip is the coverage itself. One brand can list several rates, because the rate depends on the job. RedGard, for example, has one rate for general waterproofing, a lower one for ANSI A118.10 and shower pan work, and another for crack isolation.

This page is an independent tool. It is not affiliated with or endorsed by Custom Building Products, LATICRETE, MAPEI or Schluter Systems. To plan the tile that goes on top, use the tile calculator. To plan the grout, try the TEC grout calculator.

The calculator finds the area, adds your allowance, and divides by the coverage per gallon (or the roll area for KERDI).

1. Area. The floor is length times width. The walls are the total wall length times the height. Curbs and niches are added as flat area. Openings are subtracted.
2. Allowance. Multiply by one plus your waste allowance. The manufacturer data sheets do not publish a waste figure, so the allowance is yours.
3. Gallons. Divide by the coverage for all coats. The data sheets publish the rate per coat, so divide by the number of coats. RedGard at 80 sq ft per gallon per coat, with two coats, is 40 sq ft per gallon.
4. Pails. Divide by the pail size and round up. For KERDI, divide by the roll area instead.

Wet film thickness follows from the coverage. One gallon is 231 cubic inches and one square foot is 144 square inches, so a rate of C sq ft per gallon gives a wet film of 1,604 / C mils. The RedGard and Hydro Ban sheets state limits for that film, and the verdict band checks them.

I read these figures on 2026-10-03 in the documents listed under Sources.

Take a 5 x 3 ft shower with three walls. The wall run is 5 + 3 + 5 = 13 ft, coated to 7 ft. A curb and niche add 6 sq ft. The 10 percent allowance is only an example.

1. Floor: 5 x 3 = 15.0 sq ft.
2. Walls: 13 x 7 = 91.0 sq ft.
3. Area: 15 + 91 + 6 = 112.0 sq ft. With 10 percent: 123.20 sq ft.
4. RedGard, A118.10 rate (40 per gallon): 123.20 / 40 = 3.08 gal. One 3.5 gal pail is enough, with 0.42 gal left.
5. Hydro Ban (50 per gallon): 123.20 / 50 = 2.46 gal. A 5 gal pail covers it.
6. AquaDefense (60 per gallon): 123.20 / 60 = 2.05 gal. One 3.5 gal pail covers it.
7. KERDI: 123.20 sq ft needs 2 roll of 108 sq ft, with 92.8 sq ft left.

The same area at the RedGard crack isolation rate (25 per gallon) needs 4.93 gal. That is why the purpose matters.

The same pail covers very different areas depending on the product and the purpose. This table shows what each pack covers for all coats, before any waste.

The sq ft figures are my arithmetic from the published rates. The AquaDefense row matches the 60, 210 and 300 sq ft in the MAPEI table. The Hydro Ban 5 gal row matches its 250 sq ft commercial unit.

Five things move the answer. The calculator handles the first four.

Purpose and Standard

RedGard drops from 110 to 80 sq ft per gallon per coat when you need ANSI A118.10 and IAPMO shower pan compliance. Using the wrong rate understates the quantity by about a third.

Number of Coats

The RedGard sheet calls for a minimum of two coats with a roller or brush. Hydro Ban says all areas must have two coats.

Shower Geometry

Wall height and the number of walls change the area faster than the floor does. A 7 ft wall on 13 ft of wall adds 91 sq ft to a 15 sq ft floor.

Curbs, Niches and Benches

These add surface that people forget. Enter their combined area in the extras field.

Substrate and Technique

MAPEI states that real coverage varies with substrate profile, porosity, equipment and thickness. Rough or porous backer boards use more membrane.

The four products work in two different ways. Three are liquids you roll or trowel on. KERDI is a sheet you bond with thinset.

Never compare the numbers across rows without checking the unit. A per-coat rate and a two-coat rate look similar but differ by half. The calculator converts all of them to coverage for all coats.

Buying Membrane for a Remodel

Enter the shower size and read the pails. Then add the tile quantity from the tile flooring calculator to finish the material list.

Comparing Products

Switch the product and keep the same shower. The pail count shows which membrane suits the size of your job.

Checking a Quote

If an installer quotes gallons, run the same area here. A large gap usually means a different height or coat count.

Budgeting the Whole Room

The flooring installation cost calculator helps with labor and materials once you know the quantities.

1. Using One Coverage Number for Every Job

RedGard lists four rates. Pick the one for your purpose.

2. Forgetting the Second Coat

The per-coat rate covers one coat. Divide by the coat count, as the calculator does.

3. Leaving Out the Curb and Niche

Small surfaces add up to several square feet.

4. Skipping the Allowance

The exact area assumes no spills. Add your own allowance. The material waste guide explains how.

5. Mixing Up Pail Sizes

A 3.5 gal pail and a 5 gal pail are different packs. Enter the one you will buy.

6. Forgetting Primer

The RedGard and Hydro Ban sheets describe a diluted primer coat. The calculator does not include it.

The coverage rates come straight from the data sheets, so the area math is as accurate as your measurements. Real jobs vary more than the rates do.

The Hydro Ban data sheet lists a mini unit of four 1 gal pails with a coverage of 50 sq ft. That figure is unclear next to the 250 sq ft for the 5 gal unit, so the calculator uses only the per-coat rate and the commercial unit.

Divide the area by 40 for the ANSI A118.10 and IAPMO rate. The 112 sq ft example plus 10 percent needs 3.08 gal. Add your own allowance.

The data sheet says two coats at 80 sq ft per gallon per coat for ANSI A118.10 and IAPMO. It also requires a minimum of two coats when you use a roller or brush.

It depends on the use. The data sheet lists 110 sq ft per coat for general waterproofing, 80 for A118.10 and IAPMO, 50 per coat for A118.12 and 100 for one coat of crack isolation.

The data sheet gives about 100 sq ft per gallon per coat and requires two coats. A 5 gal commercial unit is listed at 250 sq ft.

MAPEI lists 60 sq ft per 1 gal pail, 210 per 3.5 gal and 300 per 5 gal, all for two coats. The 123 sq ft with allowance needs 2.05 gal.

Divide the area with allowance by the roll area. Rolls run from 54 sq ft to 646 sq ft. Schluter says to overlap edges 2 in, so keep an allowance for the overlap.

Some give a rate per coat and some give a rate for two coats. RedGard and Hydro Ban state per coat. AquaDefense states for two coats. The calculator converts each to all coats.

Local code and the manufacturer instructions decide. This page does not publish a code figure. Enter the height your inspector or the instructions require.

No. The RedGard sheet describes one part RedGard to four parts water at 300 sq ft per gallon of diluted mix. Hydro Ban gives 250 to 300 sq ft per gallon. Add those separately.

No. The calculation runs in your browser and nothing is sent to our servers. Anything you choose to Save stays in this browser only.

• RedGard Waterproofing and Crack Prevention Membrane, TDS-104: coverage by application (Custom Building Products, published 02/14/2025, read 2026-10-03).
• HYDRO BAN data sheet DS-663-0926: coats, coverage and wet film (LATICRETE, read 2026-10-03).
• Mapelastic AquaDefense: packaging and approximate coverage per 2 coats (MAPEI document MKT 21-1206, 02/21, read 2026-10-03).
• Schluter waterproofing membranes data sheet 8.1: KERDI rolls and KERDI-BAND (Schluter Systems, dated 11/2025, read 2026-10-03).
• Schluter KERDI product page: installation steps and 2 in overlap (Schluter Systems, read 2026-10-03).