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RedGard Coverage Calculator: How Many Gallons for a Shower?

Enter the area you want to cover and pick what the membrane is for. The calculator applies the coat count and per-gallon rate Custom Building Products gives for that use in TDS-104 (published 02/14/2025), then picks the 1 gallon and 3.5 gallon pails to buy. For a shower pan or any job that has to meet ANSI A118.10, Custom calls for two coats at 80 sq ft per gallon each, which is 40 sq ft of finished membrane per gallon.

Custom RedGard Waterproofing and Crack Prevention Membrane coverage

Source: Custom Building Products Technical Data Sheet TDS-104: RedGard Waterproofing and Crack Prevention Membrane, Published 02/14/2025, read 2026-10-17.

How the calculation works

gallons = area × (1 + waste ÷ 100) × coats ÷ sq ft per gallon per coat

Every coat goes over the whole area, so two coats at 80 sq ft per gallon use as much liquid as one coat at 40. The rate depends on what the membrane is for: the data sheet lists a heavier rate for code-listed waterproofing than for crack prevention alone. The calculator then picks the fewest pails that cover the job without leaving more than one small pail of liquid unused, so a job needing 3 gallons gets one 3.5 gallon pail rather than three 1 gallon pails. Extra passes over corners, seams, drains and pipe penetrations are not included; raise the waste figure if you have many of them.

Worked example

A 100 sq ft area, Shower pan / ANSI A118.10 waterproofing, with 10% extra: 100 × 1.1 = 110 sq ft. The data sheet calls for 2 coats at 80 sq ft per gallon each, so 110 × 2 ÷ 80 = 2.75 gallons (10.4 L). In the pail sizes on sale, buy 1 × 3.5 gal pail (3.5 gal).

Custom RedGard Waterproofing and Crack Prevention Membrane key specs

What RedGard is

RedGard Waterproofing and Crack Prevention Membrane from Custom Building Products is a ready-to-use liquid that you roll, brush, trowel or spray onto the substrate before setting tile. It is an elastomeric, styrene-butadiene based coating that dries to a continuous rubbery film, so it does not need fabric in the field of a floor or wall, or in coves and corners. It goes on pink and turns dark red when it is dry, which makes it easy to see where you have already covered and when the next coat can go on.

Custom's data sheet TDS-104 lists it for interior surfaces and exterior concrete and masonry: shower pans, showers and tub surrounds, pools, spas, steam showers and steam rooms, commercial kitchens, balconies and decks, and exterior facades. It meets ANSI A118.10 for waterproofing membranes and ANSI A118.12 high performance crack isolation for cracks up to 1/8 inch, is listed with IAPMO (File 4244) as a shower pan liner, and tests under 0.5 perms for steam rooms. Suitable substrates include concrete, mortar beds, cement backerboard, coated glass mat gypsum board, existing tile, and exterior-glue plywood or OSB in interior dry areas only.

Why the amount depends on what the membrane is for

Unlike a powder sold by the bag, RedGard has no single coverage figure. TDS-104 gives a different spread rate and coat count for each job, because thicker film is needed for code-listed waterproofing than for crack prevention alone. The rates in the application sections of the data sheet are:

• Waterproofing to ANSI A118.10 and IAPMO (shower pans, wet areas that must meet code): two coats at 80 sq ft per gallon each.
• General waterproofing: 110 sq ft per gallon per coat, with at least two coats when you use a roller or brush.
• Continuous crack prevention: one coat at 100 sq ft per gallon over the whole substrate.
• Crack isolation to ANSI A118.12: two coats at 50 sq ft per gallon each.
• Vapor barrier over concrete: one full coat at 70 sq ft per gallon where vapor emission is up to 8 lb per 1,000 sq ft per day, two coats where it is up to 12 lb.

Those per-coat rates explain the short coverage chart at the end of the data sheet. One gallon is listed as 40 sq ft as an A118.10 pan liner (80 ÷ 2), 55 sq ft as a waterproofing membrane (110 ÷ 2), 100 sq ft for crack prevention and 25 sq ft for A118.12 crack isolation (50 ÷ 2). The 3.5 gallon pail figures are the same numbers times 3.5, rounded. So the most common mistake is to read "100 sq ft per gallon" and buy only 40 percent of the RedGard a code-compliant shower actually needs.

How the calculator works

Pick the application and the calculator takes its coat count and rate straight from the list above. It adds your waste allowance to the area, multiplies by the number of coats and divides by the sq ft per gallon per coat. That gives the gallons of undiluted RedGard you need. It then chooses pails: the fewest pails that hold enough, as long as you are not left with more than one 1 gallon pail's worth unused. A job needing 2.75 gallons gets one 3.5 gallon pail instead of three 1 gallon pails, while a job needing 1.2 gallons gets two 1 gallon pails rather than a 3.5. Prices are not part of the rule, because they change by store and region; compare the price per gallon where you shop before choosing between one large pail and several small ones.

The calculator does not include the extra passes Custom asks for at corners, the joints between walls and floor, drains and pipe penetrations, where you coat a strip at least 2 inches either side of the corner first and embed reinforcing tape over changes of plane and gaps wider than 1/8 inch. On a typical shower those details add a little liquid; if you have a lot of niches, benches or penetrations, raise the waste figure to 15 or 20 percent.

Measuring a shower

Measure every surface that will get membrane, not just the floor. For an alcove shower that is the floor, the three walls up to the height you waterproof (at least the shower head, often full height), the top and both faces of the curb, and the back, sides, top and bottom of any niche. A 3 x 5 ft shower with three walls to 7 ft is 15 sq ft of floor and 77 sq ft of wall. Over a tub, waterproof the wall area above the tub; Custom asks for at least 15 mils of dry film on solid walls above a tub or shower cove. Enter the total in the calculator and use the shower pan / ANSI A118.10 setting if the membrane is your pan liner or the job has to meet code.

Application notes from the data sheet

Surfaces must be structurally sound, clean, dry, sloped to drains where needed, and free of curing compounds, sealers, wax and dust. Fill cracks 1/8 inch or wider with RedGard, thinset or patch before coating. Gypsum and lightweight underlayments need a primer: 1 part RedGard to 4 parts cool, clean water, mixed at low speed and scrubbed in with a bristle broom at about 300 sq ft per gallon of mix. The same diluted primer helps on very porous surfaces and in hot or windy conditions.

Apply with a 3/8 inch rough synthetic roller, a brush, a 3/16 to 1/4 inch V-notch trowel with the ridges flattened, or an airless sprayer at 1,900 to 2,300 psi. Keep each wet coat at 40 mils or less. Let the first coat turn fully red, typically 1 to 1.5 hours and up to 12 hours in cold or humid conditions, fill any pinholes, then roll the second coat at right angles to the first. After both coats are dry the area can be flood tested. Keep the air and surface above 40°F during application and for 72 hours after. Outdoors, cover the membrane with tile within 72 hours or protect it from sunlight. Set tile with a polymer-modified mortar meeting ANSI A118.4 or A118.15, not a premixed adhesive.

Check before you buy

The numbers on this page come from TDS-104 published 02/14/2025, the version linked from Custom's RedGard product page. An older copy of TDS-104 dated 2023 is still online at a different address, so if you compare, make sure you are looking at the current one linked below. If your estimate sits right at a pail boundary, round up: running out between coats means waiting for a store run while the first coat sits uncovered.

FAQ

How many square feet does a gallon of RedGard cover?

It depends on the job. TDS-104 gives 80 sq ft per gallon per coat for waterproofing that meets ANSI A118.10 and IAPMO shower pan listing, and that use needs two coats, so one gallon finishes about 40 sq ft. For general waterproofing the rate is 110 sq ft per gallon per coat with at least two coats, about 55 sq ft finished. As a crack prevention membrane one coat at 100 sq ft per gallon is enough. Custom says its coverage chart is for estimating and real coverage varies with installation practice and jobsite conditions.

How much RedGard do I need for a shower?

Add up the floor and every wall surface you will waterproof, including the curb and any niche, then use the shower pan / ANSI A118.10 setting: two coats at 80 sq ft per gallon. A 3 x 5 ft alcove shower with its three walls waterproofed to 7 ft is 15 sq ft of floor plus (5 + 3 + 3) x 7 = 77 sq ft of walls, 92 sq ft before waste. With 10 percent extra that is about 101 sq ft, and two coats at 80 sq ft per gallon come to about 2.5 gallons, so one 3.5 gallon pail covers it. Add the curb, bench and niche faces if you have them.

Why does the coverage chart show different numbers for the 3.5 gallon pail?

The coverage chart in TDS-104 lists 40, 55, 100 and 25 sq ft for the 1 gallon pail and 140, 192, 350 and 88 sq ft for the 3.5 gallon pail. Those are finished areas with all coats applied, and each 3.5 gallon figure is 3.5 times the 1 gallon figure, rounded (192.5 becomes 192, 87.5 becomes 88). They match the per-coat rates: 80 sq ft per coat over two coats is 40 sq ft finished, 110 over two coats is 55. This calculator works from the per-coat rates and then matches the gallons to pail sizes.

Does the primer coat count toward the two coats?

No. The primer is RedGard diluted 1 part to 4 parts water and brushed in at about 300 sq ft per gallon of mix. Custom says to let it dry and then apply undiluted RedGard at the normal rate. The calculator only counts the undiluted coats. If you need a primer, one gallon of RedGard makes five gallons of primer, enough for about 1,500 sq ft, so a bathroom needs only a small amount taken from your main pail.

How long between coats, and when can I flood test or tile?

RedGard goes on pink and turns solid dark red when dry, typically 1 to 1.5 hours per coat, and up to 12 hours in cold or humid conditions. Apply the second coat at right angles to the first once the first is red and any pinholes are filled. When both coats are fully dry the area can be flood tested. Outdoors, cover the membrane with tile within 72 hours or protect it from sunlight.

Custom Building Products and Custom RedGard Waterproofing and Crack Prevention Membrane are trademarks of their owner. This page is an independent estimate based on the published data sheet; check the bag label before mixing.