SettingsSettings for this calculationUS
Only the walls the membrane actually has to reach.
Measure the walls enclosing the shower or bath and take them to the height the specification asks for, which is usually well above the shower head rather than to the ceiling. A room tanked wall to wall and floor to ceiling is a room whose membrane cost three times what the design needed; a room tanked only to waist height behind a shower is a leak with a date on it.
The whole floor in a wet room, not only the shower end.
In a curbless wet room the entire floor is tanked, because water that reaches the door reaches the joists. Where a tray is fitted the tanked floor is usually the tray's own footprint plus a margin, and the tray manufacturer's detail decides how far.
The area one unit of membrane covers in a single coat, from the tin.
Published rates for a brushed liquid membrane sit around 1 m²/L, which is roughly 40 sq ft per US gallon, and they vary by a factor of two between products and with how rough the substrate is. Take the figure from the tin you are buying rather than an average, and read whether it is stated per coat or for the full system — that distinction alone is a common doubling error.
Wall-to-floor and wall-to-wall lines that take reinforcing tape.
Run a tape along every internal angle: the perimeter where the walls meet the floor, and each vertical corner within the tanked zone. These are the lines that move as the building does, and a membrane spanning them without reinforcement is the joint that splits first. Add the tape's own overlap at every join — this figure is the bare line length and the result adds the overlap for you.
Every pipe, valve body and waste that breaks the membrane.
A shower valve is usually two, a riser one, a waste one. Each takes a preformed collar bedded into the first coat, and each is a place where the membrane stops being continuous — which is the only condition under which tanking does anything at all.
Membrane needed, both coats
8.7 gal
25% of the tanked area is floor. 3 penetration(s) counted, each needing its own collar.
- Total area tanked
- 173 sq ft
- Area covered, at two coats
- 346 sq ft
- Reinforcing tape, laps included
- 33 ft
- Preformed collars
- 3 collars
They open the calculator with your figures already in it
Wet Room Tanking and Waterproof Membrane Calculator: 8.7 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- Liquid-applied tanking is specified in two coats, the second laid at right angles to the first, and every published coverage rate is PER COAT — so the area is doubled before it is divided by the rate
- Reinforcing tape or matting is bedded into the first coat at every internal junction — wall to floor, wall to wall — and a preformed collar at every pipe penetration, because a membrane bridging a moving junction unsupported is the joint that splits first
- Primer is required on absorbent substrates such as plaster, render and sand-cement screed, and is commonly omitted on cement-based tile backer board; the manufacturer's sheet governs which applies
Inputs used
- Wall area inside the wet zone
- 130 sq ft
- Floor area to be tanked
- 43 sq ft
- Membrane coverage, per coat
- 40 ft²/gal
- Length of internal junctions
- 30 ft
- Pipe and outlet penetrations
- 3
Intermediate steps
- Total area tanked
- 173 sq ft
- Area covered, at two coats
- 346 sq ft
- Reinforcing tape, laps included
- 33 ft
- Preformed collars
- 3 collars
Confidence note: 25% of the tanked area is floor. 3 penetration(s) counted, each needing its own collar.
What this calculation does not cover
- THE MEMBRANE IS NOT THE FAILURE. Tanking does not leak in the middle of a wall; it leaks at a junction that moved, a penetration that was not collared, or a substrate that was never primed. The tape length and the penetration count on this page are the quantities that decide whether the system works, and they are the ones a quote most often leaves out.
- Coverage rates vary by a factor of two between products and with how rough the substrate is. The figure here is the one you entered from the tin, applied evenly — a sand-cement render drinks more than a primed board, and the first coat on a porous wall is always the hungry one.
- Nothing here decides how far the tanked zone extends. That is a design decision governed by the shower type, the room's use and the guidance in force, and a page that answered it from an area figure would be inventing a specification.
- Primer is not included in the headline because it is not always needed and its rate is different. On plaster, render or screed it is not optional; on cement-based backer board it is often omitted. The manufacturer's sheet governs, and the breakdown carries the area so you can apply their figure.
- Two coats is the common specification, and some systems ask for three on a floor or over a joint. Where the sheet says three, this figure is two-thirds of what you need.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Part of bigger jobs
This trade is one line of several job takeoffs. Run the whole job and every other trade comes back with it, off the same measurements.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-26 · v1.1.0
Regulatory standards & verification citations3
- Liquid-applied tanking is specified in two coats, the second laid at right angles to the first, and every published coverage rate is PER COAT — so the area is doubled before it is divided by the rate
- Reinforcing tape or matting is bedded into the first coat at every internal junction — wall to floor, wall to wall — and a preformed collar at every pipe penetration, because a membrane bridging a moving junction unsupported is the joint that splits first
- Primer is required on absorbent substrates such as plaster, render and sand-cement screed, and is commonly omitted on cement-based tile backer board; the manufacturer's sheet governs which applies
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To apply a liquid waterproofing membrane. Generic types, no brands, no prices.
Protection this work requires
- A dust mask does nothing about solvent vapour: ventilate through, and use an organic-vapour (A1) cartridge respirator in an enclosed room.
- Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.
Show the 5 tools this job needsHide tools
Essential
Brushes
Roller frame, sleeves and tray
Recommended
Mixing bucket or tub
Paddle mixer
Utility knife
Also needed as materials: replacement blades, roller sleeves.
what each waterproofing tool is for, and the spec that decides which to buy where one does.