How this waterproofing calculator works
The calculator adds up the areas that need membrane coverage — shower walls up to your set height, the shower floor, a junction allowance at floor-to-wall corners, and any additional bathroom floor area — then multiplies by your number of coats and divides by your product's coverage rate to get total litres, plus a waste allowance for offcuts, second coats on detailing and general spillage.
- Shower wall area: perimeter of the shower recess × your set wall height (1800 mm minimum under AS 3740).
- Shower floor area: full floor coverage of the shower recess, including the hob.
- Junction allowance: a small extra margin for corner and floor-to-wall junction detailing, which typically uses more membrane per square metre than flat areas.
- Additional floor area: any bathroom floor outside the shower you've told the calculator to include.
Example: a 900 × 900 mm enclosed shower, walls to 1800 mm, 2 coats, standard 3.0 m²/L coverage, with no additional floor area, needs roughly 5–6 litres of membrane including waste allowance — always round up to your product's pack sizes.
AS 3740 — what the standard actually requires
AS 3740:2021 — Waterproofing of Domestic Wet Areas is the Australian Standard that sets out where waterproofing membrane must go, minimum performance requirements, and acceptable application methods in bathrooms, showers and laundries. It's referenced by the National Construction Code, so it carries legal weight — a wet area waterproofed outside the standard is defective building work.
Key minimums this calculator is built around:
- Shower walls: membrane to at least 1800 mm above the floor, or 50 mm above the shower rose — whichever is higher.
- Shower floor: full coverage, including any hob or step-down.
- Junctions and corners: waterproofed with reinforced detailing, extending a minimum width either side of the joint.
- Bathroom floor outside the shower: full coverage required on upper-storey bathrooms; minimum 100 mm wall upturn on ground-level/slab bathrooms, though many builders waterproof the full floor regardless for extra protection.
You can read the current standard in full via Standards Australia — AS 3740:2021 isn't free to download, but Standards Australia's Reader Room offers limited free read-only access for personal, non-commercial use.
Choosing coats and coverage rate
Most liquid-applied membranes need two full coats to build an even, continuous film with no pinholes — check your product's technical data sheet for the exact number and drying time between coats. Coverage rate (how many square metres one litre covers per coat) varies by product and by how it's applied — brush and roller application typically uses slightly more product than a notched trowel or squeegee. When in doubt, use your product's stated rate rather than the calculator's defaults.
Waterproofing calculator — frequently asked questions
Do I need to waterproof the whole bathroom floor, or just the shower?
Under AS 3740, shower floors and walls (to at least 1800 mm) must always be fully waterproofed. Whether the rest of the bathroom floor needs full coverage depends on the storey — upper-floor bathrooms need the whole floor waterproofed, while ground-level bathrooms on a slab generally only need a minimum wall upturn outside the shower. Many builders waterproof the full floor anyway for extra protection, since the added cost is small.
How high up the wall does shower waterproofing need to go?
AS 3740's minimum is 1800 mm above the finished floor level, or 50 mm above the shower rose if that's higher. Where there's no fixed shower screen, many waterproofers take the membrane to the ceiling instead.
Can I waterproof my own bathroom in Australia?
Rules vary by state and by the value/scope of the work. In Queensland, waterproofing that forms part of licensable building work generally requires a QBCC waterproofing licence, and the work needs to be certified (commonly via a Form 16) for building approval, resale and insurance purposes. Check your local regulator before starting — this calculator only estimates material quantity, not compliance.
How many coats of membrane do I need?
Most liquid-applied membranes need two coats to achieve a continuous, pinhole-free film. Always check your specific product's technical data sheet, as some systems (especially sheet or reinforced systems) differ.
What happens if I get the waterproofing wrong?
Waterproofing failures are one of the most expensive defects to fix once tiling is complete, since repairs usually mean removing tiles and finishes to get back to the membrane. This is why AS 3740 exists, and why waterproofing work is licensed and certified in most states.
Related calculators
These calculators provide estimates for planning purposes only and don't replace professional advice. For structural, waterproofing, electrical or other safety-sensitive work, always check with a licensed tradie before starting your project.