Bathroom Waterproofing Calculator
Floor, walls and detail strips, with coating and membrane quantities.
Quick answer
With the default inputs: Room length 2.0 · Room width 1.8 · Wall upturn height 1.8 etc.Total waterproofing area 15.84 m²
Edit the inputs below to recalculate.Inputs
Result
Total waterproofing area
15.84m²
| Floor area | 3.60m² |
| Floor perimeter | 7.60m |
| Wall area | 12.24m² |
| Wall run after deducting door | 6.80m |
| Detail strips | 2.28m² |
| Coating required | 31.7kg |
| Drums at 20 kg | 2cans |
| Membrane with waste | 18.2m² |
| Area per m² of floor | 4.40× |
Wall area = (floor perimeter − door width) × upturn height; the doorway has no upturn, so deducting it is mandatory or you overcount by 5-10%. Code upturn heights: at least 1.8 m in showers, 0.75 m beside baths and squat pans, 0.3 m on other walls, 1.2 m at washbasins. Corners, pipe penetrations and drains take 300 mm wide detail strips, listed separately here. Finish with a 24-hour ponding test at least 20 mm deep.
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Formula
| Floor area | A = length × width |
| Wall run | L = 2 × (length + width) − door width |
| Wall area | A = L × upturn height |
| Total area | A = floor + wall |
| Coating | G = A × coverage |
| Membrane | A = total × (1 + waste) |
A 2.0×1.8 m bathroom with 1.8 m upturn, 0.8 m door and 2.0 kg/m²: 3.6 m² floor, 12.24 m² walls, 15.84 m² total, 31.7 kg coating, 18.2 m² membrane and 2.28 m² of detail strips.
How to work it out
- Floor area = length × width. That is the base, and the floor also needs a 1-2% fall to the drain.
- Wall heights follow the location: at least 1.8 m in showers, 0.3 m above a bath rim, 0.3 m elsewhere. Wall area = (length + width) × 2 × height − door opening.
- Where a shower sits in a corner, both adjoining walls go to 1.8 m, not just one. A wall shared with a bedroom wardrobe is best done full height.
- Detailing: 300-500 mm reinforcement at every internal and external corner, 200 mm around pipe penetrations, and a threshold upstand at the door.
- Total = floor + walls + detailing, times (1 + waste). Liquid membrane at 1.5-2.0 kg/m² for 1.5 mm; sheet by area ÷ roll area.
Where it goes wrong
- Waterproofing the floor only — shower spray reaches 1.5 m up the wall, so floor-only work lets water through to the room next door, where paint falls off and mould grows.
- Running 1.8 m up every wall — only the shower zone needs it; 0.3 m suffices elsewhere, and doing all walls wastes two to three times the material.
- No threshold detail at the door — without an upstand or barrier, water tracks under the threshold stone to the flooring outside; this is the commonest leak complaint.
- Tiling without a flood test — pond 20 mm for 24 hours and check the ceiling below before tiling, or the only remedy is demolition.
Material and cost
JS cementitious 15-30 CNY/m² at 1.5 mm, polyurethane 25-50, sheet 25-55. Supply and apply 40-80 CNY/m². A 4.5 m² bathroom costs 800-2000 CNY in total.
On site
- The substrate must be sound, flat, clean and free of standing water; form fillets of patching compound around pipe collars and drains and compact them.
- Key the surface after waterproofing — spatter coat or bonding agent — or the tiles will not hold and will debond.
- Drains, pipe collars and corners are the three leak hotspots; never omit the reinforcement, and keep it at least 300 mm wide.
Basis
- JGJ/T 298-2013 Technical specification for waterproofing of residential interiors, Chapter 4
- GB 50208-2011 Code for acceptance of underground waterproofing works (detail reference)
- GB 50327-2001 Code for construction of residential decoration, Chapter 7
FAQ
How high should wall waterproofing go?
Under the residential interior waterproofing code: at least 1.8 m in showers and no lower than the shower head; 0.75 m beside baths and squat pans; 1.2 m at basins; 0.3 m elsewhere. Lightweight partitions (AAC, steel stud) need the full wall to full height because they wick water. This is the most complained-about leak location — do not cut it to 0.3 m to save material.
Coating or membrane?
Floors suit liquid coating: it forms a seamless film that copes with pipe penetrations, drains and corners, and is easy to apply in tight rooms. Membrane has laps, which fail in confined and detailed areas — it belongs on large roofs and basement walls. A coating-plus-membrane build-up is more expensive but reliable. If walls are tiled, choose a coating compatible with tile adhesive or the tiles will debond.
How is the ponding test done?
Once the membrane has fully cured — about 48 hours for coatings — plug the drain and doorway, pond at least 20 mm deep for 24 hours, then inspect the ceiling below for leaks and check that the level has not dropped appreciably. No leak and no drop means pass. Any failure must be repaired and retested, and a second test is done after finishing. Both need photo records and signatures.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Bathroom Waterproofing Calculator. 2026-10-25. https://jianzhujisuan.com/en/bathroom-waterproof
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.