How the numbers are worked out
Tiles
Floor area × (1 + wastage) ÷ the area of one tile, rounded up. Wastage covers cuts, breakages and a few spares: about 10% for a straight lay and 15% for diagonal or herringbone. Small bathrooms have a high share of cut tiles, so don’t skimp.
Tile adhesive
Cement-based adhesive at roughly 3.5 kg/m² with a 6 mm notched trowel (tiles under 300 mm), 5.5 kg/m² with a 10 mm notch (tiles up to about 450 × 450 or 300 × 600 mm) and 7 kg/m² with a 12 mm notch (600 × 600 mm and larger). Large tiles also need back-buttering for full coverage, which is why we add 15% for them. Sold in 20 kg bags.
Grout
The standard manufacturer formula: kg/m² = (L + W) ÷ (L × W) × joint × depth × 1.6, with tile length L, width W, joint width and depth (tile thickness) in mm. Plus 10% for waste. Cement grout usually comes in 5 kg bags.
Waterproofing membrane and primer
Under the National Construction Code (Housing Provisions Part 10.2, which draws on AS 3740), the shower area must be waterproof, walls included to 1800 mm high. Outside the shower, a concrete or fibre-cement floor must be at least water resistant, and a timber-based floor such as particleboard or plywood must be waterproof. Most people membrane the whole floor with an upturn at the walls, so that’s what this calculator assumes: the full floor, a 150 mm upturn around the room, and 1800 mm up two walls of the shower. Liquid membranes need about 1.4 L/m² across two coats (check your product) and primer about 0.15 L/m².
Whether you can apply the membrane yourself depends on your state and the size of the job. Read can I waterproof my own bathroom? before you buy it.
Silicone
Wall–floor junctions and the shower hob or screen line are sealed with flexible silicone, not grout. One 300 mL cartridge does about 8 m of joint.
Not included
- Sand and cement screed for falls to the waste (depends on your floor)
- Floor waste, puddle flange and any plumbing, that’s licensed work
- Wall tiles (work each wall out as a separate area)
- Tile levelling clips, spacers, sealer and tools
Want the full list with quantities for other materials? Read what you need to tile a bathroom floor.
Questions
How much extra tile should I buy for a bathroom floor?
Add about 10% for a straight lay, and 15% for diagonal or herringbone patterns, small rooms with lots of cuts, or large-format tiles. Buy everything from the same batch, and keep a few spare tiles for future repairs.
How much tile adhesive do I need per square metre?
Roughly 3.5 kg/m² with a 6 mm notched trowel, 5.5 kg/m² with a 10 mm notch and 7 kg/m² with a 12 mm notch, before back-buttering large tiles. Bigger tiles need a bigger notch. Always check the coverage figure on your adhesive bag.
How is grout quantity calculated?
Grout (kg per m²) = (tile length + tile width) ÷ (tile length × tile width) × joint width × tile thickness × 1.6, with all sizes in millimetres. Smaller tiles and wider joints use much more grout.
Do I need to waterproof the whole bathroom floor?
Under the National Construction Code (Housing Provisions Part 10.2), the shower area must be waterproof, including the shower walls to 1800 mm. Outside the shower, a concrete or fibre-cement floor must be at least water resistant, and a timber-based floor (particleboard, plywood) must be waterproof. Many waterproofers membrane the whole floor anyway. Rules on who can do the work vary by state.