Understanding your result
The headline is the number of sheets. The stats beside it give the screws, the mesh tape and the thinset: bags to bed the board on a floor, or a bag to skim the seams on a wall. The table lists each in turn with the arithmetic behind it.
Sheets is the area times the waste, divided by the sheet area, rounded up. Board is cut with a scoring knife or a saw and the offcuts are often usable, so 10% is enough for a plain room; around a toilet flange, a tub and a door, 15%. Screws per sheet comes from the spacing you entered and the sheet size, starting 2 in from each edge; at the 8 in the board makers specify, a 3 × 5 sheet takes 40 and the count rises quickly if you tighten the spacing.
Seams to tape is estimated as half the perimeter of every sheet, because each edge is shared with a neighbour or meets a wall. The tape is alkali-resistant fibreglass mesh, not paper drywall tape, which the cement destroys. The thinset row depends on where the board goes: on a floor it is the bed under the board, combed with a 1/4 in notch at about 90 sq ft per bag; on a wall there is no bed and a single bag covers the taped joints. Neither is the mortar for the tile, which the thinset calculator sizes from the tile and the trowel. For a complete shower, the shower tile calculator gives the wall and floor areas that go into this page.
How we calculate this
The screw pattern starts 2 in from each edge and repeats at the spacing across and along the sheet, which is how the manufacturers’ diagrams draw it; the result is 40 screws for a 3 × 5 sheet at 8 in centres, in line with the 30 to 40 the installation sheets quote. The bed coverage of 90 sq ft is the 1/4 × 1/4 in notch figure from the mortar charts listed on the coverage reference page. The subfloor prep guide explains what the board does and does not do, and why a flexing subfloor needs plywood, not thicker cement board.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Sheet sizes | 3 × 5 ft and 4 × 8 ft (1200 × 900 and 2400 × 1200 mm) | Standard US and metric cement board formats (USG Durock, James Hardie HardieBacker, Schluter KERDI-BOARD specifications) |
| Screw spacing | 8 in (20 cm) on centre, 2 in from edges | USG and James Hardie installation instructions for floors and walls |
| Screws per box | 200 | A 1 lb box of 1 1/4 in backer board screws; boxes of 100 to 800 exist |
| Mesh tape | 50 ft rolls (15 m), 2 in wide, alkali-resistant | Standard backer board tape packaging; 150 ft (45 m) rolls are common |
| Floor bed | 1/4 in square notch, about 90 sq ft per 50 lb bag | Board maker instructions for the bed; coverage from mortar charts (Custom Building Products) |
| Seam thinset | about 300 ft of seams per 50 lb bag | A skim over 2 in tape a few millimetres thick; a judgement, generous |
| Waste | 10% | Simple rooms; 15% suggested around fixtures |
Assumptions last reviewed October 7, 2026.
The calculator does not choose the board thickness, count the waterproofing membrane, or size the plywood under the board; the subfloor plywood calculator does the last. It treats the area as a plain rectangle of sheets and does not optimise the cutting layout, which on a small bathroom floor can save a sheet if the offcuts are planned. Foam boards (Schluter KERDI-BOARD, Wedi) are counted the same way for sheets but use their own fasteners, washers and sealant, so the screw and tape rows do not apply to them.
Two worked examples
A 45 sq ft bathroom floor
3 × 5 ft sheets, 15% waste for the flange and the tub, 8 in screw spacing, 200 screws per box, 50 ft tape rolls.
- Sheets: 45 × 1.15 ÷ 15 = 3.45, rounded up to 4 sheets (60 sq ft of board)
- Screws: 5 across × 8 along = 40 per sheet; 160 in total, one box of 200
- Seams: 4 × 8 ft = 32 ft; tape with 10%: 35 ft, one 50 ft roll
- Bed: 45 × 1.1 ÷ 90 = 0.55, rounded up to 1 bag of thinset
A small floor, a short list: four sheets, a box of screws, a roll of tape and a bag of mortar for the bed, before any tile. The fourth sheet is mostly offcuts; keep them for the corners.
A 96 sq ft tub surround on walls
Three walls to 8 ft (12 ft of wall), 3 × 5 sheets, 10% waste, 8 in spacing, a 150 ft tape roll.
- Sheets: 96 × 1.10 ÷ 15 = 7.04, rounded up to 8 sheets
- Screws: 40 per sheet, 320 in total, two boxes of 200
- Seams: 8 × 8 ft = 64 ft; tape with 10%: 70 ft, one roll
- Thinset for the seams: 64 ÷ 300, rounded up to 1 bag
On a wall there is no bed, so one bag of thinset skims every seam and screw head with plenty over, and that bag is usually the same product that later sets the tile. The walls still need waterproofing before the tile goes on; that is a separate bucket or roll, sized by its own coverage.
Where to find your inputs
Area. The floor area for a floor; the wall area for a wall, from the wall tile calculator or the shower calculator. Board covers the same area as the tile.
Sheet size. 3 × 5 ft is the bathroom sheet (it fits through a door and between a tub and a wall); 4 × 8 ft is for bigger floors. Metric boards are 1200 × 900 or 2400 × 1200 mm.
Screw spacing. On the installation sheet folded into the pallet, and on the maker’s website: 8 in for the common brands.
Tape roll. On the roll: 50 or 150 ft in the US, 20, 45 or 90 m in metric markets.
Common mistakes
- No bed under floor board. The board makers require it. Without the thinset bed the board rocks and the tile cracks along the seams.
- Drywall screws. They rust and the heads snap. Use the coated, ribbed backer board screws made for the job.
- Paper tape. Cement destroys it. Use alkali-resistant mesh.
- Thicker board to stiffen a floor. Cement board adds no stiffness; a bouncy floor needs another layer of plywood.
- Board straight onto a shower without a membrane. Board is not waterproof. Membrane the board or the tile face.
- Forgetting the offsets. Board seams should not line up with the subfloor seams, and sheets are staggered; that is where the waste goes.
Questions people ask
- How many sheets of cement board for a bathroom floor?
- Divide the floor area by the sheet area and add 10 to 15% for the cuts around the toilet flange, the tub and the door. A 5 × 8 ft bathroom (40 sq ft) in 3 × 5 ft sheets is 40 × 1.15 ÷ 15 = 3.1, so 4 sheets; in 4 × 8 ft sheets it is 2 sheets.
- How many screws per sheet of cement board?
- At the 8 inch spacing the board makers specify, with the first screw about 2 inches from each edge, a 3 × 5 ft sheet takes 40 screws (5 across by 8 along) and a 4 × 8 ft sheet about 91. A 1 lb box of 1 1/4 inch backer board screws holds roughly 200, so a bathroom floor is one box.
- Does cement board need thinset under it on a floor?
- Yes. USG (Durock) and James Hardie (HardieBacker) both require a continuous bed of thinset combed with a 1/4 inch square notch under board on a floor, then screws through the board into the subfloor. The bed fills the gap between board and subfloor so the board cannot flex; without it the tile cracks. A 50 lb bag beds about 90 sq ft.
- How much mesh tape do I need?
- Every seam between sheets and every joint at a wall is taped. A good estimate is half the perimeter of each sheet, which is 8 ft of seam per 3 × 5 sheet and 12 ft per 4 × 8. Six 3 × 5 sheets need about 48 ft, so one 50 ft roll with 10% spare is tight and a 150 ft roll is the safer buy.
- Is cement board waterproof?
- No. It is water-resistant, meaning it does not fall apart when wet, but water passes through it. In a shower the board or the face of the tile needs a waterproofing membrane, either a sheet membrane or a liquid applied over the board and seams. The board calculator does not count the membrane; the maker's coverage per bucket or roll does.
- Which thickness of board should I use?
- 1/4 inch on floors over a sound plywood or OSB subfloor, where it is a tile substrate and not a stiffener; 1/2 inch on walls, where it spans between studs 16 inches apart, and on floors where the extra height is wanted to match an adjacent floor. The sheet count is the same for both; the screw length changes.