How it works
The calculator counts the boards by area, then works out the fasteners on each board from the spacing in the HardieBacker and USG Durock installation guides, and the seams that need mesh tape.
Boards = ⌈ Area × (1 + Waste ÷ 100) ÷ (Board length × Board width) ⌉
Floors: Fasteners per board = (⌈ Length ÷ Spacing ⌉ + 1) × (⌈ Width ÷ Spacing ⌉ + 1)
Walls and ceilings: Fasteners per board = (⌈ Side across the studs ÷ Stud spacing ⌉ + 1) × (⌈ Side along the studs ÷ Spacing ⌉ + 1)
Fasteners = Boards × Fasteners per board
Tape rolls = ⌈ (Seams + Corners) × (1 + Waste ÷ 100) ÷ Roll length ⌉
⌈ ⌉ rounds up to a whole number. Spacing is 8 in on floors and walls and 6 in on ceilings unless you enter your own.
Worked example in words
An 8 × 5 ft bathroom floor is 40 sq ft. A 3 × 5 ft board covers 15 sq ft, so 40 ÷ 15 = 2.67 means 3 boards. On a floor the fasteners go every 8 in over the whole board: 60 ÷ 8 = 7.5 rounds up to 8 spaces, or 9 rows, and 36 ÷ 8 = 4.5 rounds up to 5 spaces, or 6 columns – 54 per board and 162 for three. Two boards along the 8 ft side and two across the 5 ft side leave one 5 ft seam and one 8 ft seam, 13 ft in all, so one 50 ft roll of tape is enough.
Fastening schedules from the makers
HardieBacker’s guide fastens floor and countertop boards every 8 in over the entire surface, and wall boards at a maximum of 8 in on center at all supports, with studs no more than 16 in apart. USG Durock uses 8 in in both directions on floors, 8 in on walls and 6 in on ceilings. Both keep fasteners 3/8 in or more from the edges. Floors also take a bed of thinset under the boards.
Mesh tape
Joints are filled with the thinset used for the tile and covered with 2 in wide alkali-resistant glass-fiber mesh tape. USG sells its Durock interior tape in 2 in × 50 ft and 2 in × 250 ft rolls; the calculator starts with the 50 ft roll. Add the inside corners to the tape length, since USG centers tape over all joints and corners.
Worked example
Inputs
- Surface
- Floor or countertop
- Measure by
- Length × width
- Surface length
- 8 ft
- Surface width or height
- 5 ft
- Board size
- 3 × 5 ft (915 × 1525 mm)
- Inside corners and edges to tape
- 0 ft
- Tape roll length
- 50 ft
- Waste allowance
- 0 %
Result
- Area to cover
- 40 sq ft
- Boards to buy
- 3
- Fastener spacing used
- 8 in
- Fasteners per board
- 54
- Screws or nails
- 162
- Seam length
- 13 ft
- Rolls of mesh tape
- 1
Common mistakes
- Using drywall screws or paper drywall tape. USG says not to use drywall screws, nails or joint tape on Durock; use cement board screws or roofing nails and alkali-resistant glass-fiber mesh tape.
- Spacing screws like drywall. Cement board needs fasteners every 8 in (6 in on Durock ceilings) along every stud, and over the whole board on floors.
- Forgetting the floor’s mortar bed. Both makers set floor boards in a bed of thinset before fastening, which needs its own bags.
- Lining up board joints with the subfloor joints or letting four corners meet. HardieBacker asks for staggered joints, which can change how boards are cut.
- Counting a 5 ft board on 16 in studs as if its ends land on a stud. Edges that run along the framing must sit on a stud, so some boards are trimmed.
Assumptions and limits
- Sheets are counted by area plus the waste allowance; the real count depends on the layout and how offcuts are reused.
- On floors fasteners form a grid over the whole board; on walls and ceilings they run along every stud or joist the board crosses, including both edges.
- Fastener spacing is the maximum from the HardieBacker and USG Durock guides unless you enter a closer one.
- In length × width mode the boards are laid in a grid with the long side along the longer side of the surface; seams are the joints between boards.
- Thinset for the floor bed and the joints, the waterproofing membrane and the tile are not included.
Frequently asked questions
How many sheets of cement board do I need?
Divide the area by the area of one board and round up. A 3 × 5 ft board covers 15 sq ft and a 4 × 8 ft board 32 sq ft. An 8 × 5 ft bathroom floor is 40 sq ft, so it needs 3 boards of 3 × 5 ft before any extra for cuts.
How many screws per sheet of cement board?
On a floor, HardieBacker and USG Durock both fasten every 8 in over the whole board, which is about 54 fasteners for a 3 × 5 ft board. On walls with studs 16 in apart, a 3 × 5 ft board laid horizontally crosses 5 studs with 6 screws on each, or 30 screws.
What is the screw spacing for cement board?
The installation guides give a maximum of 8 in on center on walls and floors. USG Durock asks for 6 in on ceilings. Keep fasteners at least 3/8 in from the edges and 2 in from the corners.
Do I need to tape cement board seams?
Yes. Both makers fill the joints with thinset and embed 2 in wide alkali-resistant glass-fiber mesh tape in it before tiling. Paper drywall tape is not used.
Should I use 3 × 5 or 4 × 8 ft cement board?
3 × 5 ft boards are lighter and easier to carry and cut; 4 × 8 ft boards leave fewer seams to tape. The fastener count per square foot is similar, so choose by handling and layout.
Sources
- HardieBacker Cement Board Installation Guide (1/4 in and HardieBacker 500) — James Hardie Building Products (copy hosted by a distributor). Sheet sizes "3 ft x 5 ft and 4 ft x 8 ft"; floors and countertops: fasten "every 8 in over the entire surface", 3/8–3/4 in from edges and 2 in from corners; walls: studs "a maximum of 16 in on center", fasteners "a maximum of 8 in on center at all supports"; joints: "Embed 2 in wide high-strength alkali-resistant glass fiber tape" in the same mortar used to set the tile.
- USG Durock Brand Cement Board with EdgeGuard Submittal Sheet (CB321618, rev. 2-25) — United States Gypsum Company. Sizes 3 ft × 5 ft (915 × 1525 mm) and 4 ft × 8 ft (1220 × 2440 mm); floors: fasteners "spaced 8 in. (203 mm) o.c. in both directions"; walls: "maximum 8 in. (203 mm) o.c. for walls, 6 in. (152 mm) o.c. for ceilings"; maximum stud spacing 16 in. (406 mm) o.c.; "Do not use drywall screws or drywall nails. Do not use drywall joint tape."
- USG Durock Brand Interior and Exterior Tapes Submittal Sheet (CB448, rev. 7-20) — United States Gypsum Company (copy hosted by a distributor). Alkali-resistant glass-fiber mesh tape; "USG Durock Interior Tape: Rolls, 2" x 50' and 2" x 250'"; "Center USG Durock Interior Tape over all joints and corners"; "Do not overlap pieces of the tape." Tape roll default 50 ft.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 6, 2026