Cement Board Calculator for Showers & Tub Surrounds
Enter the tub or shower size and how high the tile goes. The calculator lays out each wall on 3x5 and 4x8 sheets, and gives the sheets to buy, the mesh tape for every seam and inside corner, the backer screws, and the liquid waterproofing to cover it all.
3x5 or 4x8 Sheets?
Most shower walls are built from 3x5 sheets even though they take more pieces, and the reason is weight and access. Half-inch cement board weighs roughly 3 lb per square foot, so a 3x5 sheet is about 45 lb and a 4x8 is close to 100 lb, which is hard to carry through a bathroom door and lift into a tub alcove alone. A 3x5 laid on its side is also exactly 5 feet long, the length of a standard tub, so two sheets stacked cover the back wall to 6 feet with a single horizontal seam.
The 4x8 count is lower, which is why the calculator shows both, and it pays on a tall stall where a single sheet can run floor to ceiling on each side wall with no seam at all. On the usual 60-inch alcove the difference is two or three sheets, worth a few dollars, against a much harder install.
| Layout | 3x5 sheets | 4x8 sheets | Mesh tape | Membrane |
|---|---|---|---|---|
| 60x30 tub alcove, 72 in high | 6 | 4 | 25 ft | about 1.1 gal |
| 60x30 tub alcove, to an 8 ft ceiling (81 in) | 7 | 4 | 32 ft | about 1.2 gal |
| 48x36 shower stall, 96 in high | 7 | 3 | 33 ft | about 1.5 gal |
| 60x36 shower stall, 84 in high | 7 | 4 | 33 ft | about 1.4 gal |
Each wall laid out separately without reusing offcuts, so these are buy quantities; tape for 3x5 layouts, with 10% extra. Membrane at about 55 sq ft per gallon for two coats.
How High the Backer Board Has to Go
The residential code requires shower walls, and the walls around a tub with a shower head, to have a waterproof, non-absorbent finish at least 6 feet above the floor. Above a tub that is usually set at 72 inches above the tub rim, which also clears the shower head spray. Many people run the board and tile to the ceiling instead; it looks finished, avoids a tile edge at an awkward height, and the calculator handles either. Start the board about 1/4 inch above the tub flange so it overlaps the flange without sitting on it, and leave that gap for sealant rather than grout.
Tape, Screws and Waterproofing
- Every seam and inside corner gets tape. Use 2-inch alkali-resistant mesh tape embedded in thinset, not drywall tape. The tape figure above counts each sheet joint plus the corners between walls.
- Screws, not nails, about 8 inches apart. Use corrosion-resistant backer board screws into every stud, set 3/8 to 5/8 inch in from the sheet edges. The count is based on retailer estimating figures of about 36 per 3x5 sheet and 50 per 4x8.
- Cement board is not waterproof. It does not break down when wet, but water passes through it. Either a liquid membrane over the board or a vapor barrier behind it is needed, not both, because two barriers trap moisture in the wall.
- Membrane thickness matters more than coverage. Liquid membranes must be built to a minimum thickness, usually in two coats at right angles. One widely used product lists about 55 sq ft per gallon for waterproofing; buy enough to reach that, and add fabric at the corners and around the valve if the instructions call for it.
If you are still deciding between cement board and moisture-resistant drywall, see cement board vs. greenboard. For the tile itself, the tile calculator and thinset calculator take over from here.
Frequently Asked Questions
How many sheets of cement board do I need for a tub surround?
For a standard 60x30 tub with board 72 inches up, about 6 sheets of 3x5 or 4 of 4x8, laying each wall out on its own. If you reuse offcuts carefully you can sometimes save one sheet.
How many screws do I need for cement board?
About 36 screws per 3x5 sheet and 50 per 4x8 on walls, spaced roughly 8 inches apart into the studs. A 60x30 tub surround takes around 150 screws with 3x5 sheets.
How much RedGard or liquid membrane do I need for a shower?
Figure about 55 sq ft per gallon for two coats. A tub surround of about 60 sq ft needs a little more than a gallon, so buy two gallons or a larger pail rather than coming up short on the second coat.
Does cement board need to be taped?
Yes. Every seam and inside corner gets alkali-resistant mesh tape set in thinset or in the waterproofing membrane, as its instructions specify. Untaped joints are where cracks and leaks start.