Understanding your result
The headline is the number of moisture-resistant gypsum sheets for the room, and the line under it is the cement board for the tub or shower walls. They are different materials from different aisles, fixed with different screws and taped with different tape, and the table keeps them apart: wall area, openings, the wet-area wall that goes to cement board, the wall that gets gypsum, and the two sheet counts.
The four tiles are the gypsum area, the cement-board sheets, the screws for the gypsum and the compound. Cement board screws are not counted here; they come in their own boxes, and the cement board joints are taped with alkali-resistant mesh and thinset, not paper tape and compound.
The notes are the code in plain words: cement board behind tile in wet areas (IRC R702.4.2), moisture-resistant board not over a vapour retarder in a tub or shower compartment and not on ceilings beyond the joist limits (R702.3.7). For a bathroom with no tub or shower, or a kitchen, the wet area is “none” and the room is all gypsum. The rest of the house goes through the room sheet calculator, and the thickness selector answers the same “which board here?” question for ceilings, garages and fire-rated walls.
How we calculate this
The wet-area presets are the three walls of a 5 ft tub alcove tiled to 6 ft above the tub (5 + 2.5 + 2.5 ft × 6 ft = 60 sq ft) and a 3 × 3 ft shower tiled to 7 ft (9 ft × 7 ft = 63 sq ft). A tiled wainscot around the rest of the room is not a wet area; it goes on the gypsum. Sheet areas come from the sheet sizes table, and the cement board is the common 3 × 5 ft, 1/2 in sheet. Screws and compound for the gypsum use the same GA-216 pattern and USG coverage as the other sheet calculators; the joint compound calculator switches to setting-type compound, which many finishers prefer in bathrooms because it does not soften when the room steams up before the paint goes on.
Cement board is tile backer rather than drywall, so this calculator counts it and stops. Tile, thinset and the waterproofing membrane are the flooring and tiling trades’ numbers; Floorings Calculator, our sister site, covers tile backer and tile quantities for the same room.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Tile backer in tub and shower areas | Cement board, 3 × 5 ft sheets | IRC R702.4.2 (cement, fibre-cement or glass-mat gypsum backer behind tile in tub and shower areas) |
| Moisture-resistant board on ceilings | Joists ≤ 12 in on centre for 1/2 in, ≤ 16 in for 5/8 in | IRC R702.3.7 |
| Tub alcove wet wall | 60 sq ft | 5 ft tub, three walls, tiled 6 ft above the rim |
| Shower stall wet wall | 63 sq ft | 3 × 3 ft stall, three walls, tiled to 7 ft |
| Doors and windows | 20 and 15 sq ft | Rough openings from the standard sizes table |
| Waste | 15% | Small rooms with fixtures, pipes and boxes cut up every sheet |
| Screws | 28 per 4 × 8 sheet | Gypsum Association GA-216 wall spacing on 16 in studs |
Assumptions last reviewed October 8, 2026.
The calculator does not choose between moisture-resistant and mold-resistant products, or between paper-faced and glass-mat boards, and it does not handle a steam shower, which is a wet room end to end and needs a membrane system. Where the room’s vapour retarder goes, and why green board over plastic in a shower is a mold problem waiting to happen, is in the bathroom and kitchen drywall guide; the board types themselves are in the thickness and board type guide.
Two worked examples
A 9 × 7 ft bathroom with a tub alcove
Eight-foot walls, one door, one window, a 5 ft tub with the surround tiled to 6 ft, ceiling in regular board, 4 × 8 sheets, 15% waste.
- Gross walls: 2 × (9 + 7) × 8 = 256 sq ft
- Openings: 35 sq ft; tub surround: 60 sq ft; gypsum walls: 256 − 35 − 60 = 161 sq ft
- Moisture-resistant sheets: 161 ÷ 32 × 1.15 = 5.79, rounded up to 6 sheets of 4 × 8
- Cement board: 60 ÷ 15 × 1.15 = 4.6, rounded up to 5 sheets of 3 × 5 ft
- 168 screws, 1.5 gallons of compound (one pail), 92 ft of joints
Six sheets of green board and five of cement board, where a room-only calculator would have said eight sheets of green board and put two of them in the shower.
A 12 × 10 ft kitchen, no wet area
Two doors, two windows, no tub or shower, 10% waste.
- Gross walls: 2 × (12 + 10) × 8 = 352 sq ft; openings 70 sq ft; 282 sq ft of gypsum
- Moisture-resistant sheets: 282 ÷ 32 × 1.1 = 9.69, rounded up to 10 sheets
- No cement board; 280 screws; 2.7 gallons of compound (one pail)
Many kitchens get moisture-resistant board only on the sink wall and the range wall. To count only those, enter the two walls as a “room” of their combined length and a notional width of zero, or run them through the single wall calculator and buy regular board for the rest.
Where to find your inputs
Room size. Length, width and height at the longest points. A bathroom with a separate toilet compartment is two rooms; a closet off the bathroom is dry and goes through the regular calculator.
Wet area. Tub alcove, shower stall, both, or your own figure: the tiled height times the length of the walls that will be tiled inside the tub or shower. If the tile stops at 4 ft on a tub wall, the area above it is still a splash zone and goes on moisture-resistant board.
Ceiling. Tick it only if the joists are close enough (12 in for 1/2 in board). Check in the attic or with a stud finder; most are 16 in, which means regular board on the ceiling and a bathroom paint.
Doors and windows. Count every one at 20 and 15 sq ft. A frosted window in a shower wall is in the wet area already and should not be counted twice; count only the windows in the gypsum walls.
Common mistakes
- Green board in the shower. It fails inspection and it fails in use. Cement board or another listed backer behind tile in wet areas.
- Green board over plastic in a tub surround. The vapour retarder traps moisture in the board. The code forbids it in tub and shower compartments.
- Moisture-resistant board on a 16 in ceiling. It sags. Regular or sag-resistant board and a bathroom paint on the ceiling.
- Bedding the board in the tub flange. Keep the bottom edge 1/4 in above the flange and seal the gap, or the paper wicks water from day one.
- Paper tape on cement board. Cement board joints take alkali-resistant mesh tape set in thinset.
- Counting the whole room as wet. Only the tiled walls inside the tub or shower are wet area. The rest is damp, and damp is what moisture-resistant board is for.
Questions people ask
- Where should I use moisture-resistant drywall?
- On bathroom, kitchen, laundry and utility walls that see steam and the odd splash: the walls around a vanity, behind a toilet, beside a washer, the backsplash wall before tile goes on. Not inside a tub or shower surround, where tile is set directly on the wall, and not on a ceiling over 12 in joist spacing for 1/2 in board.
- Can I use green board behind shower tile?
- Not under the International Residential Code. R702.4.2 requires cement board, fibre-cement or glass-mat gypsum backer behind tile in tub and shower areas. Moisture-resistant paper-faced gypsum board is for the rest of the bathroom; in the wet area the calculator counts 3 × 5 ft cement board instead.
- How many sheets of green board for a bathroom?
- A 9 × 7 ft bathroom with 8 ft walls, one door, one window and a 5 ft tub alcove has 256 sq ft of wall; take out 35 sq ft of openings and 60 sq ft of tub surround and 161 sq ft is left, which is 6 sheets of 4 × 8 at 15% waste, plus 5 sheets of 3 × 5 ft cement board for the surround.
- Is moisture-resistant drywall the same as mold-resistant?
- Not quite. Moisture-resistant board has a treated core and water-repellent paper; mold-resistant board adds a fibreglass or treated-paper facing that gives mold nothing to feed on. Most products sold today for bathrooms are both. Neither is waterproof, and neither replaces cement board in a shower.
- Can moisture-resistant board go on a bathroom ceiling?
- Only where the joists are 12 in on centre for 1/2 in board or 16 in for 5/8 in (IRC R702.3.7), because the treated core sags more than regular board. On 16 in or 24 in joists most finishers use regular or sag-resistant board on the ceiling and a bathroom paint, and the calculator leaves the ceiling as regular board unless you tick it.
- How much cement board for a tub surround?
- A standard 5 ft tub alcove tiled to 6 ft above the tub rim has about 60 sq ft of wall on three sides, which is four sheets of 3 × 5 ft cement board before waste and five with 15%. A 3 × 3 ft shower stall tiled to 7 ft is about 63 sq ft, also five sheets.