Three boards in one bathroom
Start with the walls: moisture- and mold-resistant drywall, like CertainTeed's M2Tech or Georgia-Pacific's Mold-Guard.
The walls around the tub or shower that get tile don't take drywall at all. The 2021 International Residential Code (IRC) lists the backers allowed behind tile in tub and shower areas: glass mat gypsum, fiber-reinforced gypsum, fiber-cement and cement board (section R702.4.2). Plain and water-resistant drywall aren't on the list.
The ceiling depends on the joists and the finish. Under the same code, 1/2 in board can be hung in either direction on joists 16 in apart. On joists 24 in apart, it has to go across them. Add a sprayed or other water-based texture on 24 in joists, and the code calls for 5/8 in board or 1/2 in sag-resistant ceiling board (Table R702.3.5, note d).
The 5 x 8 ft bathroom, unfolded
Can water-resistant drywall go on a bathroom ceiling?
Yes. The 2021 IRC permits water-resistant gypsum backing board on ceilings (R702.3.7), though not where water hits it directly or the air stays humid all the time (R702.3.7.1). Your town may add a framing limit: Cave Creek, Arizona's drywall inspection checklist allows it only where ceiling framing is no more than 12 in apart for 1/2 in board, or 16 in for 5/8 in. On 24 in joists, ask before you hang it.
Basements and garages
Manufacturers sell the same moisture- and mold-resistant board for basements, kitchens and laundry rooms as well as bathrooms. A bedroom or living room takes plain 1/2 in drywall.
A garage wall or ceiling has one job: slow a fire so it doesn't reach the house. The IRC (Table R302.6) asks for at least 1/2 in gypsum board on the garage side of any wall to the house, and 5/8 in Type X board on a garage ceiling with a bedroom or other living space above. Some states go further: New Mexico's 2015 code asks for 5/8 in Type X on the garage side of that wall too.
How many sheets you need
Wall area is the room's perimeter times its height, less 21 sq ft for each door (about 3 x 7 ft), 15 for each window (about 3 x 5 ft) and any tub and shower walls. Add 10% for cuts and divide by the sheet: 32 sq ft for a 4x8, 48 for a 4x12. In the bathroom above, 208 sq ft of wall drops to 112 once the door, window and tub walls come out.
There's no standard extra. Manufacturers' own calculators add nothing (CertainTeed and USG) or 10% (Georgia-Pacific). If your installer works to another figure, use theirs.
Mud, tape and screws
USG figures about two 4.5 gal pails of its UltraLightweight all-purpose joint compound to finish 1,000 sq ft of board, about 9 gal. Lowe's drywall chart puts tape at 350 ft and screws at 1,250 for the same 1,000 sq ft. One bathroom is a single pail, with most of it left over.
Hang the ceiling before the walls. Screws go 12 in apart on a ceiling, and 16 in apart on walls with studs 16 in apart (12 in when the studs are 24 in apart).
Where the screws go on one 4 x 8 sheet
What to ask about the drywall on a bid
Ask each contractor which board they priced for each surface. A bid that just says "drywall" hasn't told you. Ask whether the backer board sits in the tile price or the drywall price: it's easy to pay for it twice, or to find nobody priced it. Then ask which finish level the price assumes (how smooth the final coat is). Angi puts a basic hang at $0.80 to $1 per sq ft and a Level 5 skim-coat finish at $2 to $3.50.
The free bid comparison template has a Drywall & insulation line for those answers. Tiling the tub walls? The tile calculator counts the tile, and the bathroom remodel cost calculator prices the whole room.
Drywall is rarely the whole job. Finishing a basement? Our free readiness assessment gives you a preliminary estimate that covers the framing, drywall, flooring and electrical, and names the decisions still open before you ask contractors for bids.