A planning rate, not a lab assay
Joint-compound coverage varies with joint type, coat count and whether you skim the whole board. USG Sheetrock Brand ready-mix planning coverage is about 10 gallons per 1,000 sq ft of gypsum board for the full joint-finish system (0.010 gal/sq ft), the familiar "about a gallon per 100 square feet." This calculator multiplies your finished area by that rate, then by a waste factor.
Sheet count is often easier on a jobsite than measuring every wall. Each standard 4×8 panel is 32 sq ft, so twenty sheets are 640 sq ft of board. Room mode uses 2 × (length + width) × height for the walls, plus length × width when you include the ceiling. For odd floor plans, the square footage calculator adds rectangles before you convert to sheet count.
Level 5 finishes, heavy texture, large patch repairs and paperless board systems need more product than the planning rate. If your supplier publishes pounds per 1,000 sq ft for a named product, convert with that product's density rather than relying on 0.010 alone.
Buckets, waste and a room example
Ready-mix ships in pails commonly marked 3.5 or 4.5 gallons (sizes vary by brand). Enter the bucket size on the label; the calculator rounds up to whole buckets so you do not end a coat short. A 10% waste factor covers mixing loss, wiped knives and small punch-list repairs; raise it for first-time finishers or heavily damaged substrate.
Example: a 12×12 room with 8 ft ceilings and the ceiling included. Wall area = 2 × (12+12) × 8 = 384 sq ft; ceiling = 144; total = 528 sq ft. At 0.010 gal/sq ft and 10% waste: 528 × 0.010 × 1.10 ≈ 5.8 gallons, which is two 4.5-gallon buckets.
Framing and slab work use different units. Concrete estimates cubic yards for a pour; stair checks riser and tread geometry under IRC limits. This page stays on joint compound for hung board.
Ready-mix versus setting-type compounds
Ready-mix (pre-mixed) all-purpose or lightweight compound is what the gallon estimate assumes. Setting-type powders (hot mud) are sold by the bag and mixed with water on site; their coverage is stated in pounds or bags per 1,000 sq ft on the bag. Convert those figures separately if that is what you are buying.
Keep coats thin and follow the product's recoat time. Setting-type compounds harden by chemical set and are useful for first coats and repairs; ready-mix sands more easily for finish coats. The calculator does not choose the product for you.
When you are still counting board before mud, divide room square feet by 32 for a first pass at 4×8 sheets, then add waste for cuts and openings. If a staircase is part of the remodel, check coded rise and run with the stair calculator.