Coverage: from room geometry to gallons, sheets, and rolls
Every finish take-off begins with net area: measure each wall as length × height, measure ceilings as plan area, then subtract openings — a door, a window bank, and a sliding opening easily remove 40–80 ft² from an average room, and skipping the subtraction is the most common way a paint budget inflates.
Coverage rates then convert area into product, and they are honest ranges rather than constants:
- Paint: roughly 350–400 ft² per gallon on smooth, previously painted walls; 250–300 ft² on bare, textured, or porous surfaces. Primer coats the substrate, not the colour chart, and most repaints want two topcoats — so double the area before dividing.
- Drywall: a 4 × 8 sheet covers 32 ft², a 4 × 12 covers 48; edges take a screw about every 8 inches and field studs about every 12. Cuts mean sheets never divide perfectly — plan the layout so offcuts land in closets, not in the bin.
- Flooring: buy area + waste, and let the pattern set the percentage — around 10 % for tile (15 % once the layout has diagonal or herringbone cuts), 7–10 % for planks, and for carpet the roll width itself (12 or 15 feet) decides where seams fall.
- Insulation: batts are manufactured to the cavity — 15-inch batts for 16-inch stud spacing, 23-inch for 24-inch — and R-value is quoted per inch, so compressing a thick batt into a thin cavity quietly robs the wall of its rating.
Trim, baseboard, and moulding run on linear feet: total perimeter minus doors, plus a cut allowance for every inside corner and scarf joint.
Remember:Measure the room once, net it once, and let coverage rates do the converting. Subtract openings first, pick the rate that matches the actual surface (smooth painted wall versus bare texture are not the same gallon), then add material waste — three separate corrections that must not be blended into one guess.