From paint to plank, interior finishes are estimated by coverage area and waste. Reach for the paint calculator (about 350 ft² per gallon per coat), the drywall calculator (4×8 sheets for walls and ceilings), the tile calculator with cut and breakage waste, plus calculators for flooring, carpet, plywood, siding, insulation, and wallpaper.
See How to Estimate Paint, How to Estimate Tile, How to Estimate Drywall, and How to Estimate Plywood, or browse 380 answers in the interior FAQ.