Every calculator shows its math, and every factor comes from a published source.
Our factors
- Paint: 350 sq ft per gallon per coat by default (paint makers publish 350 to 400), 250 for primer, 21 sq ft per door and 15 per window.
- Wallpaper: we count strips with pattern repeat and match, not square feet.
- Rugs: standard sizes from 2 x 3 to 12 x 15 ft, a floor border you choose, and furniture rules from rug retailers.
- Curtains: 2 to 2.5x fullness, rod above and beyond the frame, standard ready-made lengths.
Sources
- Sherwin-Williams paint calculator: a gallon covers about 350 to 400 sq ft; a quart about 100 sq ft; a gallon of primer 200 to 300 sq ft
- Magnolia Home wallpaper (York): double roll 20.5 in x 33 ft = 56 sq ft
- Selected Wallpapers: European rolls vs double rolls (53 cm x 10 m, about 57 sq ft)
- InteriorDecorating.com: how much wallpaper (27 in American double rolls, about 70 sq ft)
- Room & Board: how to choose a rug size (24 in around a dining table; 18 in to the wall)
- Nebraska Furniture Mart: rug buying guide (8 x 10 for a queen, 9 x 12 for a king; 18 to 24 in beyond the bed)
- Wayfair: how to measure for curtains (rod 4 to 6 in above the frame; 84 in panels for 8 ft ceilings)
- The Home Depot: measuring for window curtains (2 to 2.5 times the window width; sheers 3 times)
- HomeAdvisor: flooring calculator (add 10% for waste under 1,000 sq ft, 7% above; 15% for diagonal layouts)
- Bali Blinds measuring guide: inside mount, narrowest width and longest height, minimum 3/4 in depth, factory deductions; outside mount 1.5 in overlap per side
- The Home Depot: measuring for window treatments (inside mount: narrowest width rounded down to 1/8 in; outside mount: about 2 in wider than the opening)
- Canadian Tire: how to hang curtains (half an inch above the floor; 6 to 8 in extra to puddle)
Sources last checked 1 November 2026.