Floor area is not paintable area
A 12 × 12 room has 144 square feet of floor and 432 square feet of wall. Confusing the two is the most common way to under-buy paint and over-buy flooring.
Two different areas
Floor area is length × width. It drives flooring, underlayment and ceiling (a flat ceiling has the same area as the floor).
Wall area is perimeter × height: 2 × (length + width) × wall height. It drives drywall on walls, primer and paint. Openings are subtracted from wall area only.
"Square footage" on a listing is floor area. If you type it into a paint calculator that expects wall area, you will buy a third of the paint you need.
Common rooms
| Room | Floor area | Wall area (8 ft) | Wall area (9 ft) |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 320 sq ft | 360 sq ft |
| 12 × 12 ft | 144 sq ft | 384 sq ft | 432 sq ft |
| 12 × 14 ft | 168 sq ft | 416 sq ft | 468 sq ft |
| 14 × 16 ft | 224 sq ft | 480 sq ft | 540 sq ft |
| 3 × 4 m | 12 m² | 34.2 m² (2.44 m) | 38.5 m² (2.75 m) |
Gross wall areas, before doors and windows.
Never subtract one from the other
Some rules of thumb subtract floor area from wall area, or add them together into a "total finish area". Neither is a real quantity. Flooring uses the floor; paint uses the walls (plus the ceiling if you are painting it, as a separate line). The planner keeps every surface separate and lets each material pick the surfaces it needs.
Ceilings
A flat ceiling equals the floor area. A symmetric gable ceiling is larger: 2 × length × √((width ÷ 2)² + rise²). For a 12 × 12 room with a 3 ft rise that is 161 sq ft, not 144 — 12% more paint and board.
Use the paint calculator for wall area and the flooring calculator for floor area, or do both at once in the planner.