Paint is bought by the can, but it is estimated by the square foot. The gap between the two is where people either run out halfway up the last wall or carry home a spare gallon.
Wall area = 2 × (length + width) × height
Paintable area = wall area − doors − windows
Gallons = paintable area × coats ÷ coverage per gallon
A 12 × 14 ft room with 8 ft walls
The room has one door and two windows, so the planner works from 416 sq ft of wall, takes off 51 sq ft for the openings, and paints 365 sq ft.
This example assumes
- 375 sq ft per gallon, the planner’s starting value
- Two coats on the walls
- No extra allowance
Worked example 12 × 14 ft room, 8 ft ceilings, 1 door, 2 windows
| Material | Calculated | Buy |
|---|---|---|
| Wall paint375 sq ft / gal, no extra allowance | 1.95 gal | 2 gallons |
The calculation gives 1.95 gal. That is just under two gallons, and you cannot buy a fraction of a can, so the list says 2 gallons. Only a little paint is left over, which is worth knowing if you expect to touch up later.
How coats change the answer
Each extra coat repeats the whole area, so the volume scales with the coat count. The buy quantity moves in whole steps:
| Wall coats | Calculated | Buy |
|---|---|---|
| 1 | 0.97 gal | 1 gallon |
| 2 | 1.95 gal | 2 gallons |
| 3 | 2.92 gal | 3 gallons |
Coverage is the number to check
The 375 sq ft per gallon figure sits in the middle of the 350 to 400 range Sherwin-Williams gives for a gallon, and the company notes that texture and the surface being painted change what you actually get. Your can’s label is a better number than any default.
What changes the estimate?
- Coverage printed on your paint: use it instead of the starting value
- Texture and porous surfaces, which soak up more paint
- A big color change, which often needs more than two coats
- Primer, which has its own coverage and is a separate line on the list
Ceilings and trim
This estimate is walls only. The ceiling is a separate area with its own coat count. Trim and doors are usually bought separately and are not in this number.