The short answer: a 12 by 10 ft room with 8 ft ceilings needs about 2 gallons for two coats, once you take off a door and a window. Here is the math so you can size any room.
The formula
paintable area = wall area − (doors × door area) − (windows × window area) paint = paintable area × coats ÷ coverage per can, rounded up to whole cans
Work out the wall area
For a single wall, multiply its length by its height. For a whole room, add the four walls at once: take the floor perimeter and multiply by the ceiling height.
wall area = 2 × (length + width) × ceiling height
A 12 by 10 ft room has a perimeter of 44 ft. At an 8 ft ceiling that is 44 × 8 = 352 sq ft of wall before any openings.
Subtract the doors and windows
You do not paint the glass or the door slab, so take them off. Standard deductions are about 21 sq ft per door and 15 sq ft per window (1.8 and 1.5 sq metres). One door and one window is 36 sq ft, leaving 316 sq ft to paint.
Coats and coverage
Two coats is standard for an even finish. At a typical 350 sq ft per gallon:
| Job | Area to cover | Gallons |
|---|---|---|
| One coat | 316 sq ft | 1 |
| Two coats | 632 sq ft | 2 |
| Two coats + primer | 632 + 316 sq ft | 2 paint, 1 primer |
Coverage is the figure that moves the result most. A smooth sealed wall stretches the paint; bare drywall, render or a dark colour underneath drinks the first coat, so lean to the higher can count.
When to add primer
Primer is a single coat over the paintable area. Add it for bare wall, patched repairs, a big colour change, or a glossy surface you want the topcoat to grip. A like-for-like refresh usually skips it.
Get your exact number
Enter your wall or room size, doors, windows and coats in the paint calculator and it returns the cans to buy in litres or gallons. For an outdoor fence instead, the fence stain guide sizes the gallons the same way.
