Paint calculator

A 12 x 12 ft bedroom with an 8 ft ceiling leaves about 343 sq ft to cover once a door and a window are taken out. At 350 sq ft per gallon, two coats is 1.96 gallons - so 2 one-gallon tins, with very little to spare.

Result

Surface to paint343 ft²

Paint needed1.96 gal

Tins2 × 1 gal

Assumptions used
  • 8.59 m2/l — Interior emulsion, one coat on a primed surface. EU tins declare up to 13-14 m2/l; US makers baseline 350-400 sq ft/gal (8.6-9.8 m2/l) and painters report 250-350 on ordinary walls. (2026-09-09)

How the number is worked out

The four walls are two pairs of rectangles, so the gross area is 2 x (length + width) x height. Doors and windows come off at their real sizes - a standard leaf and a standard casement are prefilled, and you can change both. The ceiling is optional, because half of the people measuring a room are not painting it.

The result is multiplied by the number of coats before it is divided by the spread rate, and the tin count rounds up. Running out two thirds of the way along a wall means a visible lap mark where the new tin starts, even when the colour code is identical.

The spread rate is the number that actually varies

Everything else here is geometry, and geometry does not argue. The spread rate does: the same paint covers a primed wall and bare plasterboard very differently, and the figure on the tin assumes the good case. If you are painting new board, take the bottom of the range rather than the middle.

Openings larger than the wall

If the door and windows add up to more than the wall area, the calculator says so instead of returning zero. That combination is always a typo - usually a window entered in inches while the room is in feet - and silently answering “no paint needed” would be the least helpful thing it could do.

Worked example

Bedroom, 12 × 12 ft with an 8 ft ceiling

Surface to paint
343 ft²
Paint needed
1.96 gal
Tins
2 × 1 gal

Calculated by materiacalc using the same engine that runs the tool above.

In this market

American tins are quoted in square feet per gallon, usually somewhere between 250 and 400 depending on how porous the wall is. A previously painted, primed surface sits at the top of that range; bare drywall drinks the first coat and sits at the bottom.

Questions

Should I subtract the door and windows?

Yes, and this calculator does. In a small room the openings are easily a tenth of the wall area, which is the difference between two tins and three.

One coat or two?

Two, unless you are repainting the same colour. One coat over a colour change looks solid while it is wet and patchy once it dries.

Why does my tin say a different spread rate?

Because it depends on the product and the surface. The field is editable for exactly that reason - type what your tin says and the answer follows it.

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