Paint Calculator
How much paint a room takes: walls and ceiling, the number of coats, doors and windows left out, in gallons or litres.
Paint to buy
3 gal
2.38 gal for 416 ft² × 2 coats
- Wall area
- 416 ft²
- Coats
- 2
- Gallons needed
- 2.38
- Buy
- 3 gal
- Cost
- 114.00
Kalku is coming to iPhone and Android
The app does the whole room, not just the paint: drywall, trim, wallpaper, flooring and tile from the same measurements, with each room drawn in 3D. It keeps every job with a note, shares it as a PDF, and works with no signal.
How it is worked out
The walls of a rectangular room are its perimeter times the wall height. The calculator adds the ceiling if you tick it, takes off the doors and windows you enter, multiplies by the number of coats and divides by the coverage. The result is rounded up to whole cans: that is the Buy figure. From 5 gallons up it also shows how to buy it as 5 gallon pails and single gallons.
Coverage is the number to get right. The calculator starts at 350 square feet a gallon, the figure the Kalku app uses, which is typical for one coat of wall paint on a smooth, primed surface. Switch to metric and it becomes square metres a litre (350 ft²/gal is about 8.6 m²/L) and the answer comes out in litres.
Worked example: a 14 × 12 ft room, two coats
- Walls: 2 × (14 + 12) ft × 8 ft = 416 ft².
- Two coats: 416 × 2 = 832 ft² of painting.
- At 350 ft² a gallon: 832 ÷ 350 = 2.38 gallons, so buy 3.
- At 38 a gallon the paint costs 114.
Tick Paint the ceiling too and the same room adds 168 ft² a coat: 1,168 ft² in all, 3.34 gallons, so 4 to buy. The ceiling usually takes a different paint, though, so it is often better worked out on its own.
Questions
How much paint do I need for a 12 × 12 room?
With 8 foot walls the walls are 4 × 12 ft × 8 ft = 384 square feet. Two coats at 350 square feet a gallon is 384 × 2 ÷ 350 = 2.19 gallons, so buy 3. The ceiling adds 144 square feet a coat.
How much does a gallon of paint cover?
Most wall paints cover 350 to 400 square feet a gallon in one coat on smooth, primed walls, about 8.5 to 10 m² a litre. Rough, porous or bare surfaces and strong colour changes take more. The figure on the can is the one to use; type it in as the coverage.
Do I need two coats?
For a new colour, or over new drywall, usually yes, which is why the calculator starts at two. Freshening up a wall in the same colour can take one. Primer is a separate product: work it out as its own job with one coat and the primer's coverage.
Should I subtract doors and windows?
For a room with large windows or several doors it makes a real difference, so the calculator takes their area together. A standard interior door is about 20 square feet, a typical window 12 to 15. For a room with one door and one window, leaving them in is a reasonable cushion.
