How the paint calculator works
To estimate paint, work out the wall area, remove the parts you will not paint, multiply by the number of coats, and divide by the paint’s coverage. Wall area is the total length of the walls, which for a whole room is its perimeter, multiplied by the wall height. A 12 by 12 foot room has a perimeter of 48 feet, and with 8-foot ceilings that is 384 square feet of wall. Subtract about 21 square feet for each standard door and 15 square feet for each average window. With one door and two windows the paintable area is 333 square feet. Two coats need 666 square feet of coverage, and at 350 square feet per gallon that is 1.9 gallons.
Coverage rates
Most interior wall paints cover about 350 to 400 square feet per gallon (roughly 9 to 10 square meters per liter) on smooth, previously painted walls. Coverage drops on textured, porous or patched surfaces, on fresh drywall, and when you make a big color change. Rough surfaces such as stucco or brick can take twice as much paint. Check the coverage printed on the can and enter it in the calculator; using a slightly lower number than the label gives you a safety margin.
How many coats?
Two coats are standard for an even, durable finish, even when the label says “one coat”. You may get away with one coat when repainting the same color in good condition. Going from dark to light, painting over bright reds or yellows, or covering new drywall usually needs a coat of primer plus two topcoats. Primer has its own coverage rate, often around 200 to 300 square feet per gallon on bare drywall, so calculate it separately. A tinted primer can reduce the number of topcoats needed for deep colors.
Buying the right amount
In the US paint is sold in gallons and quarts, and the calculator suggests a combination of full gallons and quarts. Buying a full gallon is often cheaper than three quarts, so round up if you are close. In metric countries, paint is usually sold in 1, 2.5, 5 and 10 liter tins. Do not forget ceilings, which are calculated as length × width, and trim, doors and cabinets, which usually use a different sheen. Keep leftover paint, labeled with the room and date, for touch-ups. Finally, buy all the paint for a room at once or mix cans together in a bucket, a technique called boxing, so the color is consistent across the walls.
Frequently asked questions
How much paint do I need for a 12x12 room?
About 2 gallons for two coats on the walls of a 12x12 room with 8-foot ceilings, one door and two windows.
How many square feet does a gallon of paint cover?
Typically 350 to 400 square feet on a smooth, primed surface.
Do I need two coats of paint?
Usually yes, for even color and durability, especially when changing colors.
Should I subtract doors and windows?
Yes. Subtract about 21 sq ft per door and 15 sq ft per window for a more accurate estimate.