How to use
- Measure length, width, and wall height in feet. For meters, multiply meters by 3.281 before you type, or convert the answer.
- Count doors and windows. The form subtracts a typical door and a typical window, not a bay window.
- Read coverage on the can. Flat paint and a dark color over white do not share one number.
- Add waste for cuts, a porous wall, and the bit left in the tray.
The formula is ordinary geometry. Two lengths and two widths, times height, is the wall area of a rectangle. Ceilings are not included. If you are painting the ceiling, add length times width once, outside this wall total, then run the coverage math again.
Worked example. A 12 by 10 foot room, 8 foot walls. Wall area is 2 × (12+10) × 8 = 352 sq ft. One door and two windows subtract about 51 sq ft, leaving 301. Two coats and 10% waste make 662 sq ft of painting. At 350 sq ft per gallon, that is 1.89 gallons. Buy 2 gallons unless the can says it covers more on a smooth sealed wall.
A 10% waste factor is a starting point, not a law. New drywall drinks the first coat. A glossy door does not. If the can says 400 sq ft, type 400. The calculator should follow the can, not a blog.
This will not price a contractor. Labor, primer, and ceiling texture are separate. Primer on a stain is another coat with its own coverage.
Do not use this for a whole exterior. Gables, soffits, and brick are not a rectangle room. Measure those faces one at a time.
FAQ
Does this include the ceiling?
No. Add length times width if you are painting it.
Why subtract 21 sq ft for a door?
That is a rough standard door. Measure yours if it is a slider.
How many coats?
Two is the usual guess. A strong color change can need three. Type what you will actually do.
Can I use meters?
Convert to feet first, or multiply the gallon answer is not the right path. Convert the inputs.
Is the gallon count exact?
No. It is an estimate from your measurements and the can's coverage.