How to Measure a Room for Paint
For walls, measure the length and width of the room along with the wall height. A room's total wall area comes from the perimeter of the room multiplied by the wall height. Doors, windows, and other areas that won't be painted can then be subtracted.
Ceilings are simpler: multiply the room length by its width. For unusual rooms or projects with several separate surfaces, breaking the job into individual walls or sections can make measuring easier.
Why Paint Coverage Rates Vary
The coverage printed on a paint can is an estimate of how much surface that particular product is expected to cover. Actual coverage can change depending on the surface and how the paint is applied.
Smooth, previously painted walls may require less paint than rough or porous surfaces. Bare drywall, masonry, textured walls, unfinished wood, and other absorbent materials can use more paint. Large changes in color can also affect how many coats are needed to achieve an even finish.
Should You Subtract Doors and Windows?
Subtracting doors and windows gives a more precise estimate of the actual wall area being painted. For a room with several large windows, sliding doors, or other substantial openings, those deductions can make a noticeable difference.
For small openings, some painters simply leave them in the estimate. The small amount of extra paint can provide material for touch-ups or help compensate for normal application loss.
Buying Paint for Future Touch-Ups
Having a small amount of matching paint left after the project can be useful. Scuffs, repairs, nail holes, and other damage may eventually require touching up the original finish.
If you keep leftover paint, label the container with the room or surface where it was used. Keeping the paint color, brand, product line, and sheen information can also make matching the finish easier if you need to buy more later.