How much paint you need depends on the size of the room, the condition of the wall surface, and the paint’s spreading rate, which is given on the manufacturer’s label. For a rough estimate of how much paint you’ll need to cover the walls:
1. Measure the total footage around the perimeter and multiply by the wall height in feet.
2. From this subtract 20 square feet for each door and 14 square feet for each window.
3. Divide by the spreading rate (usually 300 to 400 feet per gallon).
That’s the number of gallons you’ll need. Do a similar calculation for the ceiling by measuring the square footage and dividing by the spread rate.
To estimate how much paint is needed for the trim in a room, the rule of thumb is about one-fourth of what is required for the walls. It will be more or less, depending on the number of windows and doors and the amount of detail trim.