Paint is bought in whole gallons, and that single fact decides more than any of the measuring does. This paint calculator works out the wall area, applies your coats and coverage, and rounds up to what you can actually carry to the register.
What actually changes the number
A 16 by 12 foot room with 8 foot ceilings has 448 square feet of wall. At two coats and 350 square feet per gallon, that is 3 gallons. Here is what moves that figure and what does not.
Coats move it
One coat on this room needs 2 gallons, two coats needs 3, three coats needs 4. Two coats really is twice the paint by area, 873 square feet against 437, but it is not twice the gallons, because rounding up to whole units absorbs part of the increase.
Coverage moves it, at the low end
Drop the coverage from 350 to 250 square feet a gallon and the order goes to 4 gallons. Raise it to 400 and it stays at 3. Manufacturer coverage figures assume a smooth, primed, similarly colored surface, so knock them down for texture, bare drywall, or a dark color going light. Sherwin-Williams publishes 350 to 400 square feet a gallon, which is where the 350 default here comes from.
Doors and windows usually do not
This is the one that surprises people. Deducting one door and two windows takes 51 square feet off a 448 square foot room, and the answer is 3 gallons either way. Even at two doors and four windows it is still 3 gallons.
So do not agonize over the openings. A gallon covers so much that a normal room’s worth of doors and windows rarely crosses a whole-gallon boundary. The deduction earns its place on a wall of glass or a large open plan, not on a bedroom. Spend the attention on coverage and coats instead.
Buy the whole job at once
Paint is mixed in batches, and two gallons of the same color tinted weeks apart can differ enough to show on a long wall in raking light. Buy every gallon the job needs in one purchase and confirm the batch numbers match.
Keep what is left over rather than returning it. Touch-up paint from the original batch is the only thing that will ever match, and the alternative in two years is repainting a whole wall to fix a scuff.
Trim, ceilings and primer are separate orders
This page works out wall area. Trim, doors and ceilings are different products in different finishes, and they need their own quantities rather than a share of the wall total.
Primer follows the same arithmetic at one coat. Run the calculator again with the coats set to one and the coverage taken off the primer label, since primers spread differently from finish coats. For the sheets going up before any of this, the drywall calculator handles that stage.
Paint calculator FAQ
About 3 gallons for two coats at 350 square feet per gallon. The room has 448 square feet of wall at an 8 foot ceiling, and two coats means covering 873 square feet once a 10% allowance is added.
Twice the area, yes, but not always twice the gallons. On a 16 by 12 room one coat is 2 gallons and two coats is 3, not 4, because paint is sold in whole gallons and the rounding absorbs part of the increase.
It rarely changes what you buy. Taking off one door and two windows removes 51 square feet from a 448 square foot room, and the answer stays at 3 gallons. The deduction only matters on large glazed areas.
Manufacturers usually claim 350 to 400 square feet, which assumes a smooth, primed surface in a similar color. Expect closer to 250 to 300 on texture, bare drywall, or when covering a dark color, and that difference can add a gallon.