Will 1 gallon of paint cover a 12x12 room?

One gallon of paint will usually not fully cover a 12x12 room with two coats, which is standard for even finish, but it may cover one coat depending on wall height and number of doors and windows. A 12x12 room with 8 foot ceiling has about 352 to 400 square feet of wall area after subtracting door and window, plus 144 square feet if ceiling included. One gallon of quality interior paint covers about 350 to 400 square feet per coat according to label, but actual coverage is less on textured, porous, or dark walls, often 300 to 350 square feet. So one gallon may be enough for one coat on walls of 12x12 room if walls are in good condition and you are painting similar color. For two coats, which is recommended for durability and even color, you will need about 2 gallons for walls alone. If you include ceiling, you need additional gallon. If painting over dark color with light color, you may need primer plus 2 gallons. If trim included, add quart. So answer is one gallon is enough for one coat on 12x12 walls, but not enough for proper two-coat job, you should buy 2 gallons for walls to be safe, and you can return unopened. Always buy same batch.