One gallon of paint covers about 350 to 400 square feet in a single coat, and most jobs need two coats, so a gallon realistically covers around 175 to 200 square feet of finished wall. A typical 12 x 12 room with 8 ft ceilings takes 2 gallons for the walls, plus about a gallon for the ceiling.
Here is the arithmetic, the gallons by room size, and the things that quietly change coverage.
Gallons by room size (walls, two coats)
Each row assumes 8 ft ceilings and deducts one door and two windows, the standard allowances of 21 and 15 square feet:
| Room | Wall area after openings | Gallons |
|---|---|---|
| 10 x 10 ft | 269 sq ft | 2 |
| 12 x 12 ft | 333 sq ft | 2 |
| 12 x 15 ft | 381 sq ft | 3 |
| 15 x 20 ft | 509 sq ft | 3 |
The paint calculator takes your exact dimensions, ceiling height, and the real number of doors and windows, and it handles coats and coverage rate rather than assuming.
The math behind it
Multiply the distance around the room by how tall the walls are. A 12 x 12 room measures 48 ft around, so 48 x 8 = 384 square feet of wall. Subtract 21 square feet for the door and 15 for each window, which brings it to 333. Two coats is 666 square feet of coverage, and at 350 square feet per gallon that is 1.9 gallons, so 2 gallons.
Note what does not appear in that calculation: the floor area. A long narrow room and a square room of the same square footage need different amounts of paint, because perimeter is what matters. This is why "gallons per square foot of house" rules of thumb are unreliable.
Ceilings and trim are separate purchases
Ceilings use flat ceiling paint and are simply the floor area. A 12 x 12 ceiling is 144 square feet, which at two coats is well inside one gallon. A 15 x 20 room needs 2 gallons. Buy ceiling paint rather than using wall paint: it is formulated to spatter less and to hide roller lap marks on a surface you see in raking light.
Trim, doors, and windows are sold and calculated separately, usually in quarts, because trim paint is a different product in a different sheen. A rough figure is one quart per 6 to 8 standard doors, or per about 60 to 70 linear feet of baseboard and casing at two coats. Trim paint goes further than you expect because the surfaces are narrow.
Do you really need two coats?
Two is the working assumption above because it is right for most jobs, but the honest answer depends on what is underneath.
One coat is genuinely enough when the wall is already in good condition, the new shade barely differs from the old one, the sheen is unchanged, and the paint is a quality product. Refreshing a beige room in a nearly identical beige is the classic case.
Two coats covers everything else: a color change of any consequence, a wall that has been patched, or a switch in sheen. It also gives a more uniform film, which is what makes the finish look even under raking light rather than merely opaque.
Three coats show up with deep saturated colors, particularly reds, oranges, and bright yellows, whose pigments hide poorly no matter what you pay. If you are going dark, budget for three and treat a tinted primer as the cheaper first one.
The practical planning rule: buy for two coats, and expect to use less on an easy repaint and more on a dramatic color change. Running short mid-wall is the outcome worth avoiding, because a visible stop line rarely disappears under the next coat.
Quarts, gallons, or a five
Container size is worth a moment because the per-gallon price moves a lot. A gallon typically costs meaningfully less per unit than four quarts, and a 5 gallon bucket less again per gallon, often by 15 to 25 percent.
That makes the decision simple once you have a number. If the job lands near 4 gallons, pricing the bucket is usually worth it even if you finish with some left over, and a bucket also removes any question of consistency between containers. Below about 3 gallons, individual cans are the sensible buy.
Quarts earn their place on trim, doors, and accent walls, where you need a different product or color in a small quantity and a full gallon would simply go off in the garage.
The paint calculator also covers what shifts real coverage on your particular walls, texture, bare drywall, and application method, which is what decides whether your job lands at the optimistic or pessimistic end of these figures.
Prepping the room first? The drywall calculator covers the board and compound, and the baseboard calculator sizes the trim that gets its own paint.