Paint Calculator

By Stephen Morrison, Editor Last reviewed October 30, 2026

Enter your room size and get gallons and quarts to buy for walls and ceiling, with doors and windows taken out. Add a price to see the cost.

Quick answer: a 12 × 12 ft room with 8 ft walls, one door and two windows has 334 sq ft of wall. Two coats at350 sq ft per gallon need 1.91 gallons, so buy 2 gal.

How the calculator works

Walls: 2 × (length + width) × height
Openings: doors × 20 + windows × 15 sq ft
Area: walls − openings (+ length × width for the ceiling)
Gallons: area × coats ÷ coverage per gallon
Buy: round up to whole quarts; 3 quarts become 1 gallon

Paint for common room sizes

8 ft walls, one door, two windows, two coats, 350 sq ft per gallon.

Room	Wall area	Walls: buy	Ceiling: buy
10 × 10 ft	270 sq ft	2 gal	1 gal
12 × 12 ft	334 sq ft	2 gal	1 gal
12 × 14 ft	366 sq ft	2 gal + 1 qt	1 gal
14 × 16 ft	430 sq ft	2 gal + 2 qt	1 gal + 2 qt
16 × 20 ft	526 sq ft	3 gal + 1 qt	2 gal

What changes how much you need

Texture and porosity. Rough plaster, popcorn ceilings, bare drywall and brick drink paint. Lower the coverage to 250–300 sq ft.
Big color changes. Dark to light, or a bright red or yellow, can take a third coat. Tinted primer helps.
Trim. Baseboards, door casings and window trim are usually a different sheen, so buy them separately. A quart covers a lot of trim.
Touch-ups. Keep a little extra from the same batch. Later batches can differ slightly in color.

Common questions

How much does a gallon of paint cover?

Sherwin-Williams puts it at about 350–400 square feet per coat on a smooth wall. We default to 350 because textured, porous or freshly patched walls soak up more. If your can's label gives a different number, enter it under "More options".

How much paint for a 12 × 12 room?

With 8 ft walls, one door and two windows, the walls are 334 sq ft. Two coats need 1.91 gallons, so buy 2 gal. That's walls only; the ceiling is another 0.82 gallons for two coats.

Do I really need two coats?

Usually, yes. A second coat evens out the color and sheen, especially for a big color change or over patched spots. Some paint-and-primer products claim one-coat coverage over similar colors, so if you are repainting the same shade, one coat may be enough. Set "Coats" to 1 to see the difference.

Why subtract doors and windows?

You don't paint them, and on a small room they make up a real share of the wall. We take off 20 sq ft per door (a standard 36 × 80 in door) and 15 sq ft per window (3 × 5 ft). Change both under "More options" if yours are different.

Gallons or quarts: what should I buy?

The calculator rounds up to whole quarts, then swaps three quarts for a gallon because three quarts usually cost about the same as a gallon. If you need more than about 4 gallons of one color, a 5-gallon bucket is usually cheaper still.

Do I need primer?

Use primer on bare drywall, fresh patches, stained areas, glossy surfaces, or when going from a very dark color to a light one. Primer covers less area per gallon than paint, so check its label for coverage.