Paint Calculator (Gallons for Walls and Ceilings)

How much paint for a bedroom? Enter the room, the doors and windows, the number of coats and the coverage on the can, and this calculator gives the wall paint, ceiling paint and primer in gallons and quarts, plus the cans to buy. Choosing the surface fills in a typical coverage rate; Sherwin-Williams says a gallon typically covers 350 to 400 sq ft on a smooth surface.

Paint calculator

How it works

Wall area = 2 × (length + width) × height − 20 sq ft per door − 15 sq ft per window
Gallons = area × coats ÷ coverage per gallon
Ceiling = length × width × coats ÷ coverage, as a separate paint
Cans: whole gallons plus quarts for the remainder; a fourth quart becomes another gallon

Worked example

A 14 × 12 ft room with 8 ft ceilings, two doors and two windows, two coats at 400 sq ft per gallon:

Units: Feet and inches
Room length: 14 ft
Room width: 12 ft
Wall height: 8 ft
Doors: 2
Windows: 2
Paint the ceiling too: Yes (separate ceiling paint)
Coats: 2
Surface: Smooth, previously painted (400 sq ft per gal)
Coverage per gallon: 400 sq ft
Primer coats: 0
Price per gallon (optional): $0

The calculator gives:

Wall paint: 1.73 gal (6.5 L): buy 1 gal + 3 qt
Ceiling paint: 0.84 gal: buy 1 gal
Wall area: 346 sq ft (32.1 m²) after 2 doors and 2 windows, 2 coats
Ceiling area: 168 sq ft (15.6 m²)
Rate used: 400 sq ft per gallon per coat

The walls are 346 sq ft after the openings, so two coats need 1.73 gallons: a gallon and three quarts (or two gallons, which is usually cheaper). The ceiling takes 0.84 gallons, so one gallon of ceiling paint. Textured walls at 350 sq ft per gallon would need two full gallons for the walls.

Material and waste notes

Coverage on the can is for a smooth, sealed surface. New drywall, textured walls and bare masonry take more; the surface choice adjusts it.
Two coats are the norm; a dark-to-light change or new drywall wants a primer coat as well.
Trim, doors and windows are painted separately, usually in a quart or two of a different sheen.
Buy all the paint for a room at once and box (mix) the cans together so the colour matches wall to wall.
A gallon short at the end costs a trip; a quart over is a touch-up supply.

Frequently asked questions

How much paint do I need for a 12×12 room?

With 8 ft walls, one door and two windows, about 334 sq ft of wall: 1.7 gallons for two coats at 400 sq ft per gallon. Buy two gallons.

How many square feet does a gallon of paint cover?

About 350 to 400 sq ft per coat on smooth walls (Sherwin-Williams), less on texture or bare surfaces.

How many coats of paint do I need?

Two coats for an even colour. Add a primer coat over new drywall, patches, stains or a big colour change.

How much ceiling paint for a 12×12 room?

144 sq ft × 2 coats ÷ 400 = 0.72 gallons: one gallon of flat ceiling paint.

Do I subtract windows and doors when calculating paint?

Yes, about 20 sq ft per door and 15 per window. The calculator does it automatically.

Sources

Sherwin-Williams paint calculator: a gallon of paint typically covers about 350 to 400 square feet

Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.