How much paint do I need for a 12x12 room?

For a 12 x 12 ft room with 8 ft walls and two coats, the wall area is 384 sq ft before subtracting openings. After subtracting one door and two windows, the painted surface is about 302 sq ft, or 604 sq ft across two coats. At 350 sq ft per gallon that is about 1.73 gallons, so buy 2 gallons.

How many square feet does a gallon of paint cover?

Most interior wall paints list coverage around 350 to 400 sq ft per gallon per coat on smooth, properly primed surfaces. Porous or heavily textured surfaces cover less, so use the lower end of the range when planning.

Should I subtract doors and windows from the wall area?

This calculator subtracts 20 sq ft per door and 15 sq ft per window, which is a common estimating convention. If you prefer a conservative estimate, count fewer openings or add a little extra paint.

Does this calculator include primer?

No. Primer is estimated separately because it depends on the surface and the paint being used. If you plan to prime, calculate the primer using the same paintable area and the primer's own coverage.

Paint Calculator

Enter your room dimensions, the number of coats, and the doors and windows on the walls. The calculator returns the paintable area and the number of gallons to buy, rounded up to the nearest whole gallon.

Paint needed

2 gallons

Based on 604 sq ft of painted surface.

Wall area: 352 sq ft
Openings subtracted: − 50 sq ft
Paintable area: 302 sq ft
With coats: 604 sq ft
Exact gallons: 1.73

Planning note

Gallons are rounded up because paint is sold in whole containers. Buy a little extra for touch-ups, and confirm coverage on the label before ordering.

How this calculator works

The calculator converts a rectangular room into wall area, removes the area taken up by doors and windows, optionally adds the ceiling, multiplies by the number of coats, and divides by the paint coverage.

wall area      = 2 × (length + width) × wall height
openings       = (doors × 20) + (windows × 15)
ceiling        = length × width        (optional)
paintable area = wall area − openings + ceiling
surface        = paintable area × coats
gallons        = surface ÷ coverage

Assumptions

The room is rectangular, and the walls are the same height throughout.
Each door removes 20 sq ft and each window removes 15 sq ft from the wall area.
The default coverage is 350 sq ft per gallon per coat. Change it to match the can.
Results cover one color on the measured surfaces and do not include primer, trim, or the ceiling unless you enable it.
Gallons are rounded up because paint is sold in whole containers.

Worked example

A 12 ft by 10 ft room with 8 ft walls has a wall area of 2 × (12 + 10) × 8 = 352 sq ft. Subtracting one door (20 sq ft) and two windows (30 sq ft) leaves 302 sq ft. Two coats is 604 sq ft. At 350 sq ft per gallon, that is 604 ÷ 350 = 1.73 gallons, so you would buy 2 gallons.

Common mistakes

Forgetting the second coat. Two coats is the norm for a uniform finish.
Using the floor area instead of the wall area.
Assuming a coverage number without checking the label for the specific product.

Frequently asked questions

How much paint do I need for a 12x12 room?: For a 12 x 12 ft room with 8 ft walls and two coats, the wall area is 384 sq ft before subtracting openings. After subtracting one door and two windows, the painted surface is about 302 sq ft, or 604 sq ft across two coats. At 350 sq ft per gallon that is about 1.73 gallons, so buy 2 gallons.
How many square feet does a gallon of paint cover?: Most interior wall paints list coverage around 350 to 400 sq ft per gallon per coat on smooth, properly primed surfaces. Porous or heavily textured surfaces cover less, so use the lower end of the range when planning.
Should I subtract doors and windows from the wall area?: This calculator subtracts 20 sq ft per door and 15 sq ft per window, which is a common estimating convention. If you prefer a conservative estimate, count fewer openings or add a little extra paint.
Does this calculator include primer?: No. Primer is estimated separately because it depends on the surface and the paint being used. If you plan to prime, calculate the primer using the same paintable area and the primer's own coverage.

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Last updated: September 25, 2026