Quick Answer

A room with four 12-foot walls and 8-foot ceilings (384 sq ft), minus one door and two windows, requires approximately 2.3 gallons for two coats at 350 sq ft/gallon coverage.

Wall Height (feet)

Wall Widths (feet, comma-separated)

Enter the width of each wall, separated by commas

Number of Doors

Number of Windows

Number of Coats

Coverage per Gallon (sq ft)

Common Examples

Input	Result
8 ft ceilings, walls 12, 12, 12, 12 ft, 1 door, 2 windows, 2 coats	1.8 gallons (333 sq ft paintable, 666 sq ft with 2 coats)
8 ft ceilings, walls 14, 14, 10, 10 ft, 2 doors, 3 windows, 2 coats	2.6 gallons (549 sq ft paintable)
9 ft ceilings, walls 20, 20, 15, 15 ft, 1 door, 4 windows, 1 coat	1.6 gallons (549 sq ft paintable)
8 ft ceilings, walls 10, 10, 10, 10 ft, 1 door, 1 window, 2 coats	1.6 gallons (284 sq ft paintable)

How It Works

The Formula

Gallons Needed = (Paintable Area x Number of Coats) / Coverage per Gallon

Where:

• Paintable Area = Total Wall Area - Door Area - Window Area
• Total Wall Area = Wall Height x (sum of all wall widths)
• Door Area = Number of Doors x 21 sq ft (standard door opening)
• Window Area = Number of Windows x 15 sq ft (standard window opening)
• Coverage per Gallon = typically 350 sq ft for standard interior latex paint

Standard Deductions

The industry standard for a door opening (including frame) is approximately 21 square feet (roughly 3 ft x 7 ft). A standard window opening is approximately 15 square feet (roughly 3 ft x 5 ft). These are averages; actual openings may vary. For large picture windows or sliding glass doors, count those separately.

Coverage Rates

Most interior latex paints cover 350 to 400 square feet per gallon on smooth, previously painted surfaces. Coverage varies based on surface texture, color, and paint quality. Rough or textured surfaces, bare drywall, and dark-to-light color changes typically require more paint. Primer coats usually cover about 300 to 400 square feet per gallon.

Number of Coats

Most painting projects require two coats for full, even coverage. A single coat may suffice when repainting the same color or a very similar shade. Going from a dark color to a light color may require three coats or a coat of primer plus two coats of finish paint.

Worked Example

For a room with 8-foot ceilings and four walls measuring 12, 12, 14, and 14 feet: Total wall area = 8 x (12 + 12 + 14 + 14) = 8 x 52 = 416 sq ft. With 2 doors and 3 windows: deductions = (2 x 21) + (3 x 15) = 42 + 45 = 87 sq ft. Paintable area = 416 - 87 = 329 sq ft. For 2 coats at 350 sq ft/gallon: Gallons = (329 x 2) / 350 = 658 / 350 = 1.88 gallons. Rounding up, purchasing 2 gallons provides enough paint with a small reserve for touch-ups.

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Frequently Asked Questions

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

A 12 x 12 room with 8-foot ceilings has 384 square feet of wall area. After subtracting one door (21 sq ft) and two windows (30 sq ft), the paintable area is about 333 square feet. For two coats at 350 sq ft/gallon, that requires approximately 1.9 gallons. Two gallons is a practical purchase for this room size.

How many square feet does a gallon of paint cover?

Standard interior latex paint covers approximately 350 to 400 square feet per gallon on smooth, previously painted surfaces. The exact coverage depends on the paint brand, surface texture, and application method. Check the paint can label for the manufacturer's specific coverage estimate.

Do I need to account for primer separately?

If priming is needed (bare drywall, stains, or dramatic color changes), calculate primer gallons separately. Primer coverage is similar to paint, typically 300 to 400 square feet per gallon. This calculator estimates finish paint only. Add primer as a separate coat if required.

Why does the calculator use 21 sq ft for a door?

The 21 square foot figure is an industry standard estimate for a standard interior door opening including the frame (approximately 3 feet wide by 7 feet tall). For larger openings like double doors or sliding glass doors, you may want to reduce the number of standard doors entered and manually subtract the actual opening area from your total.

Is it better to buy more paint than the estimate?

Having a small surplus is practical. Extra paint is useful for touch-ups after the project and for future repairs. A good rule of thumb is to round up to the next full gallon. Stored properly with the lid sealed tightly, latex paint can last for several years.