How to Calculate Paint for a Room: Coverage Guide
Learn how to calculate exactly how much paint you need for any room. Includes wall area formulas, coverage rates, tips for doors and windows, and a free calculator.
The Basic Paint Calculation Formula
The fundamental formula is: Gallons Needed = (Total Wall Area in sq ft - Unpainted Areas) / Coverage Rate per Gallon. A standard gallon of paint covers approximately 350-400 square feet with one coat. To find total wall area: measure the perimeter of the room (sum of all wall lengths) and multiply by the ceiling height. A 12x14 foot room with 8-foot ceilings has a perimeter of 52 feet and a total wall area of 52 x 8 = 416 square feet. After subtracting doors and windows, you typically need about 1 gallon per coat.
Accounting for Doors Windows and Trim
Subtract unpainted areas from your total wall area. A standard interior door is approximately 21 square feet (3x7 feet). A standard window averages about 15 square feet (3x5 feet). A sliding glass door is roughly 42 square feet. For the 416 square foot room example with two windows and one door: 416 - 30 - 21 = 365 square feet of paintable wall area. For trim, baseboards, and crown molding, calculate their area separately as they often use a different paint type and finish. Baseboards in a 52-foot perimeter room at 6 inches tall equal about 26 square feet.
How Many Coats Do You Need
Most paint jobs require two coats for full, even coverage. Plan for two coats when: painting a light color over dark, using a color with poor coverage (reds, yellows, bright oranges), painting new drywall or patched areas, or switching between drastically different colors. You may get away with one coat when: using paint-and-primer-in-one over a similar color, touching up small areas, or applying a dark color over a similar dark shade. For our 365 square foot room with two coats: 730 total square feet / 375 average coverage = 1.95 gallons. Buy 2 gallons.
Paint Coverage Variables
Advertised coverage of 350-400 square feet per gallon assumes smooth surfaces and proper application. Rough or textured surfaces like stucco or brick can reduce coverage by 20-50%, requiring 200-300 square feet per gallon. Porous surfaces like new drywall or bare wood absorb more paint and need a primer coat first. Dark colors covering light surfaces may need a tinted primer to reduce the number of topcoats. Spraying uses 20-30% more paint than rolling due to overspray. When in doubt, buy an extra quart to ensure consistent color from the same batch.
Ceiling and Accent Wall Calculations
For ceilings, simply multiply room length by width. A 12x14 room has a 168 square foot ceiling, requiring about half a gallon per coat. Ceiling paint is typically flat white and covers well, often needing only one coat if the existing ceiling is already white. For an accent wall, calculate just that single wall's area and buy a quart (covers about 90-100 square feet) instead of a full gallon. For two-tone rooms or wainscoting, measure each section separately. Always keep leftover paint for touch-ups — store tightly sealed in a cool, dry place where it can last 2-10 years.
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Frequently Asked Questions
How much does it cost to paint a room?
For a DIY job, paint costs $25-50 per gallon for quality interior paint. A typical 12x12 room needs 2 gallons for two coats on walls, totaling $50-100 for paint. Add $20-40 for supplies (roller, tray, tape, drop cloth). Professional painters charge $300-800 per room depending on size and complexity. The biggest cost savings in DIY is the labor — professional painting is roughly 85% labor cost.
Should I buy a quart or a gallon?
A quart covers about 90-100 square feet (one coat). Buy a quart for: a single accent wall, small bathroom walls, or touch-ups. Buy a gallon for: any room larger than a small bathroom, multiple coats, or when you want matching backup paint. Gallons are significantly cheaper per square foot of coverage. Avoid buying multiple quarts when a gallon would be cheaper and ensure better color consistency.
Do I need primer before painting?
Use a separate primer when: painting over dark colors with a light shade, covering stains or water marks, painting bare drywall or wood, dealing with glossy surfaces, or switching between oil-based and latex paints. For repainting a wall in a similar color over existing paint in good condition, a quality paint-and-primer-in-one product is usually sufficient and saves a full step.