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How to Calculate Paint for a Room
Knowing how much paint to buy prevents wasted trips to the store and leftover gallons cluttering your garage. Here's the industry-standard method.
Basic Formula
Step by Step
- Calculate wall area: Perimeter (2 × length + 2 × width) × wall height
- Add ceiling if painting it: Length × width
- Subtract openings: Doors (~21 sq ft each), windows (~15 sq ft each)
- Multiply by coats: Most color changes need 2 coats
- Divide by coverage: Typically 300–400 sq ft per gallon depending on finish
Coverage by Paint Finish
- Flat / Matte: ~400 sq ft per gallon — best for ceilings and low-traffic areas
- Eggshell: ~350 sq ft per gallon — most popular for living rooms and bedrooms
- Satin: ~350 sq ft per gallon — great for kitchens, bathrooms, hallways
- Semi-Gloss: ~350 sq ft per gallon — ideal for trim, doors, and cabinets
- Gloss: ~300 sq ft per gallon — high durability for trim and accent pieces
Do I Need Primer?
Primer isn't always necessary, but it helps in several situations:
- New drywall: Always prime bare drywall to seal the surface
- Dark to light color change: A tinted primer reduces the number of top coats needed
- Stains or odors: Use a stain-blocking primer for water stains, smoke, or pet odors
- Glossy surfaces: Primer helps paint adhere to slick finishes
If you're painting a similar color over a previously painted wall in good condition, you can usually skip primer and go straight to two coats of paint.
Frequently Asked Questions
How much does a gallon of paint cover?
One gallon covers approximately 350–400 square feet on a smooth surface with one coat. Rough or textured surfaces may require more paint per coat.
How many coats of paint do I need?
Two coats is standard for most projects. You may get away with one coat if you're painting the same color. Dramatic color changes (especially dark to light) may need three coats or a coat of primer first.
How much paint for a 12×12 room?
A 12×12 room with 8-foot ceilings has about 384 sq ft of wall area. After subtracting a door and two windows, you need roughly 2 gallons for two coats.
Should I buy extra paint?
Yes. Buy at least 10% extra for touch-ups and future repairs. Store leftover paint in a cool, dry place — it can last years when sealed properly.