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For a standard 12×12 room with 8-foot ceilings, you need approximately 2 gallons of finish paint (two coats) plus 1 gallon of primer. The exact amount depends on wall area, the number of doors and windows, surface porosity, and number of coats — all handled automatically by the calculator above.
Estimating paint quantity is one of the most common and costly DIY mistakes. Over 60% of DIYers either run out of paint mid-project or end up with more than one full gallon leftover. Running short forces a second hardware store trip — and risks a dye-lot mismatch that leaves a visible seam. Buying too much wastes money on material that often ends up in the trash.
Most interior latex paints cover approximately 350–400 square feet per gallon on a smooth, previously painted surface (per Benjamin Moore and Sherwin-Williams product specifications). On raw drywall or porous surfaces, coverage drops to 300 sq ft because the surface absorbs more material on the first coat. A 10% waste factor should always be added for roller loading, cut-in overlap, and touch-ups. Our calculator applies this automatically.
Primer is essential when painting over bare drywall, dark colors, or stains. It seals the surface, improves adhesion, and reduces the number of finish coats needed — often turning a three-coat job into two. One gallon of primer typically covers 200–300 sq ft at standard application. Skipping primer on new drywall is the #1 reason amateur paint jobs look uneven. Always calculate primer and finish coat quantities separately.
Flat (matte) has no sheen and hides wall imperfections best — ideal for ceilings and low-traffic rooms. Eggshell has a slight glow and is the most popular for living rooms and bedrooms. Satin is more washable, making it a good choice for hallways and children's rooms. Semi-gloss is highly washable and moisture-resistant — the standard for kitchens, bathrooms, and trim. High-gloss is reserved for doors, cabinets, and detailed woodwork. The sheen level doesn't change your quantity calculation, but higher sheens require more uniform application and typically benefit from two full coats.
Interior paints are formulated for washability and low VOC levels. Exterior paints use UV-resistant resins and mildewcides designed to handle freeze-thaw cycles. Never substitute one for the other. Exterior paint on interior walls off-gasses toxic compounds; interior paint on outdoor surfaces peels within a single season. For garage floors, porches, and surfaces that bridge interior and exterior, use specialty paints designed for that specific application.
A standard 12×12 room with 8-foot ceilings has approximately 320 sq ft of wall area. After subtracting a door (21 sq ft) and two windows (30 sq ft), net area is roughly 269 sq ft. At 350 sq ft/gallon, that's about 1 gallon per coat. For two coats of finish paint, buy 2 gallons — plus 1 gallon of primer if painting bare drywall or making a dramatic color change. Always round up and keep one extra for touch-ups.
Most projects require two finish coats for even, opaque color coverage. When going from a light to a dark color (or vice versa), two coats over primer is the minimum. Flat paints hide imperfections better than semi-gloss but are harder to wash. Professionals almost always do two coats regardless of color change for a uniform sheen and lasting durability.
A 9-inch, 3/8-inch nap roller is the standard for smooth to lightly textured walls. Use a 1/2-inch nap for heavier textures like orange peel or knockdown. For ceilings, a 3/4-inch nap provides better coverage. Buy quality roller covers — cheap ones shed fibers that end up embedded in the paint film.
Light sanding with 220-grit paper between coats is good practice on trim and doors — it removes brush marks and dust nibs for a smoother final coat. On walls, it's optional but recommended in high-gloss applications. Always remove sanding dust with a tack cloth before applying the next coat.
Most interior latex paints are dry to the touch in 1 hour and ready for a second coat in 2–4 hours. Oil-based paints take 6–8 hours between coats. Applying the second coat too early traps solvents and causes wrinkling or peeling. Cold or humid conditions can double the drying time — plan accordingly.
A standard 12×12 room requires roughly 2–3 gallons of finish paint at $35–$65 per gallon, putting material cost at $70–$195 depending on brand and quality. Add $20–$30 for primer, $25–$40 for roller covers, tape, and drop cloths. Budget $150–$250 total for a typical bedroom.
Common questions answered — straight from the job site.
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