Paint Calculator

Paintable area is the perimeter of your room (2 × length + 2 × width) multiplied by ceiling height, minus a standard deduction for each door (21 sq ft) and window (15 sq ft). If you include the ceiling, length × width is added. That paintable area is then multiplied by the number of coats to get total square footage of paint film required.

We divide by 350 sq ft per gallon and round up to the nearest whole gallon, because paint stores sell gallons (or quarts), not square feet. Buying half-gallons sounds efficient but introduces dye-lot risk: paint mixed from two cans can show a faint seam under raking light. For walls over about 200 square feet, buy one extra gallon and 'box' (combine) it with the rest before rolling.

Perimeter is 2 × (12 + 14) = 52 ft, times 9 ft ceilings = 468 sq ft of wall. Subtract one standard door (21) and two windows (30) and you get 417 sq ft of paintable wall. At two coats, that is 834 sq ft of paint film, which divides to 2.38 gallons — round up to 3 gallons.

Add the 12 × 14 = 168 sq ft ceiling and you need an additional 1 gallon (168 × 2 ÷ 350 ≈ 0.96, round up). At $45/gallon for a quality interior latex like Behr Marquee or Sherwin-Williams ProClassic, the total paint bill is $180. Add roughly 15% for primer, brushes, rollers, and tape and you land near $210 in materials for a same-color refresh — substantially less than the $400–$600 a contractor would charge for the same room.

Add 10–15% for textured walls (knockdown, orange peel, stucco). Texture eats paint by exposing more surface area than a flat wall of the same nominal footprint.

Add a full extra coat when going from dark to light, or any color to a strong red or yellow. Reds and yellows have notoriously low hiding power because their pigments are translucent.

Subtract nothing for trim and doors — paint them separately with a quart of semi-gloss or satin. Mixing sheens on the same roller mark guarantees a touch-up nightmare later.

If you are painting in two sessions (e.g., walls today, trim next weekend), record the dye-lot number from the can label. Two cans with the same color name but different lots can show a faint stripe at the cut line.

Sheen affects coverage. Flat and matte paints hide imperfections but show fingerprints; they cover at the full 350 sq ft / gallon spec. Eggshell and satin (the most common interior choice) sit at the same coverage. Semi-gloss and high-gloss reveal every surface flaw and often need an extra coat to look uniform — plan for 300 sq ft / gallon and budget the extra gallon.

Primer is mandatory for bare drywall, raw wood, glossy old paint, water-stained ceilings, and any color change of more than two shades. A single coat of dedicated primer (about 350 sq ft / gallon) is cheaper per gallon than finish paint and seals the substrate so your finish color lays down evenly. 'Paint and primer in one' products work for color refreshes on already-primed walls — not for new construction.

Order: cut in (brush the edges around trim and ceiling) first, then roll the field. Working in 4 × 4 ft sections and keeping a 'wet edge' (rolling into still-wet paint) prevents lap marks. Two thin coats always outperform one thick one, regardless of what the can promises.