Paint Calculator

How to measure for paint

For walls, measure each wall's width and multiply by ceiling height, then add them up and subtract about 20 square feet per door and 15 per window. A faster method pros use: measure the room's perimeter × ceiling height, then subtract 20% for openings — it's accurate enough for estimating.

For ceilings, it's just length × width. For exteriors, measure each wall's length × height to the eaves, add gable triangles (½ × base × height), and subtract large openings. Don't forget soffits and fascia if you're painting trim — estimate trim separately at roughly 1 gallon per 6–8 windows/doors.

The primer question

Primer is the step DIYers skip and pros never do. You need primer (or a paint-and-primer combo on previously painted surfaces) when painting new drywall, patched spots, bare wood, or making a drastic color change. A $25 gallon of primer routinely saves a $50 gallon of topcoat by preventing the first coat from soaking in unevenly.

For dark-to-light changes, have the primer tinted toward the finish color — gray-tinted primer under light colors dramatically improves hide. For stain blocking (water marks, smoke, tannin bleed), only a shellac or oil-based primer works; latex primers won't stop bleed-through.

Pro tips that save a coat

• Buy quality paint. Premium paints with higher solids cover in two coats where budget paint needs three. The "cheap" gallon is the most expensive paint you can buy.
• Box your paint. For large jobs, mix all gallons together in a 5-gallon bucket so color is perfectly uniform across batches.
• Cut in, then roll. Cut edges with a brush, then roll each wall section while the cut-in is still wet to avoid flashing lines.
• Don't stretch it. Rolling paint too thin to "make it cover" is why DIY jobs need a third coat. Load the roller properly.
• Save a labeled quart of each color for touch-ups. Write the room, brand, color name/number, and sheen on the lid.
• Sheen guide: flat for ceilings, eggshell/satin for walls, semi-gloss for trim and doors, high-gloss only for flawless surfaces.

What affects painting cost

Paint runs $25–$70 per gallon; a typical 12×12 room needs 2 gallons of wall paint plus a quart of trim paint — $60–$150 in materials. Professional interior painting runs $2–$6 per square foot of floor space ($300–$900 per average room), with prep work (patching, sanding, caulking) driving the spread.

Exteriors run $3,000–$7,000 for an average home, heavily dependent on height, condition, and how much scraping is needed. Multi-story homes cost more per square foot simply because of ladder and lift time.

Common painting mistakes to avoid

• Skipping prep. Paint doesn't hide problems — it highlights them. Wash, patch, sand, and caulk before the first drop of paint; prep is half the job.
• Painting in bad conditions. Direct sun dries paint before it levels (lap marks); high humidity or temps below 50°F prevent curing. Follow the can's temperature and humidity specs.
• Cheap brushes and rollers. A $15 brush lays paint smoothly; a $3 brush sheds bristles into your finish. Good tools pay for themselves on the first wall.