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Paint Calculator
Gallons for walls, ceilings, primer, and trim — one room or the whole house, with the coverage math shown, not hidden.
Field measurements
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Printed from build-your-house.com/calculators/paint — estimate only.
Methodology
How this takeoff is figured
SHEET TO-05
Wall area, one room. Perimeter times height: 2 × (length + width) × ceiling height, then 21 sq ft deducted per door and 15 per window. A 12 × 12 room with 8-ft ceilings, one door, and two windows nets about 333 sq ft of paintable wall.
Wall area, whole house. Paintable wall area is estimated at 2.7 × floor area — the industry heuristic for whole-interior repaints of typical layouts. It bakes in interior partitions (painted on both sides) less the openings, closets, and tile you will not paint.
Gallons. Area × coats ÷ 350 sq ft per gallon, rounded up to whole cans. That 350 is the coverage printed on the can and holds on smooth drywall — textured walls drink roughly 25% more.
Primer. New drywall gets one coat of PVA primer over every wall and ceiling. Primer spreads about 15% worse on bare board than paint on a sealed surface — figure roughly 300 sq ft per gallon instead of 350.
Trim. Enamel for doors, casing, and base is estimated at about half a gallon per 150 sq ft of floor per coat, rounded to the nearest half gallon with a one-gallon minimum — trim is cut-in work, and coverage per gallon matters less than having enough to finish every side of every door.
What's not here. Exterior paint, cabinet finishing, wallpaper, and labor. If you are still hanging board, run the drywall calculator first — its square footage is the same one this sheet paints.
Assumptions
Coverage, deductions, and prices
ADJUST ABOVE
- Coverage350 sq ft/gal per coat
- Textured surfaces~25% more paint
- Door deduction21 sq ft each
- Window deduction15 sq ft each
- Whole-house wall area2.7 × floor area
- Primer coverage~300 sq ft/gal on bare board
- Trim enamel0.5 gal per 150 sq ft floor
- Wall paint$38–$65/gal, Aug 2026
Questions
Owner-builders ask
FAQ
How much paint for a 12×12 room?
A 12 × 12 room with 8-ft ceilings has 384 sq ft of wall — 2 × (12 + 12) × 8. Deduct 21 sq ft for the door and 15 for each of two windows and you are down to about 333 sq ft. Two coats means covering 666 sq ft, and at 350 sq ft per gallon that rounds up to 2 gallons of wall paint. Painting the 144 sq ft ceiling too adds one more gallon for two coats.
How many square feet does a gallon of paint cover?
The number on every can — and the one this calculator uses — is 350 sq ft per gallon per coat on smooth drywall. Textured surfaces run about 25% lower, so on knockdown or orange peel figure closer to 260 sq ft per gallon. Primer spreads about 15% worse on bare board than paint does on a sealed wall, roughly 300 sq ft per gallon.
Do I really need primer on new drywall?
Yes — on fresh drywall it is non-negotiable. Unprimed board flashes the joints through the finish: paper face and joint compound absorb paint at different rates, so every seam and screw head reads as a dull or shiny stripe under the topcoat, no matter how many coats you roll. One coat of PVA drywall primer ($18–$28 a gallon) seals the whole surface to one porosity, and your finish coats behave the same everywhere.
One coat or two?
Two coats is the honest default for color changes and new drywall — one coat of even premium paint rarely hides a color shift, and primer is not a finish coat. The one-coat case is a refresh in the same color and sheen on walls already in good shape. When in doubt, buy for two: an unopened gallon can go back to the store, but a Saturday spent re-rolling a streaky wall cannot.
How much does it cost to paint a house interior yourself?
Materials are what this takeoff prices: quality interior latex runs $38–$65 a gallon as of August 2026, primer $18–$28, and trim enamel $45–$70. Add $50–$150 for a first-time setup of brushes, rollers, tape, and drop cloths. Hiring it out typically runs $2–$6 per square foot of floor area — the gap is labor, and painting is the finish trade where owner-builder labor most reliably substitutes for a pro, if you accept that prep and cutting-in take longer than the rolling.