Paint Calculator
Calculate how many gallons of paint you need for walls, ceilings, and trim — accounting for doors, windows, and coats.
Your details
Openings
Coverage
Cost
This calculator runs entirely on your device. Nothing you enter is uploaded, stored, or sold. How this works
Paint needed
3.43 gal
Buy 4 gallons for 2 coats over 643 ft²
Includes your inputs, the full breakdown, and every row of the table.
Wall area
435 ft²
87 ft² of openings deducted
Ceiling area
208 ft²
Estimated cost
$192
At $48 per gallon
Trim & doors
0.50 gal
134 linear feet
- Wall perimeter
- 58.0 ft
- Gross wall area
- 522 ft²
- Doors (2 × 21 ft²)
- - 42 ft²
- Windows (3 × 15 ft²)
- - 45 ft²
- Ceiling
- + 208 ft²
- Paintable area
- 643 ft²
- × 2 coats
- 1,286 ft²
- ÷ 375 ft² per gallon
- 3.43 gal
- Gallons to buy
- 4
What this means
- One gallon covers 350-400 square feet on smooth, previously painted drywall. Textured, porous, or bare surfaces can drop that to 250-300.
- Two coats is standard. Going from a dark colour to a light one, or covering bare drywall, usually needs primer plus two coats.
- At this volume a five-gallon bucket is typically cheaper per gallon and guarantees consistent colour across the whole job.
How the paint calculation works
Paint quantity comes down to paintable square footage divided by coverage per gallon, times the number of coats. The wall area is the room perimeter times ceiling height, minus roughly 21 square feet per door and 15 per window.
Coverage claims on the can assume ideal conditions. Real-world coverage on textured walls, over a colour change, or with a heavy nap roller runs meaningfully lower. When the calculation lands close to a round number, buy the extra gallon — touch-up paint from the same batch is worth having.
Frequently asked questions
How much paint for a 12x12 room?
With 8-foot ceilings, a 12×12 room has about 384 square feet of wall before openings. Two coats needs roughly two gallons, plus another gallon if you are painting the ceiling.
Do I need primer?
Yes on bare drywall, wood, or patched areas, and when making a dramatic colour change. Over the same colour in good condition, a quality paint-and-primer product is usually enough.
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Disclaimer. This calculator is provided for general information and educational purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Results are estimates based on the inputs you provide and the assumptions described above. Confirm any figure with a qualified professional before acting on it.