Paint Calculator
Gallons of paint a room needs by size, coats, doors and windows.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Buying paint by eye almost always means a second trip to the store mid-project or a half-used can in the garage forever. This calculator turns room dimensions, coats, and openings into a clean gallon count.
How it works
Perimeter × ceiling height, minus ~21 sq ft per door and ~15 per window, times coats, divided by 350 sq ft per gallon.
The math starts from wall area — perimeter times ceiling height — then subtracts the spots you won't paint: roughly 21 square feet per door and 15 per window. Multiply by the number of coats, divide by a gallon's real-world coverage of about 350 square feet, and round up to whole cans.
Coverage of 350 sq ft per gallon assumes a smooth, primed surface and a roller. Heavily textured walls, raw drywall, or deep-to-light color changes all eat more paint, which is exactly what the second coat in the calculator accounts for.
Wall area = 2 × (length + width) × height; net area = wall area − (doors × 21) − (windows × 15); gallons = net area × coats ÷ 350, rounded up.
Worked examples
A 12 ft × 12 ft bedroom with 8 ft ceilings, two coats, one door, two windows. → 2 gallons.
Walls = 2 × 24 × 8 = 384 sq ft. Minus 21 for the door and 30 for two windows = 333 sq ft. Two coats = 666 sq ft ÷ 350 ≈ 1.9, which rounds to 2 gallons.
A 16 ft × 20 ft living room with 9 ft ceilings, two coats, one door, two windows. → 4 gallons.
Walls = 2 × 36 × 9 = 648 sq ft, minus 51 for openings = 597 sq ft. Two coats = 1,194 ÷ 350 ≈ 3.4, rounded up to 4 gallons.
Tips & gotchas
- Buy all your paint in one trip from the same batch — store-mixed colors can drift slightly between cans, and a mid-wall seam will show it.
- Keep about a cup of leftover paint labeled with the room and date for touch-ups; it's far cheaper than re-matching later.
- A quality roller and a true second coat hide more sins than one heavy coat ever will, and they last longer too.
- Ceilings aren't counted here — add the room's floor area as extra square footage if you're painting the ceiling as well.
FAQ
Do I need primer?
Fresh drywall or a big color change usually needs primer plus two coats.
How many coats do I really need?
Two is the safe default for full coverage and an even sheen. You can sometimes get away with one when repainting the same color over a clean surface, but color changes and fresh drywall always want two.
Does the calculator include the ceiling?
No — it only counts the walls. If you're painting the ceiling, figure it separately at length × width and add the result to your gallon need.
Should I buy primer separately?
Yes, when you have bare drywall, stains, or a dramatic color change. Primer-and-paint-in-one products work for minor refreshes but rarely replace a dedicated primer coat on raw surfaces.
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