FitAndFillPaint Calculator: How Many Gallons for a Room
Work out how much paint a room needs from its size, doors, windows, number of coats and the coverage printed on the can.
How the calculation works
The calculator finds the area you actually paint, then divides by the coverage on the can.
wall area = 2 × (length + width) × ceiling height paintable walls = wall area − doors × 21 − windows × 15 gallons = (paintable walls + ceiling) × coats ÷ coverage × (1 + extra), rounded up Doors count as 21 sq ft (3 ft × 7 ft) and windows as 15 sq ft (3 ft × 5 ft). If your openings are much bigger or smaller, adjust the counts or measure them yourself.
Worked example
A 12 ft × 12 ft room with 8 ft ceilings has 2 × (12 + 12) × 8 = 384 sq ft of wall. One door and one window remove 36 sq ft, leaving 348 sq ft to paint.
Two coats at 350 sq ft per gallon need 348 × 2 ÷ 350 = 1.99 gallons, so buy 2 gallons.
If you also paint the 144 sq ft ceiling, the total is 492 sq ft and you need 2.81 gallons, so 3 gallons.
Assumptions and limits
- The room is rectangular with a flat ceiling.
- A standard door counts as 21 sq ft (3 ft × 7 ft) and a standard window as 15 sq ft (3 ft × 5 ft). Measure large or unusual openings yourself.
- Trim, baseboards, closets and built-ins are not included.
- Coverage is the sq ft per gallon for ONE coat on your surface. Rough, porous or very dark surfaces cover less.
- Primer is not included; bare drywall, stains or big color changes may need it.
Frequently asked questions
How much does a gallon of paint cover?
It depends on the product. Behr, for example, says one gallon of its Ultra or Premium Plus interior paint covers 250 to 400 sq ft, and many other manufacturers quote about 350 to 400 sq ft on a smooth, primed surface. Rough, porous or textured walls and big color changes cover less. This calculator starts at 350; change it to the coverage printed on the can you plan to buy.
How many coats do I need?
Two coats is the usual plan for a repaint. A single coat can work when you are painting a similar color over a clean, sound surface with a product designed for one-coat coverage, but check the label.
Do I need primer?
Bare drywall, patched areas, stains and large jumps from dark to light often need primer. This calculator does not include it; calculate it separately using the primer's coverage.
Does this include trim, baseboards or closets?
No. Add those areas separately or buy a small extra can for trim.
Why does it say my openings are larger than the walls?
The doors and windows you entered add up to more area than the walls have, which usually means a typo in the room size or the counts. The paintable wall area is then shown as zero.
Sources
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