Paint Calculator
Find out how many gallons of paint you need for a room — walls, ceiling, and trim — including doors and windows subtracted.
What is Paint Calculator?
The paint calculator tells you exactly how many gallons to buy for a room. It accounts for the wall perimeter and ceiling height, subtracts typical door and window openings, and multiplies by the number of coats you plan to apply.
Default coverage is 350 square feet per gallon — typical for high-quality interior latex paint on primed drywall. Textured or porous surfaces may cover 250 sq ft/gal, and you may need a primer coat first.
Formula
Wall area = perimeter × height, where perimeter = 2 × (length + width).
Subtract approximate opening areas:
- Standard door ≈ 21 sq ft
- Standard window ≈ 15 sq ft
Gallons needed = (wall area × coats) ÷ coverage per gallon
Worked example
A 14' × 12' bedroom with 9' ceilings, 1 door, 2 windows, 2 coats at 350 sq ft/gal:
- Perimeter = 2 × (14 + 12) = 52 ft
- Gross wall = 52 × 9 = 468 sq ft
- Minus openings (21 + 30 = 51) = 417 sq ft
- Gallons = (417 × 2) / 350 ≈ 2.4 → buy 3 gallons
How to use this calculator
- Measure room length, width, and ceiling height.
- Count doors and standard-size windows.
- Choose 2 coats for a new color, or 1 coat if you are refreshing the same color.
- Check the paint can label for coverage — premium paints often cover more. The default 350 sq ft/gal is a safe average.
Buy whole gallons; touch-up jobs always benefit from having a quart of the same batch set aside.
Frequently asked questions
Do I need primer?
Primer is needed when: painting over bare drywall or wood, making a major color change (dark → light), or covering stains. Most modern "paint and primer in one" products work fine for color refreshes on already-painted walls — but do not count on them for the hard cases.
How much coverage do ceiling paints get?
Ceiling paint is formulated for one-coat coverage over flat white primer. Typical coverage is 300–400 sq ft/gal. Use flat finish on ceilings; satin or semi-gloss will reflect every imperfection.
What sheen should I pick?
Flat/matte hides wall imperfections best, good for bedrooms and ceilings. Eggshell is the most popular all-around choice. Satin handles scrubbing, good for kitchens and kids' rooms. Semi-gloss is used on trim, doors, and bathrooms.
How much does the cost example account for?
Just the paint. Add brushes, rollers, drop cloths, painter's tape, and primer — budget about 30–40% extra beyond the paint cost for supplies if you are starting from scratch.