Buying too little paint means a second trip to the store and a potential color mismatch between batches. Buying too much wastes $30 to $60 per extra gallon. A quick paint calculator formula eliminates the guesswork and gets you within a quart of what you actually need. Here is how to measure, calculate, and account for every variable including windows, doors, texture, and multiple coats.
The Basic Formula
One gallon of paint covers approximately 350 to 400 sq ft of smooth, primed wall surface in a single coat. For planning purposes, use 350 sq ft per gallon to build in a small buffer. The formula is straightforward:
Total wall area (sq ft) / 350 = gallons needed per coat
Multiply by the number of coats (usually two for a color change, one for a refresh of the same color), then round up to the nearest gallon or quart.
How to Measure Your Walls
Measure the length of each wall in feet and multiply by the ceiling height. A standard room with 8-foot ceilings and four walls measuring 12, 14, 12, and 14 feet has a total wall perimeter of 52 feet. Multiply 52 by 8 to get 416 sq ft of gross wall area.
Now subtract openings. A standard interior door is about 21 sq ft (3 feet wide by 7 feet tall). A standard window averages 15 sq ft (3 feet wide by 5 feet tall). If the room has two windows and one door, subtract 51 sq ft. Net paintable wall area: 365 sq ft. That is just over one gallon for a single coat, or about 2.1 gallons for two coats. Buy 2.5 gallons to be safe.
Ceiling Paint Calculation
Ceiling coverage uses the same 350 sq ft per gallon rate. Multiply room length by room width. A 12×14-foot room has a 168 sq ft ceiling, which needs about half a gallon per coat. Most ceilings need just one coat of flat white ceiling paint. Buy one gallon and save the leftover for touch-ups.
Trim, Baseboards, and Doors
Trim paint covers slightly less because of the detailed brush work and absorption into raw or previously painted wood. Plan on 150 to 200 sq ft of coverage per quart for trim paint. Measure the linear feet of baseboard (multiply by 0.5 feet for standard 6-inch baseboard to get sq ft), add door and window casing areas, and divide by 175.
A single interior door, both sides, consumes about 40 sq ft of trim paint. Crown molding adds roughly 0.5 sq ft per linear foot. For a 12×14-foot room with baseboards, one door, two window casings, and crown molding, expect to need about one quart of trim paint per coat.
Adjustments for Special Surfaces
Textured Walls
Knockdown, orange peel, and heavy texture increase paint consumption by 15% to 25%. The peaks and valleys of textured drywall create more surface area than the flat measurement suggests. If your paint calculator says 2 gallons, buy 2.5 gallons for textured surfaces.
Dark to Light Color Change
Covering a dark wall with a light color typically requires a coat of primer plus two coats of finish paint. Some high-hide paints like Benjamin Moore Aura or Sherwin-Williams Emerald can skip the primer and cover in two heavy coats, but three coats total is safer for dramatic color shifts like dark red to bright white.
New Drywall
Unpainted drywall absorbs significantly more paint than a previously painted surface. Always prime new drywall first. A primer coat uses about 300 sq ft per gallon, slightly less than finish paint. Skipping primer wastes expensive finish paint and results in uneven sheen.
Sample Room Calculations
- Small bedroom (10×12, 8-ft ceilings): 352 sq ft gross walls minus 36 sq ft for one door and one window equals 316 sq ft. Two coats = 1.8 gallons. Buy 2 gallons.
- Living room (16×20, 9-ft ceilings): 648 sq ft gross walls minus 72 sq ft for two doors and two windows equals 576 sq ft. Two coats = 3.3 gallons. Buy 3.5 or 4 gallons.
- Bathroom (6×8, 8-ft ceilings): 224 sq ft gross minus 36 sq ft openings equals 188 sq ft. Two coats = 1.1 gallons. Buy 1.5 gallons (one gallon plus one quart).
How Much Paint to Keep for Touch-Ups
Always keep at least a quart of each color for future touch-ups. Store it in the original can with the lid hammered on tightly, or transfer to a clean mason jar with a tight seal. Label it with the room, brand, color name, and sheen. Properly stored latex paint lasts 5 to 10 years. Oil-based paint lasts even longer. Store in a climate-controlled area, never in a garage or shed where temperatures swing below freezing.
Money-Saving Tips
Buy in 5-gallon buckets when painting multiple rooms the same color. The per-gallon cost drops by 10% to 20% compared to individual gallons. Ask the paint counter for a volume discount on orders over 10 gallons. Mistints and returned paint are sold at 50% to 75% off at most home improvement stores. For rental properties and utility spaces, mistint paint is an unbeatable deal. Using a paint calculator before shopping prevents the most common waste: returning partially used, non-refundable custom-tinted gallons.
