How to use this calculator
Measure the room at the floor, in feet, and the ceiling height in inches — 96 for a standard 8 foot ceiling, 108 for 9 foot, 120 for 10. Count the doors and the windows. That is all the calculator needs to give you a usable number.
Then decide the coats before you read the result. One coat and two coats are different orders, and the honest answer is almost always two.
The coverage number on the can
350 sq ft per gallon
That figure is measured on smooth, sealed, previously painted drywall, applied by a professional with a roller at the right nap. It is realistic for a repaint in a normal room and it is the default here. A 12 by 14 room with 8 foot ceilings has 416 square feet of wall before openings; take off a door and a window and you are at 380, which is just over a gallon per coat and therefore 2.5 gallons for two coats.
When coverage drops
New drywall with fresh joint compound drinks the first coat — think 250 to 300 square feet. Knockdown or orange peel texture adds surface area you cannot see: subtract 15%. Raw wood, bare masonry or a wall with heavy patching can halve it. Spraying rather than rolling uses 15% to 20% more because of overspray and a thinner film.
If you drop the coverage figure below 300 the calculator will say so. That is not a warning about your input, it is a reminder that primer is cheaper than a third coat of finish paint.
The other number the can does not mention is the roller. A 3/8 inch nap on smooth drywall lays a proper film at the stated spread rate. Drop to a 1/4 inch nap and you are laying about 20% less paint per pass, which reads as poor hide and sends people back for a third coat they did not need. On light texture use 1/2 inch, and on knockdown or masonry 3/4. Match the nap to the surface before you blame the paint.
Primer: when you actually need it
Prime bare drywall, bare wood, any repair larger than a business card, water stains, smoke damage, glossy surfaces you have not sanded, and any drastic colour change. Do not prime a clean previously painted wall going from one mid-tone to another — a quality paint is self-priming over itself and the money is better spent on the finish coat.
Where primer earns its place is colour change. Going white over a deep red without primer takes three or four coats of finish paint. A tinted grey primer plus two coats covers it and costs less. The full decision tree is in how many coats of paint.
Ceilings and trim
The ceiling area is the room footprint: 168 square feet for a 12 by 14 room. Two coats of flat ceiling paint at 350 square feet per gallon is one gallon. Ceiling paint is a different product from wall paint — dead flat to hide the roller lap, and thicker so it does not spatter.
Trim is measured in linear feet, not square feet, and a gallon of trim enamel goes a long way: figure 1 gallon per 300 linear feet of baseboard and casing for two coats. If you are working out what trim to buy at the same time, the baseboard and trim measuring guide covers the linear foot method.
Work out the order in one pass so you make one trip. Wall colour first, by the gallon; ceiling paint second, as a separate flat white product; trim enamel third, at roughly one gallon per 300 linear feet for two coats; primer last, only where the surface demands it. A typical 12 by 14 bedroom with 8 foot ceilings, one door and one window comes out at 2.5 gallons of wall paint, 1 gallon of ceiling, and about a third of a gallon of trim enamel — so buy three gallons and a quart, and expect to have most of a quart left for touch-ups.
Common mistakes
- Using the floor area instead of the wall area. A 168 square foot room has around 400 square feet of wall. This is the single most common paint estimating error.
- Buying quarts. Past one gallon, the price per gallon drops sharply at the gallon and again at the five gallon pail. Two quarts cost more than a gallon.
- Not boxing the paint. If you buy three gallons, mix them together in a pail before you start. Tint varies between cans and the difference shows on a long wall.
- Cutting in the whole room first. Cut in one wall and roll it while the cut line is wet. Dry cut lines flash differently from the rolled field.
- Forgetting the closet. It is another 100 square feet of wall and it needs the same two coats.
- Recoating too soon. Four hours for latex in normal conditions, longer if it is cold or humid. Rolling over a half-cured coat pulls it off the wall.