By EA Paint & Light Carpentry7 min read
At a glance
- Coverage
- About 350–400 sq ft per gallon
- Coats
- Plan on two
- 12×14 room walls
- About 2 gallons
- Trim
- About a quart for a typical room
How much paint do I need for a room?
Multiply the room’s perimeter by the wall height, subtract about 21 square feet per door and 15 per window, then double it for two coats. Divide by the coverage on the label, commonly about 350 to 400 square feet per gallon. A 12×14 room with 8-foot ceilings needs roughly two gallons for the walls.
The formula for wall area
You do not need an app. You need a tape measure, the height of the walls and the label on the can. Work in feet and square feet throughout, and do the walls, the ceiling and the trim as three separate sums, because they usually take three different paints.
- Add up the length of every wall to get the perimeter. For a rectangular room, that is twice the length plus twice the width.
- Multiply the perimeter by the wall height, measured from the floor to the ceiling.
- Subtract about 21 square feet for each standard door (3 by 7 feet) and about 15 for each average window (3 by 5 feet). Measure anything bigger, such as a slider or a wide opening.
- Multiply what is left by the number of coats — two, for almost every repaint.
- Divide by the coverage on the label, then round up to the next quart or gallon.
Tip: Coverage is printed on every can and on the product data sheet. It changes from product to product, so use the number for the paint you are actually buying.
Worked example: a 12×14 room with 8-foot ceilings
Take a 12 by 14 foot bedroom with 8-foot ceilings, one door and two windows. The perimeter is 12 + 14 + 12 + 14, or 52 feet. Multiplied by the 8-foot height, that is 416 square feet of wall.
The door takes off 21 square feet and the two windows take off 30, so 51 square feet come out. That leaves 365 square feet of paintable wall. Two coats double it to 730 square feet.
At 400 square feet per gallon, 730 divided by 400 is about 1.8 gallons, and two gallons covers it with a little to spare. At 350 square feet per gallon, 730 divided by 350 is about 2.1 gallons, and two gallons would leave you just short — buy two gallons and a quart. Either way, the honest answer for this room is about two gallons for the walls.
How many gallons of paint per room size
The chart below uses the same method and the same assumptions for every row, so you can check it with a calculator: 8-foot ceilings, one door and two windows (51 square feet subtracted), two coats, and a conservative 350 square feet per gallon. The ceiling column is the floor area times two coats, divided by the same 350.
If the label on your paint promises more coverage, you will have some left over. If your ceilings are taller, or the room has more windows or doors, redo the sum for your room.
10 × 10 ft
- Wall area to paint
- 269 sq ft
- Walls, two coats
- About 1.5 gal — buy 2 gal
- Ceiling, two coats
- About 0.6 gal — buy 1 gal
10 × 12 ft
- Wall area to paint
- 301 sq ft
- Walls, two coats
- About 1.7 gal — buy 2 gal
- Ceiling, two coats
- About 0.7 gal — buy 1 gal
12 × 14 ft
- Wall area to paint
- 365 sq ft
- Walls, two coats
- About 2.1 gal — buy 2 gal + 1 qt
- Ceiling, two coats
- About 1.0 gal — buy 1 gal
14 × 16 ft
- Wall area to paint
- 429 sq ft
- Walls, two coats
- About 2.5 gal — buy 3 gal
- Ceiling, two coats
- About 1.3 gal — buy 2 gal
15 × 20 ft
- Wall area to paint
- 509 sq ft
- Walls, two coats
- About 2.9 gal — buy 3 gal
- Ceiling, two coats
- About 1.7 gal — buy 2 gal
Ceilings, trim and doors are separate
Ceilings are usually painted in a flat ceiling paint, so they get their own sum: length times width, times two coats, divided by the coverage. In the 12×14 example that is 168 square feet, 336 for two coats, or about one gallon. A ceiling that has never been painted, or is being covered after a stain, can drink more than the label suggests, so a ceiling that comes out at exactly one gallon is one to round up on.
Trim is measured in running feet rather than area, and it takes very little paint. In a typical room, a quart of trim enamel is usually enough for two coats on the baseboard and the door and window casings. A standard door slab is about 21 square feet per side, so painting both sides with two coats adds roughly 84 square feet. If you are painting the door as well, buy a second quart, or step up to a gallon of trim paint if you are doing several rooms.
Do you need primer, and how much?
A repaint over sound paint of a similar color usually does not need primer — two coats of finish paint is the job. Primer earns its place in a few specific situations, and when it does, you prime only what needs it.
- New drywall or fresh joint compound, which soaks up finish paint unevenly and flashes through if it is not sealed
- Patches, repairs and spots where old paint has been scraped back
- Water stains, smoke staining and marker, which need a stain-blocking primer or they bleed through
- A big change in color — very dark to very light, or the other way — where a tinted primer can save a coat
- Glossy old oil-based paint, which needs to be de-glossed and primed with a bonding primer before a latex topcoat
Tip: Primer coverage is often lower than finish paint coverage on raw drywall and patched walls. Check the primer’s own label rather than reusing the finish paint’s number.
Why some New England rooms take more paint
The 350 to 400 square feet per gallon on most labels assumes a smooth, sealed, previously painted surface. Older houses do not always offer that. Plaster that has been patched many times, textured ceilings, walls that were skim-coated in places and unpainted wood all absorb more on the first coat, and a deep color over a light one often needs a primer or a third coat to cover evenly. In Northborough, half of the homes were built before 1980, and plenty of rooms in houses that age have a few decades of patches under the current paint.
The other local factor is storage. Latex paint can be ruined by freezing, and an unheated garage, shed or basement bulkhead in a Massachusetts winter gets cold enough to do it. Keep leftover paint inside the heated part of the house, with the lid sealed tight.
Buying paint, leftovers and what a painter works out
Round up rather than down, and buy all of a color at once. Paint mixed in separate batches can differ slightly, and the difference shows on a wall lit from the side. If you need more than one gallon of a color, pour the cans together into a bucket and stir — painters call it boxing — so the whole room comes from one consistent mix.
Keep a quart of every color for touch-ups, and write the room, the color name or formula and the sheen on the lid.
If you hire the work out, the painter does this math as part of the estimate, and the paint quantity matters less to you than what the estimate says will be painted: walls, ceilings, trim, doors, closets, the number of coats, whether primer is included on repairs and stains, and the products being used. Those are the lines to compare between bids.
Talk to a local crew
EA Paint & Light Carpentry is a painting and light carpentry company based in Northborough, MA. If you would like the house looked at, call (508) 466-5846 or request a free written estimate. Related services: Interior Painting.
