Standing in the paint aisle asking yourself how much paint do I need is how most people end up with either four unopened gallons in the garage or a half painted wall at 7pm on a Sunday. Neither is a small problem. The spare gallons are money sitting on a shelf, and running short mid project is worse than it sounds, because the replacement can comes from a different batch.
The arithmetic takes about three minutes with a tape measure and a phone calculator. What follows is the formula, the coverage figures to plug into it, a worked example you can copy, and the adjustments that catch people out.
The formula
Everything comes down to one line:
(Wall area in square feet x number of coats) divided by coverage per gallon = gallons needed
To get wall area, measure the perimeter of the room and multiply by the ceiling height. For a rectangular room that is simply two times the length plus two times the width, multiplied by the height.
Then round up to whole cans. You cannot buy 2.34 gallons.
Coverage figures worth planning around
Manufacturers publish coverage on the can, and it varies by product, so check yours. For planning purposes these ranges are typical, per gallon and per coat:
- Interior wall paint: roughly 350 to 400 square feet
- Ceiling paint: roughly 350 to 400 square feet
- Primer: roughly 225 to 275 square feet
Primer covers noticeably less ground than paint, which surprises people planning a bare drywall job. Budget for it separately rather than assuming one gallon does the same work.
Use 400 as your planning number on smooth, previously painted, sealed walls. Drop to 350 on anything textured, porous or freshly patched.
A worked example
Take a bedroom measuring 12 feet by 14 feet with 9 foot ceilings.
- Perimeter: 12 plus 14 plus 12 plus 14 equals 52 feet
- Wall area: 52 x 9 equals 468 square feet
- Two coats: 468 x 2 equals 936 square feet of coverage needed
- Divide by 400: 2.34 gallons
- Buy: 3 gallons, or 2 gallons plus 2 quarts if you want to trim the excess
For the ceiling in the same room, 12 x 14 is 168 square feet. Two coats is 336 square feet, which divided by 400 gives 0.84 gallons. One gallon covers it.
For the baseboard, 52 linear feet of 4 inch board is about 17 square feet of surface. Two coats is 34 square feet. A single quart handles that with plenty spare for the door casing.
One practical note on buying. A 5 gallon bucket is usually cheaper per gallon than five separate cans, so if your total lands anywhere near 4 gallons, price both options before deciding.
Should you subtract windows and doors?
Most guides tell you to. In practice, for a normal room, it is usually a mistake.
A standard interior door is roughly 20 square feet of wall you are not painting. An average window is around 15. In our 468 square foot example, one door and two windows remove about 50 square feet, which is a little over 10 percent of the job.
That 10 percent is not spare paint. It is the margin that covers cutting in around the frames, the extra passes on the wall behind where furniture sat, the touch ups three weeks later, and the simple fact that you do not spread paint as evenly as a laboratory roller does. Subtract the openings and you convert your safety margin into a second trip to the store.
Subtract only when the openings are genuinely large: a wall of sliding doors, a bay window, a double height feature window, or a fireplace and chimney breast. For a standard door and a couple of windows, leave them in. If you want to check what a standard opening actually measures, our guide to standard door sizes has the figures.
How many coats do you actually need?
Plan for two. Nearly every job needs two, and the few that do not are easy to identify.
Two coats, no argument: any colour change, bare or newly patched drywall, going from dark to light, any deep or saturated colour, and anywhere the finish needs to look even under raking daylight.
One coat might genuinely do: repainting the exact same colour over a sound, clean, previously painted surface, in a low light area, using a quality paint.
Deep reds, strong blues and vivid yellows are the awkward ones. Those pigments have poor hiding power and can need three coats over a light base. Priming in a tinted grey first usually saves a coat, which saves both money and a full day of drying time.
Paints advertised as one coat can work, but they do so by being thicker, which means each gallon covers less area. Read the coverage figure on the can rather than assuming you need half as much.
The adjustments that change the number
- Texture: add 20 percent to your square footage. Textured walls have more actual surface than their flat dimensions suggest, and knockdown or popcorn finishes drink paint.
- New drywall: prime it properly first. Unprimed drywall and joint compound absorb paint unevenly, and no amount of extra topcoat fixes the patchy result.
- Dark walls going light: prime, then expect two topcoats on top. Skipping the primer here usually costs you an extra coat of the expensive paint.
- Patched areas: spot prime them. Fresh filler soaks up paint at a different rate to the wall around it and shows through as dull patches.
- Sloped or vaulted ceilings: the sloped surface is longer than its floor footprint, so measure along the slope rather than across the floor. Our guide to attic bedroom ideas covers working with those angles.
- Stairwells: the tall wall is taller than you think. Measure from the bottom tread to the ceiling, not from the landing, and check the figures in our guide to stair dimensions if you are planning the space.
Ceilings and trim are separate jobs
Do not roll the whole room into one number. Walls, ceiling and trim use different products in different finishes, and each needs its own calculation.
Ceiling area is simply length times width. If the ceiling is flat and you are painting it white over white, one coat is often enough, so check before buying for two.
Trim is estimated from linear feet. Measure the run of baseboard, door casing, window casing and crown, then multiply by the width of the moulding to get square footage. Trim paint covers similar ground to wall paint, but you apply it in thinner passes, so a quart goes a long way. For a single average room, one quart of trim paint is usually the right purchase.
Why running short is worse than overbuying
Tinted paint is mixed in batches. Two cans of the same colour code, mixed on different days from different base stock, can differ very slightly in shade. On trim or in separate rooms nobody notices. On one continuous wall, a batch change halfway along can show as a faint band in certain light, and the only fix is recoating the entire wall.
Two habits prevent it. First, buy all the paint for a room in one go, so the cans come from the same mixing session. Second, box the paint: pour every can for that room into one large container, stir it thoroughly, and work from the mixed supply. Any batch variation is then spread invisibly across the whole room instead of landing on one wall.
Write the room name and the colour code on the lid of whatever is left over, and keep it sealed for touch ups. Note the finish too, because matching the sheen matters as much as matching the colour.
Frequently Asked Questions
How much does one gallon of paint cover?
Roughly 350 to 400 square feet per coat for interior wall and ceiling paint. Primer covers less, around 225 to 275 square feet. Always check the figure printed on the can, because it varies by product and finish.
How much paint do I need for a 12 by 14 room?
With 9 foot ceilings, the walls come to 468 square feet. Two coats need about 2.34 gallons, so buy 3 gallons. Add 1 gallon if you are painting the ceiling and 1 quart for the trim.
Do I subtract windows and doors from the wall area?
Not for ordinary openings. A door is about 20 square feet and a window about 15, and that allowance usually gets absorbed by cutting in, touch ups and uneven application. Subtract only for genuinely large features such as sliding door walls, bay windows or a chimney breast.
Can I get away with one coat?
Only when repainting the same colour over a clean, sound, previously painted surface. Any colour change, any bare drywall, any deep pigment and any strong natural light all call for two. Deep reds and vivid yellows sometimes need three.
Why should I mix my cans together before painting?
Because tinted paint varies slightly between mixing batches. Combining all the cans for a room into one container and stirring, known as boxing, spreads any variation evenly rather than leaving a visible line where you opened the second can mid wall.
Three minutes with a tape measure
Measure the perimeter. Multiply by the ceiling height. Multiply by two coats. Divide by 400. Round up.
That single sequence covers the overwhelming majority of interior painting jobs. Everything else in this guide is an adjustment to one of those five steps, and the only one most rooms need is the 20 percent you add for texture.
Do the sum before you leave the house, not while standing in front of the colour chips. The aisle is the worst possible place to think clearly about arithmetic.