How Much Paint Do I Need?

If you are asking how much paint do i need, the safest place to start is not the colour chart but a tape measure. Paint is bought by volume, but it is used by area, so every sensible estimate turns the walls you can see into a number of square feet, allows for the coats you plan, and then divides by what a gallon actually covers. Guess that chain and you either run short on the last wall or store half a gallon for years. Work through it once, slowly, and the answer is usually accurate to within a single tin. If you would rather skip the arithmetic, our paint calculator does the same sums instantly and lets you change the ceiling height, the openings and the coverage until the result matches your room.

The direct answer

One US gallon of standard wall paint covers about 350 square feet per coat on smooth, previously painted walls. To find your amount, measure the wall area by multiplying the room perimeter by the ceiling height, subtract about 21 square feet for each standard door and 15 square feet for each standard window, multiply what is left by the number of coats, and divide by 350. That is the whole method. Everything else in this guide is detail that helps you apply it honestly: what to do about ceilings, when primer counts as its own purchase, and why two coats is the normal planning figure rather than a hopeful one.

Keep two cautions in mind from the start. First, 350 square feet per gallon is a planning figure for a typical eggshell or satin finish, not a promise printed on every tin. Flat paint can go a little further, semi-gloss often covers a little less, and rough or thirsty walls can pull the figure down noticeably. Second, round up when you buy. Paint is sold in whole gallons and quarts, batches can vary very slightly, and a small amount left over for touch-ups is far more useful than a wall that is one roller-load short.

Quick amounts by room size

The table below assumes walls only, 8 foot ceilings, two coats, and a rough deduction already made for doors and windows. Use it for a fast first estimate, then confirm your own room with the step-by-step method that follows.

Room	Paint for walls, 2 coats
10 x 12 ft bedroom	About 2 gallons
12 x 12 ft bedroom	About 2 gallons
15 x 20 ft living room	About 3 gallons
Ceiling of a 12 x 12 ft room	Adds about half a gallon per coat

How to work it out, step by step

Step 1: Measure the walls

Measure the length and width of the room in feet, add them together, and double the result to get the perimeter. Multiply the perimeter by the ceiling height and you have the gross wall area. A room that is 12 feet by 12 feet with 8 foot ceilings has a perimeter of 48 feet and a gross wall area of 384 square feet. For a room that is not a simple rectangle, measure each wall separately and add the pieces together instead. Alcoves, chimney breasts and long hallways are all easier handled wall by wall than forced into one formula.

Step 2: Deduct doors and windows

Subtract about 21 square feet for each standard door and about 15 square feet for each standard window. These are averages, so very large picture windows, glazed double doors or a wall of wardrobes deserve their own quick measurement. Do not deduct small windows, narrow sidelights or areas you may paint around anyway, because keeping a little extra in the estimate protects you against waste. Once you have deducted the openings, the figure left is the area that will genuinely receive paint, and it is the only area you should pay for.

Step 3: Decide the number of coats

Plan on two coats as the standard. The first coat seals the surface and does most of the colour work, while the second evens out the sheen and gives the finished depth you saw on the sample card. One coat can work for a same-colour refresh on walls that are clean, sound and already painted in a similar shade, especially in a low-traffic room. It rarely works over repairs, filler, a noticeably different colour, or walls that have not been painted for years. If you are changing colour dramatically, budget for two coats and treat a possible third as a contingency rather than a surprise.

Step 4: Apply the primer rules

Primer is a separate product with a separate job, and it is not needed on every job. Use it on bare plaster or new drywall, where the surface would otherwise drink the topcoat unevenly. Use a stain-blocking primer over water marks, smoke damage or other marks that could bleed back through ordinary paint. Consider a tinted primer when you face a dramatic colour change, because it gives the topcoat a more helpful starting point. Sound, previously painted walls in a similar colour usually need no primer at all, just cleaning, light sanding where the finish is rough, and filling before you start. If you do prime, remember that primer also has to be bought, and it covers at roughly its own rate rather than the rate of your wall paint.

At this point you can run your final numbers through the paint calculator as a check. Enter the same measurements, coats and coverage you used on paper. If the two answers differ, look for a missed wall, a ceiling you counted twice, or a deduction you applied in one place and forgot in the other.

Worked example: a 12 x 12 room

Take a square bedroom that measures 12 feet by 12 feet, with 8 foot ceilings, one standard door and one standard window. The perimeter is 48 feet, so the gross wall area is 48 multiplied by 8, which is 384 square feet. Deduct 21 square feet for the door and 15 square feet for the window, a total of 36 square feet, and the net wall area is 348 square feet. For two coats you need to cover 348 multiplied by 2, which is 696 square feet of coating. Divide 696 by the planning coverage of 350 square feet per gallon and you get 1.99 gallons. In the shop, that means 2 gallons. The example is deliberately ordinary, and that is the point: most standard bedrooms follow exactly this pattern.

Ceilings, trim and sheen are separate

Ceiling paint and trim paint are separate purchases from wall paint, and mixing them into one total causes most buying mistakes. Ceilings are normally painted with a flat ceiling paint that hides imperfections and splashes less, worked out from length multiplied by width for a single coat at a time. Trim, skirting, doors and window frames normally take a tougher gloss, satin or semi-gloss product in a much smaller quantity. Work out each surface on its own, buy each product on its own, and keep any leftovers labelled by room.

Sheen affects coverage slightly as well. Flatter finishes tend to spread a little further and hide more, while glossier finishes can cover a little less and show every flaw beneath them, which is why preparation matters more as the sheen rises. If you are working in metric, or your paint is sold in litres, read our guide on how many litres of paint per square metre you should plan for. And before you trust the number on any tin, read our companion piece on paint coverage per litre, which explains why real walls rarely match the laboratory figure.

Finally, buy the whole job in one go when you can, and keep the receipt. A final run through the paint calculator with your real coverage figure, your real ceiling height and a sensible waste allowance will tell you whether one more quart is a waste of money or the cheapest insurance in the whole project. In most rooms, it is insurance worth having.

Frequently asked questions