How much paint do I need for a room?

Start with the wall area

Paint is measured by area, so the first job is finding how many square feet of wall you have. You do not measure each wall separately unless the room is an odd shape. Walk the perimeter and use the ceiling height.

Measure the length and width of the floor in feet, add them, double that for the full perimeter, then multiply by ceiling height.

• Wall area = perimeter x ceiling height = 2 x (length + width) x height
• Length and width are the floor dimensions in feet
• Height is floor to ceiling, usually 8, 9, or 10 feet

Subtract doors and windows

You do not paint glass or a door slab, so that area comes off the total. Instead of measuring every opening exactly, most estimates use round figures that are close enough for buying paint: about 20 square feet per standard door and 15 square feet per average window.

Add up your openings, subtract them from the wall area, and what is left is your paintable area. For oversized windows or double doors, measure those individually and use the real number instead of the round one.

Account for coats and coverage

A gallon of interior paint typically covers 350 to 400 square feet on a smooth, previously painted wall. Textured walls, deep colors, and fresh drywall pull that toward the low end, so planning around 350 keeps you from running short.

Most rooms need two coats for an even finish, especially when changing color or covering patched spots. Multiply your paintable area by the number of coats before dividing by coverage.

• Paint (gallons) = (paintable area x number of coats) / coverage per gallon
• Use 350 square feet per gallon as a safe planning figure
• Plan on two coats unless you are refreshing the identical color

A worked example: 12 x 12 room, 8 ft ceilings

Here is the full calculation for a common bedroom size: a room 12 feet by 12 feet with 8 foot ceilings, one door, and one window.

Perimeter is 2 x (12 + 12) = 48 feet. Wall area is 48 x 8 = 384 square feet. Subtract one door at 20 and one window at 15, which removes 35 square feet, leaving 349 square feet of paintable wall. For two coats, double that to 698 square feet. At 350 square feet per gallon, that is about 2 gallons.

That two-gallon figure is an estimate, not a guarantee. Real usage shifts with roller technique, wall texture, and how thickly you apply each coat, so treat it as a shopping target.

Should you include the ceiling?

The wall formula above leaves out the ceiling. If you are painting it too, calculate it separately by multiplying length times width. For the 12 by 12 example, the ceiling is 144 square feet, well under one gallon.

Keep the ceiling on its own line because it usually gets a different product. Flat ceiling paint hides imperfections and resists spatter, and ceilings often stay white while walls take color. Bundling the two makes it easy to buy the wrong amount of the wrong finish.

Common mistakes

Most paint-buying errors fall into a few predictable traps. Knowing them ahead of time saves a return trip and keeps you from overspending.

• Forgetting the second coat: buying for one coat, then finding the color looks patchy, is the most common way to run out halfway through.
• Ignoring primer on new drywall: bare drywall soaks up the first coat and needs primer or a self-priming paint, and primer is a separate purchase, not part of your color-coat total.
• Underbuying to save money: coming up short mid-wall means a second can that may be tinted slightly differently, leaving a visible seam.
• Overbuying by ignoring the openings: skipping the door and window subtraction inflates the total and leaves you with cans you do not need.
• Assuming full coverage on deep colors: strong reds, blues, and dark shades often need an extra coat, so plan for it rather than discovering it on the wall.

A metric worked example and converting coverage

Outside the United States, paint is sold in liters and rooms are measured in meters, but the method is identical. Take a room 4.2 m by 3.6 m with 2.4 m walls, one door, and two windows, planning two coats. The perimeter is 2 x (4.2 + 3.6) = 15.6 m, so the gross wall area is 15.6 x 2.4 = 37.44 square meters. The tool's openings deduction is about 1.86 square meters per door and 1.39 per window, so you subtract 1.86 + 2 x 1.39 = 4.64, leaving 32.8 square meters. Two coats make 65.6 square meters. At 10 square meters per liter, that is 6.56 liters, so you would buy 7.

Coverage figures do not convert one-to-one with the US numbers in this guide. A US gallon is 3.785 liters, so 350 square feet per gallon equals about 8.6 square meters per liter and 400 square feet per gallon equals about 9.8. A 10 square meters per liter assumption is therefore on the generous side, close to 407 square feet per gallon. If you plan with 8.6 instead, the same room needs 65.6 / 8.6 = 7.63 liters, which rounds up to 8. Always take the coverage printed on your specific can over any default, and when in doubt, use the lower figure.

A quick reference for common room sizes

A few more worked rooms give you a sense of scale. These all assume two coats and 350 square feet per gallon, with the tool's standard deductions of 20 square feet per door and 15 per window, and the tool rounds up to the nearest quarter gallon.

These are planning values. Treat any room more than a few feet different from the examples as needing its own calculation, and remember that heights matter as much as floor size, since a 10 foot ceiling adds a quarter to the wall area of an 8 foot one.

• 10 by 10 ft, 8 ft ceilings, 1 door, 1 window: 285 sq ft of wall, 570 sq ft for two coats, 1.63 gallons, buy 1.75
• 12 by 15 ft, 8 ft ceilings, 1 door, 2 windows: 382 sq ft of wall, 764 sq ft for two coats, 2.18 gallons, buy 2.25
• 20 by 15 ft, 10 ft ceilings, 3 doors, 5 windows: 565 sq ft of wall, 1,130 sq ft for two coats, 3.23 gallons, buy 3.25

Odd shapes, gable ends, and oversized openings

The perimeter-times-height formula assumes four straight walls under a flat ceiling. Anything else needs extra pieces. A gable end, the triangular wall under a pitched roof, has an area of half the base times the height. A wall 14 feet wide with a peak 5 feet above the flat top has a triangle of 0.5 x 14 x 5 = 35 square feet. Add that to the rectangle below it. Two matching gable ends add 70 square feet.

Stairwells, vaulted ceilings, and alcoves are the same idea. Break the surface into rectangles and triangles, calculate each area, and add them. An accent wall painted a different color is simply its own line item: its width times its height, then its own coats and gallon count, so you do not end up with half a gallon of the wrong color.

The standard 15 square foot allowance for a window is an average. A large picture window of 6 by 4 feet is 24 square feet, and a sliding glass door can run to 20 square feet or more. For rooms with several big openings, measure them and subtract the real areas, otherwise you will overbuy. Conversely, if a room has no windows to speak of, do not subtract phantom area that is not there.

Trim, doors, and cabinets are a separate calculation

The wall formula leaves out baseboards, window casings, doors, and crown molding, which usually take a different paint, often a semi-gloss or satin enamel. Keep them on their own line. For baseboards, take the room perimeter, subtract door openings, and multiply by the height. In the 12 by 12 room, 48 feet minus a 3 foot door is 45 feet, and a 6 inch baseboard is 0.5 feet tall, so 45 x 0.5 = 22.5 square feet per coat, or 45 square feet for two.

A standard interior door is about 20 square feet on one face, so painting both faces is roughly 40 square feet per coat, plus edges. Paneled doors and louvered closet doors have more surface than their dimensions suggest. Casings add a few more square feet around each opening.

Even a trim-heavy room rarely needs more than a quart. A quart is a quarter of a gallon and covers about 87.5 square feet at 350 square feet per gallon, which handles the baseboard in the example with room to spare. Trim also tends to take more coats than walls, since it is often changing color or being repainted over old gloss, so check the sheen and surface prep before settling on a number.

Surfaces, color changes, and how you apply it

Three variables can push your real consumption away from the formula. The first is the surface. Fresh drywall, unprimed patches, and textured walls such as orange peel or knockdown drink more paint than a smooth, sealed wall, so plan around 350 square feet per gallon or a little less rather than counting on 400. Priming the bare surface first usually means your color coats then cover closer to their stated rate.

The second is the color change. Going from a dark wall to a light one, or from a bold color to white, can take three coats of the final color unless a tinted primer does part of the work. Running the 12 by 12 example at three coats gives 1,047 square feet, which is 2.99 gallons, or effectively 3 instead of 2. Saturated reds, yellows, and deep blues are especially known for needing extra coats.

The third is the method. Rolling is the baseline most coverage numbers assume. Spraying can use more paint because of overspray and the need to back-roll in some cases, though it covers large areas quickly. Using a cheap roller cover that sheds or soaks up paint wastes some, and so does a deep tray left unevenly loaded. None of this is precise, but if you are spraying or working with a difficult surface, lean toward the next larger container.

Container sizes, tinting, and leftovers

Paint comes in quarts, gallons, and often five-gallon pails. A pail holds about 1,750 square feet of single-coat coverage at 350 square feet per gallon, which makes it economical for a whole floor of rooms or a big exterior job, though only if you will use it all. For a single room, the practical choice is usually a gallon and a quart, not a pail.

Tinted paint is mixed to order and often cannot be returned, so be sure of your color and quantity before the shop adds the pigment. Batches mixed separately can vary slightly even from the same formula. If you need more than one can, you can pour them into a single larger bucket and stir, often called boxing, so any small differences blend evenly instead of showing as a seam between cans.

Keep the color name, the manufacturer, the sheen, and the formula number from the lid label. Once you finish, a small amount saved in a clean, sealed container is useful for touch-ups. Store it where it will not freeze or get hot, and note the room and date on the lid. Properly stored latex paint can stay usable for years, though it is worth stirring and testing a small spot before relying on it.