Construction

Paint Calculator

Work out how many gallons of paint you need for walls, a ceiling, or both. Enter the room’s size, its doors and windows, and the coverage on your can; the paint calculator rounds up to whole cans and suggests a quart mix when that trims the leftovers.

Your room

Gallons needed

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Enter your room to see how much paint to buy.

Paintable area (1 coat)

—sq ft

Coverage, all coats

—sq ft

Estimated cost

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Surface breakdown

Walls —Ceiling —

Show the math

Paint = [2 × (L + W) × H − doors − windows + ceiling] × coats ÷ coverage, rounded up

Estimate only — actual paint needed varies with surface texture, paint color/quality, and application method. Estimated cost covers paint only, not primer, supplies, or labor.

How much paint do I need?

Paint is sold by volume (gallons and quarts in the US, litres elsewhere), but a room is measured in length, width and height. To get from one to the other, work out the wall area from the room’s perimeter and height, add the ceiling if it’s getting painted, take off the doors and windows, multiply by the number of coats, and divide by the coverage printed on the can. The answer is rounded up, because stores sell whole cans.

The two inputs that move the answer most are coats and coverage. Behr’s product pages list interior paint coverage of 250–400 sq ft per gallon per coat, depending on how it’s applied and how porous the surface is; this page defaults to 350, a common label figure. Rough or thirsty surfaces sit at the low end of that range. If a room is irregular (an L-shape, a bay window, a stairwell), break it into rectangles with the square footage calculator and enter the pieces one at a time.

Worked example: a 12×10 ft bedroom

Take a 12×10 ft bedroom with 8 ft walls, one door, two windows, and two coats of a paint rated at 350 sq ft per gallon (the default values above):

Walls: 2 × (12 + 10) × 8 = 352 sq ft.
Openings: 1 door × 20 sq ft + 2 windows × 15 sq ft = 50 sq ft, leaving 302 sq ft to paint.
Two coats: 302 × 2 = 604 sq ft of coverage.
Gallons: 604 ÷ 350 = 1.73, rounded up to 2 gallons.
Quarts: one quart covers about 350 ÷ 4 = 87.5 sq ft, so 604 ÷ 87.5 = 6.9, rounded up to 7 quarts, or 1 gallon + 3 quarts.

Tick “Include ceiling” and the 120 sq ft ceiling joins the total: 422 sq ft per coat, 844 sq ft for two coats, and 3 gallons (2.41 rounded up). In practice ceilings are often done in a dedicated flat ceiling paint, so you may prefer to run the walls and the ceiling as two separate estimates.

Formula & methodology

Wall area = 2 × (Length + Width) × Height, minus a typical door (20 sq ft) for each door and a typical window (15 sq ft) for each window. Ceiling area = Length × Width, added only if “Include ceiling” is checked; doors and windows are never subtracted from it. That paintable area is multiplied by your number of coats (2 is standard for most walls), then divided by the coverage per gallon on your paint can and rounded up. Anything past an exact number of gallons, even 0.1, becomes one more can, which is why the hero card also shows a quart option.

In metric mode the same steps run in metres: a door counts as 1.86 m², a window as 1.39 m², coverage is in m² per litre, and the result is rounded up to whole litres. Open “Show the math” under the results to see each step with your own numbers.

Quarts vs gallons: how much to buy

A US quart is a quarter of a gallon (0.946 L), so it covers roughly a quarter of the gallon figure: about 87.5 sq ft per coat for a 350 sq ft paint. For small jobs (an accent wall, a bathroom, trim, a door) or for topping up the last bit of a big one, quarts can save you from opening a second or third gallon you’ll barely use. The table below is per coat at 350 sq ft per gallon; double the area for two coats.

Quarts and gallons needed per coat at 350 sq ft per gallon
Area per coat	Quarts only	Gallons only	Tightest mix
50 sq ft (4.65 m²)	1	1	1 quart
100 sq ft (9.29 m²)	2	1	2 quarts (or 1 gallon)
200 sq ft (18.58 m²)	3	1	1 gallon
400 sq ft (37.16 m²)	5	2	1 gallon + 1 quart
800 sq ft (74.32 m²)	10	3	2 gallons + 2 quarts

Compare the shelf prices of quarts and gallons before you choose the mix. Rounding to gallons only is simplest; the quart mix just means less paint sitting in the garage afterwards.

Do you need primer?

Primer seals the surface so the finish coats go on evenly and stick. Common painting guidance is to prime when the surface can’t give paint a uniform base:

Bare drywall or new plaster: the paper face and the joint compound absorb paint at different rates, which shows up as blotchy patches (“flashing”).
Patched or repaired areas: spackle and filler soak up paint, so spot-prime them before the full coats.
Big color changes: going from dark to light, or covering a strong color, a tinted primer can save a finish coat.
Stained or glossy surfaces: water marks, smoke and marker need a stain-blocking primer; glossy paint needs a bonding primer (or a light sanding) so the new coat grips.

To estimate primer, run the calculator again with coats set to 1 and the coverage figure from the primer can. Paint-and-primer products can replace a separate primer over sound, previously painted walls, but they don’t replace a dedicated stain blocker.

Metric: litres and m²

Outside the US, paint is sold in litres and coverage is printed in m² per litre. One US gallon is 3.785 L, so a 350 sq ft per gallon paint works out to about 8.6 m²/L; switch to metric above and the calculator converts your dimensions, coverage and price for you. Many UK and European emulsions quote 10–13 m²/L on smooth walls, while 9 m²/L is a cautious all-round figure, so check the tin and enter its number. The result is in whole litres; buy the tin sizes that add up to it with the least left over.

Time & cost to paint a room

As a rough rule of thumb, one person paints about 100–150 sq ft per hour including prep and cutting in, so a typical bedroom takes 4–6 hours for two coats plus drying time in between. Enter your paint’s price per gallon above (optional) for a material-only cost. Hiring out the job is a separate number: professional interior painting runs roughly $2–$6 per sq ft of floor area (rough 2026 US range; prices vary by region, so get local quotes). If the room is also getting new floors, the flooring calculator uses the same length and width you measured here, so paint first and lay the floor after.

Tips for painting

Buy everything for one room at once and box the cans (pour them into one bucket and mix) so a slight tint difference between cans can’t show on the wall; keep the leftover from that same mix for touch-ups.
Dark or accent colors, and covering a much lighter existing color, often need a third coat; raise the coats field above to match.
For exterior walls, texture matters: stucco and brick absorb more paint than smooth siding, so enter a lower coverage figure than the label’s smooth-surface number.
Measure wall height from floor to ceiling, not to the top of the baseboard; trim is usually painted separately in a different sheen.
Painting a bathroom or kitchen before new tile goes in? Size the tile with the tile calculator and paint only the wall area the tile won’t cover.

Frequently asked questions

How much paint do I need for a 12x12 room?

With 8 ft walls, one door and two windows, a 12×12 room has about 334 sq ft of wall to paint. Two coats at 350 sq ft per gallon come to 668 sq ft, so 2 gallons covers the walls. Add the 144 sq ft ceiling and the two-coat total rises to 956 sq ft, which means buying 3 gallons.

How many gallons of paint does a typical room take?

Most bedrooms and home offices between 10×10 and 12×14 ft with 8 ft ceilings take 2 gallons for two coats on the walls. Living rooms, rooms with 9–10 ft ceilings, or jobs that include the ceiling usually push that to 3 gallons or more. Enter your own measurements above, since door and window count shifts the total.

How long does it take to paint a room?

As a rough rule of thumb, one person paints about 100–150 sq ft per hour once prep and cutting in are counted. A 12×10 ft bedroom has around 300 sq ft of wall, so two coats take roughly 4–6 hours of working time, plus however long the label says to wait between coats.

Is one gallon of paint enough for a bedroom?

Usually only for a single coat. A 10×10 ft bedroom with 8 ft walls, one door and two windows has about 270 sq ft of wall, which one gallon rated at 350 sq ft can cover once. Two coats need about 540 sq ft of coverage, so plan on 2 gallons, or 1 gallon plus 3 quarts if your store sells quarts.

How much does a paint job cost?

Material cost is gallons × price per gallon; enter a price above to see it. Primer, tape, rollers and drop cloths are extra. Hiring a professional for interior painting runs roughly $2–$6 per sq ft of floor area as a rough 2026 US range, covering labor and usually paint. Prices vary by region, so get local quotes.

How much paint do I need for a ceiling?

Ceiling area is simply length × width, so a 12×12 ft room has a 144 sq ft ceiling. Two coats need 288 sq ft of coverage, which one gallon at 350 sq ft per gallon handles. Check the coverage on the ceiling paint you buy, and tick “Include ceiling” above to add it to the wall total.