HowMuch

Paint calculator

Enter the room and this gives you gallons for the walls or the ceiling, with the doors and windows taken out. The first coat and the later coats are costed separately, because a porous wall drinks the first one.

Paint to buy 2 gal Rounded up to whole gallon cans.
Paint actually needed 1.95 gal
Surface to cover 348 sq ft
First coat 0.93 gal
Remaining coats 0.93 gal
Taken off for doors and windows 36 sq ft

How the number is worked out

Wall area is the perimeter times the ceiling height: a 12 × 12 room with an 8 ft ceiling is 2 × (12 + 12) × 8 = 384 sq ft. Take out one door (21 sq ft) and one window (15 sq ft) and you are painting 348 sq ft, not 384.

Then each coat is divided by the coverage printed on the can. Sherwin-Williams publishes 350–400 sq ft per gallon on the data sheets for SuperPaint Interior Satin, ProMar 200 and CHB Interior Latex Flat, measured at 4 mils wet. Behr publishes 250–400 sq ft per gallon for Premium Plus Interior, and says outright that the figure depends on "surface texture, porosity and application method". Both statements are on the same page as each other: 375 is a fair working number on a wall that is already painted and smooth.

The two mistakes that cost you a trip to the store

Nobody deducts the openings. A 12 × 12 room loses about 10% of its wall area to one door and one window. On a 12 × 20 living room with a patio door and three windows it is closer to 20%. Every calculator that asks only for length, width and height is quietly selling you an extra gallon.

The first coat is not the same as the second. Bare drywall, joint-compound patches and any texture are porous — the first coat sinks in and you get nowhere near the number on the can. That is the whole reason Behr's published range runs down to 250 sq ft/gal. Doubling the area and dividing by 375 under-buys the first coat and is exactly why people run out with one wall to go. This calculator prices the coats separately.

The clean fix is to prime bare board first. A gallon of primer is cheaper than a gallon of colour, it kills the porosity, and the two colour coats then behave like the can says.

Buying it

Paint comes in gallons and quarts, and a quart is roughly a quarter of the price of a gallon. If the answer lands at 1.2 gallons, buy one gallon and one quart rather than two gallons — it is a real saving on a small room, and you still have touch-up paint left.

Above about three gallons the maths flips: five-gallon buckets are cheaper per gallon, and mixing everything from one bucket removes any risk of a batch mismatch showing up on the wall. If you do buy two separately-tinted gallons, box them — pour both into one bucket and stir — before you start.

Trim and ceiling are separate orders. Ceiling paint is a dead-flat product that is deliberately thick, so it covers less per gallon than wall paint; drop the coverage figure to around 350 when you switch this calculator to the ceiling.

Common cases

Questions people ask

How many square feet does a gallon of paint cover?
Between 350 and 400 sq ft for one coat on a smooth, primed wall — that is the figure Sherwin-Williams prints on its interior latex data sheets. Behr publishes a wider 250–400 range and says it depends on how porous and how textured the surface is. Treat 350–400 as the best case, not the average.
How much paint do I need for a 12x12 room?
About 2 gallons for two coats on walls that are already painted. The walls are 384 sq ft, less 21 for a door and 15 for a window, so 348 sq ft — two coats at 375 sq ft/gal is 1.9 gallons. If the drywall is bare or heavily patched, the first coat drinks more and you will need a third gallon or a coat of primer first.
Do I have to subtract doors and windows?
Yes, if you want the number to be right. A standard interior door is 3 × 7 ft = 21 sq ft and a typical window is about 15 sq ft. In a small bedroom that is 10% of the wall area, and 10% of a two-coat job is most of a gallon.
Is one coat ever enough?
Only when you are repainting the same colour on a sound, previously painted wall. Anything else — a colour change, bare drywall, patch repairs — needs two, and going light over a dark or saturated colour usually needs a tinted primer plus two coats. Self-priming paint does not change this; it just means the first of your two coats is doing the priming.
Should I buy an extra gallon just in case?
Buy the quart instead. Paint is tinted to order, so a top-up mixed weeks later can be a shade off, but a leftover quart from the same batch is perfect for touch-ups. Keep it sealed and label the lid with the room name.

Related calculators

Last reviewed