Understanding your result
The headline is the paint you need for the number of coats you chose, including the allowance. Under it is the line that matters at the store: what to buy, rounded up to cans, and the total those cans hold. The three figures beneath show the paintable area (walls minus openings), the paint for a single coat, and the can list again.
The paintable wall area is the number to remember. It does not change when you change your mind about the paint, and every other quantity on this site starts from it: the primer calculator takes the same area, and the paint cost calculator prices it. If you are painting the ceiling as well, the ceiling paint calculator uses length times width and its own coverage, because ceiling paint and texture behave differently.
The breakdown table lists the wall area before openings, the openings subtracted, the coverage used, the paint per coat, the allowance, the total, the cans and the leftover. A big leftover means the rounding was unkind: a need of 1.05 gallons buys a gallon and a quart, with most of the quart to spare. The can size optimizer compares the prices of the combinations if that matters.
How we calculate this
The wall area is the perimeter of the room times the wall height: a 12 × 12 room with 8 ft ceilings has a 48 ft perimeter and 384 sq ft of wall. The openings are subtracted at standard sizes, 20 sq ft for a door and 15 sq ft for a window, listed with the other defaults on the standard sizes page. If your windows are unusually large, count a picture window as two.
Coverage is the one input that changes the answer most, and it is the one people least often check. The figure on the can is measured on a smooth, sealed test panel; real walls are rougher and thirstier, which the coverage guide explains in detail. The coverage rates table lists what the main manufacturers print for each product class and what to use on textured or bare surfaces. In metric mode the calculator takes metres, square metres and square metres per litre; 350 sq ft per gallon is 8.6 m² per litre, and the conversions are exact.
Rounding to cans is the last step. Paint is sold in quarts, gallons and 5-gallon pails (and in 1, 2.5, 5 and 10 litre cans elsewhere), so 2.09 gallons becomes two gallons and a quart. The calculator always rounds up, and shows what is left over.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Coverage per coat | 350 sq ft per gallon (8.6 m²/L) | The lower end of the 350–400 sq ft per gallon that Sherwin-Williams, Benjamin Moore and Behr print on interior wall paint data sheets for smooth, previously painted surfaces |
| Door opening | 20 sq ft | A standard 36 × 80 in interior door, the most common US residential size |
| Window opening | 15 sq ft | A typical 3 × 5 ft double-hung window; large windows count as two |
| Wall height | 8 ft (2.4 m) | The standard US ceiling height since the mid-20th century; newer houses are often 9 ft |
| Coats | 2 | Manufacturers’ recommendation for a colour change or a new surface |
| Allowance | 5% | Roller and tray loss plus a touch-up jar; a round figure, not a measurement |
Assumptions last reviewed October 7, 2026.
The calculator does not model the extra paint a bare, patched or textured wall absorbs on its first coat, the extra coat a deep colour may need, or the primer a colour change from dark to light calls for. Those are decisions rather than arithmetic, and the guide to coats and the primer guide cover them. It also does not count ceilings, trim or doors, which take different products.
Two worked examples
A 12 × 12 bedroom, one door, one window
Standard room, 8 ft ceilings, a colour change so two coats, paint rated at 350 sq ft per gallon.
- Wall area: 2 × (12 + 12) × 8 = 384 sq ft
- Openings: 1 × 20 + 1 × 15 = 35 sq ft, leaving 349 sq ft
- Per coat: 349 ÷ 350 = 0.997 gallons
- Needed: 0.997 × 2 × 1.05 = 2.09 gallons
- Buy: 2 × 1-gallon cans + 1 quart (2.25 gallons), leaving 0.62 quarts over
This is the classic case where two gallons is just short. The quart costs a few dollars and saves a second trip; the alternative is to skip the allowance and accept that the second coat may finish thin in the last corner.
A 15 × 18 living room with 9 ft ceilings
Two doors, three windows, a better paint rated at 400 sq ft per gallon, two coats.
- Wall area: 2 × (15 + 18) × 9 = 594 sq ft
- Openings: 2 × 20 + 3 × 15 = 85 sq ft, leaving 509 sq ft
- Per coat: 509 ÷ 400 = 1.273 gallons
- Needed: 1.273 × 2 × 1.05 = 2.67 gallons
- Buy: 2 × 1-gallon cans + 3 quarts (2.75 gallons), leaving 0.31 quarts
Three quarts cost about the same as a gallon, so most people would buy three gallons here and keep the extra for touch-ups. The same room in metric is 4.6 × 5.5 × 2.7 m; typed that way with 9.8 m² per litre, the calculator gives the same answer in litres.
Where to find your inputs
Length and width. Measure along the floor at the baseboard, wall to wall, and round to the nearest half foot. For an L-shaped room, measure the full perimeter instead and enter it as a rectangle with the same perimeter (for example a 60 ft perimeter as 15 × 15); the wall square footage calculator takes up to six walls individually if the shape is awkward.
Wall height. Floor to ceiling, not floor to the top of the baseboard. Sloped ceilings: measure the average height, or use the square footage calculator wall by wall. The measuring guide covers alcoves, bay windows and stairwells.
Doors and windows. Count every opening you will not paint, including closet doors and the doorway you came in through. Built-in bookcases and tiled areas count as openings too: estimate their area and enter it as windows at 15 sq ft each.
Coverage. On the back of the can or the product’s technical data sheet, usually as a range. Take the lower number unless the wall is smooth and already painted in a similar colour.
Common mistakes
- Dividing floor area by 400. Walls, not floors, get painted. A 12 × 12 room has 144 sq ft of floor and 384 sq ft of wall.
- Measuring one wall and multiplying by four. Rooms are rarely square. Measure the perimeter or both dimensions.
- Forgetting the second coat. Nearly every job needs two coats; the second takes almost as much paint as the first.
- Trusting 400 sq ft per gallon on a textured wall. Orange peel and knockdown textures take 10 to 20% more; use 300 to 325.
- Buying exactly the calculated amount. Keep a 5% allowance and a sealed quart for touch-ups; the colour will not match perfectly if you buy it a year later.
- Including the ceiling in the wall figure. Ceiling paint is a different product with different coverage. Run the ceiling calculator separately.
- Mixing feet and metres. Switch the units toggle before typing; the defaults change with it.
Questions people ask
- How much paint do I need for a 12 × 12 room?
- With 8 ft walls, one door and one window, a 12 × 12 room has 384 sq ft of wall and 349 sq ft to paint. Two coats at 350 sq ft per gallon come to 2.09 gallons including a 5% allowance, so buy two gallons and a quart, or three gallons if you want a full can left for touch-ups.
- Do I subtract the doors and windows?
- Yes. The calculator takes off 20 sq ft for each door (a standard 36 × 80 in slab) and 15 sq ft for each window. For a room with one of each that is 35 sq ft, about a tenth of the walls, which is a tenth of a gallon per coat. Small openings under 10 sq ft are not worth subtracting.
- How many coats of wall paint do I need?
- Two for a colour change, new drywall or patched walls, and the calculator defaults to two. One coat is enough only when you are refreshing the same colour over sound paint. Deep reds and yellows over a light wall often take three, or two over a tinted primer.
- What coverage figure should I use?
- The one on the can. Most interior wall paints print 350 to 400 sq ft per gallon per coat, measured on a smooth, sealed surface. Use the lower figure for a textured wall or a dark colour, and 250 to 300 for bare drywall if you are not priming first.
- Why is the result in gallons and quarts rather than one number?
- Because paint is sold that way. A need of 2.09 gallons cannot be bought; the calculator rounds it up to two gallons and a quart, shows the leftover, and lists the cans. If the leftover is a whole quart you may prefer to round down and keep a touch-up jar from the main cans instead.
- Does this include the ceiling, trim and doors?
- No. Ceilings take flat ceiling paint and are estimated by length times width in the ceiling paint calculator; trim and doors take enamel and are counted in quarts in the trim and door calculators. Keeping them separate matches how the cans are bought.