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See all Interior paint how-to »

Determine How Much Interior Paint to Buy

Skill:  Novice DIY'er

Figuring out how much paint to buy doesn't have to be tricky. It's important to purchase enough paint to complete your project, but not have a large amount of paint leftover.

For Best Results:

Specialty BEHR® products, such as Texture Paint or Faux Finishes, have unique application techniques and will require different amounts of product. Consult the back of the label for specifics.

For very small or very large quantities of paint, BEHR paint can be purchased in quarts or five gallon pails, in addition to the standard one gallon size.

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  1. Step 1

    One gallon of Interior BEHR PREMIUM PLUS ULTRA® Paint and Primer in One, or BEHR PREMIUM PLUS® is enough to cover 23 to 37 m2 (250 to 400 sq ft) of surface area with one coat.

    To determine how much paint you'll need for your project, follow the steps outlined below or use the handy paint calculator on

    Less paint may be needed if you are painting a smooth, primed or sealed surface, or your color change is subtle. Also, the more "unpaintable" features the wall has, such as windows and doors, the less paint you will need.
  2. Step 2

    To estimate the size of the wall area, first measure the perimeter of the entire room. Next, measure the height of your wall. Multiply the perimeter measurement by the height of the room.


    Subtract the doors and windows from your measurement. For reference, an average window is 1.4 m2 (15 sq ft) and the average door is 1.9 m2 (20 sq ft).

    If you are painting a porous, rough, textured, previously unpainted surface or you are making a dramatic color change, you may need more paint for those additional coats.
  3. Step 3

    For trim, measure the total length of trim for all expanses: baseboard, door, window and molding. The average trim width is six inches, so multiply your trim length by 0.15 m (0.5 feet) to get a square footage estimate for trim.


    If you are painting the ceiling along with your walls, follow these simple steps and add the ceiling size to your total room square footage. First measure the room's width then the room's length. Multiply the width and the length measurements to determine the total ceiling area.