How Many Gallons of Paint Needed

Interior Painting Estimate Calculator

Interior Paint Calculator
Wall Height ft.
ft. wide
      by ft. long
perimeter = ft.
      and area = sq. ft.
Area of Doors
and Windows
sq. ft.
Number of
Paint Coats
coats on the walls
coats on the ceiling
Paint Coverage sq. ft. per gallon

You need gallons of paint.

When painting indoors, accurate estimates will help you buy the right amount of paint so that you don't overspend or fall short. The number of gallons of paint you need depends on the dimensions of the walls, how many coats you apply, and how much coverage one gallon of paint can provide. You can estimate paint gallons with the calculator on the left, or by applying the formulas described below.

The first step is the estimate the square footage of the doors and windows in the room. These are the surfaces that you do not want to paint. The area of a door or window is length times width. For example, if your room has two windows that are 4 by 3 feet and a door that is 2.5 by 7 feet, then the total unpaintable area is

(3)(4) + (3)(4) + (2.5)(7) = 41.5 sq. ft.

Next, calculate the area of the walls with the formula


where P is the perimeter of the room and H is the height of the walls. If your room is a rectangle, the perimeter is 2(L+W), where L is the length and W is the width. For example your room is 12 feet by 14 feet and the walls are 8 feet height, then the wall area is 2(12+14)8 = 416 sq. ft.

Now subtract the window and door area from the wall area to obtain the total paintable area. In our example, 416 - 41.5 yields 374.5 sq. ft. of paintable wall space. Call the paintable area A

The next step is to convert this number to gallons. If the number of coats is C, and the coverage is V sq. ft. per gallon, then the total number of gallons is


Continuing our example, suppose we apply 2 coats and our paint can cover 250 sq. ft. per gallon. Then we need

2(374.5)(250) = 2.996 gallons.

You should round up and add an extra half gallon for good measure so that you have enough paint to finish your project. Also note that coverage varies by brand, thickness, and overall quality of the paint. High-quality interior paints usually cover 300 to 350 square feet.

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