Question: This may seem like a very unusual question, but whenever I am faced with a painting project at home, I never know how much paint to buy. Is there some formula that I can refer to that will help me to be more prepared when I visit the paint store?

Your question is a good one, really, and one that all of us have asked ourselves from time to time when the call of the paintbrush is just too strong to resist. In fact, I’ve often wondered myself how professional painters know the right amount of material to order for a job. For them the answer probably lies more in experience than anything else, but for the layman there is a formula that you can follow.

Your first calculation should deal with the perimeter of the space you intend to paint. The perimeter is just a fancy word for the actual distance around the room. Multiply this figure by the height which will give you the total area in square feet.

If you plan to use a different color for the trim, then subtract from your total the area of doors (which is approximately 21 sq. ft. each), and windows (which are approximately 15 sq. ft. each). The ceiling area is found by multiplying the length by the width of the room.

Next you need to divide your wall area by 400 sq. ft. which happens to be the approximate area one gallon of paint will cover. The result is the number of gallons of paint that will be required for one coat. (And I’ve yet to see any satisfactory results that are achieved with less than 2 coats and dark colors will often require 3 – 4.) This same procedure can be used to figure how much trim and ceiling paint will be needed as well.

To illustrate the use of this formula, lets take an actual room size and work our way through the process. Our sample room measures 12 x 15 ft. and has an 8 ft. high ceiling. The room has 2 doors and 2 windows. The walls are to be painted in satin flat, the rim in semi-gloss and the ceiling in satin flat.

Wall area including doors and windows is 54 (perimeter) x 8 (height) = 432 sq. ft.

Trim area is 42 (2 doors) + 30 (2 windows) = 72 sq. ft.

Wall area excluding doors and windows is 432 – 72 = 360 sq. ft.

Ceiling area is 15 (length) x 12 (width) = 180 sq. ft.

Amount of paint needed for walls is approximately 1 gallon; for trim approximately 1/5 gallon (you can purchase 1 quart) and for ceiling approximately ½ gallon which translates to buying 2 quarts.

It really is as simple as that and with this formula tucked safely into the back pocket of your painter’s overalls, you’ll always be able to order paint with the confidence of a pro.