How to Calculate the Quantity of Deck Paint Needed

Measure the length and width of your deck. Multiply the length by the width and write the answer down in your notepad. If your deck has an irregular shape, break it into squares, rectangles, triangles and circles and calculate each shape accordingly. The formula for finding the area of a triangle is the base (width) multiplied by the height then divided in half. For circles, multiply the radius of the circle by itself, then multiply that by 3.14, also known as "pi." Add each shape together to get the full surface area. Set that calculation aside.

Measure the width and height of a baluster, multiply them together, then multiply that number by four to get the area for each of the four faces. If the baluster is rounded rather than square, use the widest point for your width. Multiply the area by the number of balusters, then add that to your total area.

Measure the width and height of a deck support post and multiply them together, then multiply that by four to include each face. Multiply the result by the number of deck support posts and add it to your total area.

Continue measuring and adding the surface area of other deck parts you need to paint, such as benches or roof posts.

Select the paint you want to use on your deck and check the label on the can to find the average coverage per gallon. For wooden deck surfaces, this is generally between 350 and 400 square feet per gallon. Rougher surfaces will require more paint than smooth surfaces. The label should show high and low estimates of coverage. Use the low estimate for rough surfaces and the high estimate for smooth surfaces.

Divide your total area by the average coverage per gallon for your chosen paint. This will determine how many gallons of paint you need. It is a good idea to add an extra quart to the total to allow room for mishaps.