Barns come in many different shapes and sizes. And the cost of painting one depends on such factors as the square footage and condition of the walls, and the type of paint you want to use. If your barn is large, measuring the walls will likely take the help of a friend. If you have a small utility barn, you may be able to complete the measurements on your own.

Measure the height and width of each wall. For example, let’s say that the barn is an A-frame, 25-feet wide and 50 feet at the apex. The other two walls are 30-feet high and 25-feet wide.

Multiply 50 by 25 to get the square footage of each A-shaped wall. This comes to 1,250 square feet. Multiply by two for both walls (2,500 square feet). Multiply 30 by 25 for the other walls (750 square feet). Multiply by two (1,500 square feet). Add the totals: the barn is 4,000 square feet.

Inspect the wood. If it is dried out and old, it will need a coat of primer and likely two coats of paint. If the wood is new, it will need a coat of primer and likely one coat of paint. For our example, let's assume you need one coat of primer and two coats of paint.

Determine how much the exterior primer will cost. Paint manufacturers supply data sheets that tell you the approximate square footage each gallon of paint can cover. This information is also on the can. One can of primer will cover about 350 square feet. Divide the square footage of your barn by the square footage one gallon of your primer can cover. For our example, divide 4,000 by 350, which is 11.4. So the barn will need approximately 12 gallons. Multiply the cost of the primer per gallon by the total amount needed. Let’s assume $15 per can. For our example, you will invest $180 in primer.

Use the same method for determining the cost of paint. In some cases, paint will cover less area than primer. Again, you can refer to the data sheet or the can. Let's assume 250 square feet per can at $20 per gallon. Because we need two coats, you need 32 gallons of paint at a cost of $640.

Add the cost of renting a scaffold -- a metal walkway that makes it easier to paint higher walls. You can rent scaffolding from many paint supply retailers. You can save money by using ladders, if you're willing to climb up and down them with your supplies.

Add the cost of supplies such as paint rollers and brushes. You can save money and time by buying or renting an airless spray painter.

Add the cost of labor if you are paying anyone to help you paint the barn.