When you want to paint the exterior of your home or a building, buying the correct amount of paint is important. If you purchase too much, you'll be stuck with a shade of leftover paint that you probably won't be able to use elsewhere. Get too little and it's very unlikely you'll be able to match the color exactly. It can be tricky to calculate exterior paint quantities because the amount required depends on the type and condition of the surface to be painted as well as the area needing coverage.
Things You'll Need
 Calculator
 Measuring tape
 Camera

Measure the length of the walls to be painted. Include in the total length any alcoves or projecting sections. For the height of the walls, you can measure if you wish, but it's adequate to simply assume 10 feet per story. Add two feet for overhangs that will be painted. If the height of a wall changes from one end to the other, use the average height.

Multiply the length of the walls by the height to find the square footage of surface to be covered. For example, if the total wall length of a singlestory home is 180 feet, and you have an overhang along 120 feet, you have (180 feet x 10 feet) + (120 feet x 2 feet) for a total of 2040 square feet.

Measure the height and width of each window and door frame. Multiply each height and width to find the area. Most professional painters simply add up the results to arrive at an estimate of the total area to be covered. Usually a single gallon of trim paint is sufficient. If you have more than 400 square feet (200 square feet if applying two coats), you'll need a second gallon of trim paint. On rare occasions, the door/window area will be greater. Buy one gallon for every 400 square feet of trim area (200 square feet for two coats).

Decide if one or two coats of paint are required. The surface you are covering affects this. If there is a strong color contrast, you'll probably need two coats. That's also the case if the surface is porous (likely with a new wall or one that has not been painted in a very long time) or is an unusually rough texture.

Calculate the amount of paint you need to buy. You can expect to cover about 400 square feet with one gallon of quality paint. For example, if you need to cover an area of 2040 square feet (from Step 2) divide 2040 by 400, which equals 5.1 gallons. Round up to allow a reserve and make it 6 gallons. For two coats, divide 2040 by 200 (10.2 gallons) and round up to 11 gallons.

Consult with someone experienced at making exterior paint estimates at the store where you purchase the paint. Because of the multiple factors that affect how you calculate exterior paint needed, it's a good idea to get an experienced opinion. It's helpful to take a few photographs of the surface you'll be painting to show its current color and condition.