Find the length and height of each wall using a tape measure. If you are painting the exterior of a building, avoid climbing up and down a ladder by estimating the height as 10 feet for each story. If a wall has an overhang that must be painted, add an extra 2 feet to the height.
Multiply the length by the height of each wall to find square feet. Then, add the square feet of all the walls to find the total area to be painted. For example, if you are painting a 15 X 12 foot room with an 8 foot ceiling, you have two walls that are 120 square feet each (158) and 2 walls that are 96 square feet each (128) for a total of 432 square feet.
Determine the square feet of a ceiling by multiplying the length and width of the room (from the wall lengths measured in step one). For a 15 by 12 foot room, the ceiling area would be 15*12 = 180 square feet. Since this example is an interior room, add the wall area of 432 square feet for a total area of 612 square feet.
Measure the height and width of any area that requires trim paint. This includes window trim, baseboards, and doors. Add up the figures to find the square feet. Don't try to measure and deduct for the actual window space. Professional painters generally don't because the difference isn't enough to change the amount of paint you'll need to buy.
Evaluate the color, texture, and condition of the surface to be painted. If you are going to be painting over a dark surface with a light color, you will probably need two coats. The same is true if the surface is rough textured or hasn't been painted in a long time and is worn. New surfaces are most likely going to need two coats as well. You may want to consult with an expert where you buy the paint in situations where you aren't sure.
Calculate the number of cans of paint to purchase. A good quality of paint will cover about 400 feet with one gallon (200 square feet for double coats). Divide the number of square feet by 400 (or 200 for 2-coat projects). For example, If you have 1800 square feet to cover with a single coat, you have 1800/400 = 4.5 gallons. Paint is sold by the gallon, so round up to five gallons. Remember to calculate the amount for trim paint separately.
Figure the cost of paint. The price of a gallon of paint varies, but the calculation is easy. Just multiply the number of gallons by the price per gallon. For example, if you need five gallons of paint and the price is 18.95/gallon the cost for paint is 89.75 (plus sales tax if applicable).