wikiHow Paint Calculator To calculate the amount of paint you need to paint a room, you'll need to determine the area of the walls and ceiling that you plan to paint. Once you have that, you can calculate the amount of paint you'll need based on the coverage rate of the paint you've chosen. Here are the steps to follow: 1. Measure the height and length of each wall in feet, and multiply the height by the length to get the square footage of each wall. Add up the square footage of all the walls to get the total square footage of the walls. 2. Measure the length and width of the ceiling in feet, and multiply the length by the width to get the square footage of the ceiling. 3. Add the total square footage of the walls and ceiling together to get the total surface area that you'll be painting. 4. Determine the coverage rate of the paint you've chosen. This is typically listed on the paint can or on the manufacturer's website. It's usually listed as the number of square feet that one gallon of paint will cover. 5. Divide the total surface area by the coverage rate of the paint to get the amount of paint you'll need in gallons. Round up to the nearest whole number. Here's an example to illustrate how this formula works: Let's say you're painting a room that has four walls, each measuring 8 feet high by 12 feet long, and a ceiling that measures 12 feet by 14 feet. The coverage rate of the paint you've chosen is 350 square feet per gallon. 1. The total square footage of the walls is (8 x 12) x 4 = 384 square feet. 2. The square footage of the ceiling is 12 x 14 = 168 square feet. 3. The total surface area is 384 + 168 = 552 square feet. 4. The coverage rate of the paint is 350 square feet per gallon. 5. You'll need 1.58 gallons of paint to cover the room, which rounds up to 2 gallons to account for additional coats and spills. So in this example, you would need 2 gallons of paint to cover the room. Page