Calculate your costs.Having an idea of the cost of paint you need to purchase will help obtain quotes from a contractor. “Paint coverage is based on how many square feet of surface one gallon will cover,” says Bunting. “Most paints will cover approximately 400 square feet. Tally this by multiplying in feet the wall’s height by its width, minus windows and doors.” Determine how much paint is needed to complete the project. “To calculate the cost of a project, factor in the day rates, the size of the property, any architectural features, and the paint finish needed,” says Rance.