A gallon of paint typically covers 400 to 600 square feet. However, everything varies depending on the type of paint and its brand. Read the product instructions for a better guide. Alternatively, let a professional handle it. To calculate how many square feet I need to cover per room, multiply the width by the height of each wall, and at the end add the results for each wall. Keep in mind that for each layer of paint that must be applied, it will add or double the final result. That is, if the area to be painted is 400 square feet, and you will need to apply at least two coats of paint. It is recommended to have paint to cover 800 square feet. That is, 2 gallons. But once again, it’s a general result. The choice of paint ultimately determines everything. Some have less coverage than others. But you should always choose paints with an integrated primer.