To figure out how much it will cost to paint a room, you have to start by choosing the paint that you want. Different types of paint have different prices. You then need to move on to the size of the room. Measure the length and the height of each wall that will be painted. Then add these numbers together to get the total amount of square feet. Each can of paint will cover a certain amount of space, and you can use this number to figure out how many cans you will need for the walls and the trim.