Measure the surface area to be painted. Measure both the length and the width of the surface.
Multiply length times width of the surface area, this will give you square footage. Combine the square footage of all surface areas--ceilings, walls, floors and trim.
Choose a paint and color. Remember that your paint surfaces will also need to be primed. Paint has a chart on the side of the can that will tell you how much square footage the can will cover.
Divide the total square footage of surface area to be painted by the coverage capability of one can. This will give you the total number of paint cans necessary to paint the surface area. Keep in mind that you will need the same amount of primer as paint.
Multiply the cost of one can by the total number of cans and you will have the cost of paint and primer. Also account for the cost of painting tools: roller refills, roller and handle, brushes, tape, and roller pans. Many of these items are reusable, so they are a one-time cost for the first room you paint.