I hired a painting company to paint my house and garage. Price was about $5,000.Sherwin Williams paint T-1000. When he was questioned about the paint peeling two days after he painted the garage he quit even with a contract not a very professional person. So,now I have to find someone else to paint my house.After 3 weeks of them trying to paint around the rain. The original time frame was 3-4 days.Yes 3 weeks and all that got done was a small two car garage that could have been done in two days.I know he did not prep correctly and properly I will end up having to redo the garage. I went to Sherwin Williams and got the colors.So beware even with a contract and good references.
After you have done the multiplication, subtract about 40 sq feet per window, and about 60 sq feet per door. Divide by 400 (interior paint covers 400 sq feet per gallon). The number that remains, is how many gallons you will need. If doing multiple wall colors, then you should do this process for each room (or sets of rooms) with a particular color. 
Painting interior walls does not require professional aptitude, but it is still a skill that arrives with a learning curve. The most challenging part of any interior painting project is the prep work, which often involves cleaning the walls, masking off windows, doorways and baseboards, then carefully “cutting in” the paint around the edges. Once the masking and cutting in are finished, volunteers with handheld or roller brushes can usually fill in the rest of the space with little trouble.

Most of a projects price is the cost of labor at $25 to $75 per hour. However, most contractors bid out based on the entire project which includes all work hours. Hourly rates are only useful if your pro asks to do the job as “time and materials.” However, be wary of this pricing structure, it usually shows the contractor is uncertain how long the project will take.