Per Square Foot, Per Room, or Hourly?
Most painters quote by the job. The method you use to get to that number is what separates a profitable bid from a guess.
Price off measured area, then quote one job number. $2 to $6 per square foot of floor area for interior walls, ceilings, and trim; $1 to $3 per square foot of wall area for walls only; $1.50 to $4 per square foot of surface for exterior.
Good for fast residential quotes. $300 to $800 per average room walls-only, plus ceilings and trim. Customers understand it instantly, but measure first so a big room doesn't blow your number.
A painter hourly rate of $40 to $90 per hour depending on solo vs crew. Use it for wallpaper removal, repairs, and jobs where the prep is unpredictable. Most painters keep an hourly rate in their back pocket and quote by the job up front.