The idea of a per square footage quote is a simple one. The typical painter will have experience with many contracted jobs, or will have been employed by another painter who has contracted jobs. Based on this experience, they will know, roughly how much the materials will cost as well as what their labor costs will be for themselves or their employees.
Painting is a skill and an experienced painter is likely to do a better job than an inexperienced painter would do. Contracting an experienced and professional painter will often cost more per square foot, because the painter is able to charge a higher price for the increased quality of the workmanship.
There are many other factors that go in to determining what a painter will charge you on a per square foot basis. Painting an interior wall, for instance, will typically require less prep work, which means reduced labor costs for your painter -- and for you. Painting an old exterior wall, on the other hand, may require old paint to be removed, and the surface of the wood to be resurfaced and primed. These extras will significantly increase the cost of a paint job.
Another factor that will determine what your painting costs per square foot will be are the condition that interior walls are in before painting begins. If your painter has to do a significant amount of prep work on your walls, by removing wall paper, for example, then your per square foot costs are going to go up, whereas painting on new drywall will involve very little prep work for your painter. By doing this kind of prep work yourself you can help lower your painting costs.
To get a quote over the phone, your prospective painter may ask you what the square footage is of the room or house to be painted. To calculate the square footage of a single room, multiply the length and width of the room together. To find the square footage of the entire house, multiply every room by its width and length and add them all together. Do this for every floor of the house. This is assuming a standard eight foot ceiling.