Does anyone have any guidelines/formulas for estimating the cost of interior painting?

It will depend on how many gallons of paint you are going to use and any other supplies that may be needed for repair or preparation of the surface to be painted. I.e. spackling compound, masking tape, drop cloths, abrasive paper, brushes, rollers, etc.

Another factor that determines how many gallons you will be using is your method of application, roller, brush or by spraying.

I like to use a minimum of two coats of paint. Plus, you may need to apply a primer to an area or room. So, that will also affect your cost.

If the surfaces that you are going to paint are textured at all, expect to use more paint.

The following is from the site Howstuffworks "Estimating Interior Paint".

Estimating Interior Paint

Estimating the paint you'll need for a job is easy. Take a few minutes at home to measure the area to be painted. A gallon of paint will typically cover 450 square feet according to the manufacturer's calculations. It's safer to figure 400 square feet of coverage per gallon of paint. If you're buying 2 or more gallons of the same color, it's a good idea to mix them all together at home so color variations don't show up in the middle of a wall.

To determine the amount of paint required to cover a wall, multiply the height of the wall by its length, then divide by 400. This means a gallon of paint will cover a 10 X 15-foot room (two 10-foot walls and two 15-foot walls, 8 feet high) with one coat. Two coats will take 2 gallons. However, there are other factors you should consider when calculating coverage:

Textured walls. When a wall is textured or rough-troweled, it will require more paint than if it were a smooth wall. This is because the texture represents added surface to be covered, even though it does not contribute to the size of the area. Just how much more surface area there is depends on just how textured the surface is, but for medium-rough, porous, or previously unpainted walls, you can safely estimate 300 to 350 square feet of coverage from a gallon of paint.

Doors and windows. Most walls have doors or windows or other areas that are not painted. If the nonpainted area is a single window or door, ignore it in your calculations. Two or three windows, a door and a window, multiple sliding doors, or a fireplace reduce the paint you'll need. By how much? Multiply the lengths by the widths of these nonpainted areas to get the total square footage that you can subtract from your overall surface figures, or you can subtract about 15 square feet for typical windows and 21 square feet for typical doors. (These figures also can be used to estimate the paint you'll need for each if you plan to use a different color or surface finish.) If you're painting the ceiling, figure its square foot area at width times length, too.

Speed factor. Estimating the time you'll have to put in on any given paint project is less precise. Some people work faster than others, so there's no way to estimate individual differences in speed. On the average, you should be able to cover about 120 square feet of flat surface in about an hour. For bare wood or plaster, figure about 100 square feet. In a typical 12 X 15-foot room, you're likely to spend four or five hours on the job, including trim work, for the first coat. The second coat, if it's necessary, will go faster, but you'll have to wait for the first coat to dry, anywhere from 2 to 36 hours.

The following is from the site How Much Paint Will You Need?

ESTIMATE YOUR ROOM size and paint needs before you go to the store. Running out of a custom color halfway through the job could mean disaster. For the sake of the following exercise, assume you have a 10x15-foot room with an 8-foot ceiling. The room has two doors and two windows.
FOR WALLS

Measure the total distance (perimeter) around the room:
(10 ft. + 15 ft.) x 2 = 50 ft.

Multiply the perimeter by the ceiling height to get the total wall area:
50 ft. x 8 ft. = 400 sq. ft.

Doors are usually 21 square feet (there are two in this exercise):
21 sq. ft. x 2 = 42 sq. ft.

Windows average 15 square feet (there are two in this exercise):
15 sq. ft. x 2 = 30 sq. ft.

Take the total wall area and subtract the area for the doors and windows to get the wall surface to be painted:
400 sq. ft. (wall area)
- 42 sq. ft. (doors)
- 30 sq. ft. (windows)

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328 sq. ft.
As a rule of thumb, one gallon of quality paint will usually cover 400 square feet. One quart will cover 100 square feet. Since you need to cover 328 square feet in this example, one gallon will be adequate to give one coat of paint to the walls. (Coverage will be affected by the porosity and texture of the surface. In addition, bright colors may require a minimum of two coats.)

FOR CEILINGS

Using the rule of thumb for coverage above, you can calculate the quantity of paint needed for the ceiling by multiplying the width by the length:
10 ft. x 15 ft. = 150 sq. ft.

This ceiling will require approximately two quarts of paint. (A flat finish is recommended to minimize surface imperfections.)

FOR DOORS, WINDOWS, AND TRIM

The area for the doors and windows has been calculated above. Determine the baseboard trim by taking the perimeter of the room, less 3 feet per door (3 ft. x 2 = 6 ft.), and multiplying this by the average trim width of your baseboard, which in this example is 6 inches (or 0.5 feet).
50 ft. (perimeter) - 6 ft. = 44 ft.
44 ft. x 0.5 ft. = 22 sq. ft.

Add the area for doors, windows, and baseboard trim.
+42 sq. ft. (doors)
+30 sq. ft. (windows)
+22 sq. ft. (baseboard trim)

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=94 sq. ft.
One quart will probably be sufficient to cover the doors, windows, and trim in this example.

Wow That Was Some Figuring Right There, Let Me Be Totally Honest With You. If Your Painting For Someone Else, You Have To Decide What Your Happy With Making, For Your Basic Room, Painting Walls, Trim And Ceiling, And Some Doors, Id Charge You Someone Around $450 + Materials
You Just Have To Figure How Much Time Your Going To Have In The Project Adn What Your Time Is Worth.
On Average I Charge $85 An Hour For Most Of Our Work. That Is For 2 People
Also To Consider , Can You Paint A Straight Line Or Do You Have To Tape Things Off, You'll Have A lot Of Time In Just That. Are Your Using Good Brushs, Good Paint, Just That Has A lot To Do With How Long It Takes You.
One Last Thing , There Is No 1 Coat Paint, It May Say It But You Need To Put Two Coats On There.
So To Recap, Charge What You Feel Comfortable Making Without Gouging,
If Your Not Making Any Money Your Not Going To Feel Like Doing A Good Job, And Lastly Latex Doest Stick To Oil

Figuring cost is much easier than that first answer, more exact than the second. I have been a painting contractor for over 29 years, here is my formula.

1. . 60 - .70 cents per square feet of wall.
2. $30 - $40 per door.
3. $1 dollar per linear foot of baseboards.
4. $2 dollars per linear foot of crown.

Approximately $400 per room, including paint.

Sharonjenny, your post is appreciated, but your numbers only apply to the work in your area (and may or may be correct there, too)... not correct for mine or many other states!

Estimating costs properly comes from experience, overhead and competition in your particular area of the country... not something that can be learned from a post online!

Thanks again!

Mark