How do I calculate the price of painting a wall

If you are referring to how much paint you should buy then use a rule of thumb whereby a gallon of paint will cover between 300 and 400 square feet of wall or ceiling space. Surface prep is important for a long lasting paint job without problems, such as peeling or crackling. Deglossing by sanding or chemical products need to added in. If the walls have not been painted for a while then make sure you include the cost of a tinted primer sealer. If your walls or ceiling are stucco double the amount. Pop corn ceilings would be about 1 and a half times as much. If you just intend on throwing on a coat of paint the figure above should suffice.

How do I calculate the price of painting a wall
I charge 50 cents per square foot for wall space - (drywall or plaster.) That is only labor for painting...
Prep, paint and supplies are extra.

Snakebite's quote is right on my number, pros may charge 75 to 1.00 labor only.

There are some factors that are unknown here, such as: Is the wall textured in any way, and if so, what kind of texture? Is the paint to be put on to be applied with a brush, roller or by spray of some kind? Plus, we don't know how many coats or the sheen of the paint that is to be applied or whether it will be latex or oil-based, depending on the texture of the wall. Is this interior or exterior? And, how large of an area is this seemingly just a single wall? Perhaps it is the outside wall of a large building!

Many factors to be considered here...

It would be helpful to have more information before coming up with definitive conclusions as far as prices. Prices will also vary greatly depending on the location where the work is to be performed.

Just my take on things...

Thanks for putting up with me!