Q: How much paint do I need to buy to paint a room?

A: Measure the length of each wall and add them together; multiply this times the height of the room.  Measure the door and window openings and subtract them rom the total.  If the ceiling is being painting then measure the length times width and add to the total square footage.  You will get roughly 350 sq/ft out of a gallon.  

Things to consider:
Will the color cover in 1 coat?  If it does not cover, don't forget to account for this when buying paint.  Additionally, the second coat tends to use up slightly less paint then the first coat.  

What is the surface of the wall like?  Walls that have previously been painted with a glossy finish like eggshell or semi-gloss will suck up a lot less paint then walls with no sheen.  Dry walls that have no gloss will suck up more paint. Rough surfaces like cinder block or foundation tend to suck up more paint.  

Rule of thumb for an average bedroom:  If you are doing 1 coat of paint on walls, a gallon should do it.  If you are doing 2 coats on walls for an average bedroom, you most likely will need 2 gallons.

Q: How do I prep a room before painting?

A: Properly prepping a room before painting will make or brake the job.  The idea is to get the walls as smooth as you can and to eliminate cracks and openings before applying the finish paint.

First, examine the walls.  Are there big holes or dents that need patched with spackle? Is there water damage or peeling paint? Are their cracks between the baseboard and the wall?  Are there water stains or bare drywall that will need sealed out with primer?  

Start with the things that will take the longest to dry.

Deeper holes that need patched are usually first.  As you are patrolling the walls for holes that need patched, you should also be searching for other imperfections that can be sanded out with sandpaper.  It is good practice to give the walls a light sand to catch unwanted bumps and boogers on the walls that may be hard to see.  Remember, if imperfections get overlooked now, they will stand out like a sore thumb after you apply the finish paint.

After sanding the walls and patching the holes it is a good idea to caulk the baseboard and/or crown molding to eliminate cracks and let the caulking dry while you finish the rest of the prep. Be sure to let the caulking dry before you paint it or else it will crack out. 

After you have acknowledged the imperfections by sanding, patching, and caulking it is a good time to spot areas like water stains, bare drywall or bare wood that will bleed through the finish coat.

I cannot stress the importance of prepping walls before paint.  A good paint job is only as good as the prep job.  However, a good prep job can leave your paint job looking like new.  It is amazing how a good prep job and paint job can absolutely transform a room.