As you tackle a painting project, there are many details to remember to ensure your finished job is a quality one. After the painstaking selection of paint colors to create just the look you desire in your room, you must then calculate the square feet of the surface to be painted. If you take the time to figure your square footage accurately, you will end up buying the right amount of paint.
Things You'll Need
 Tape measure
 Paper
 Pencil
 Calculator

Measure, in feet, the length of one wall you will paint. Measure the height of that wall, in feet. Write down both measurements. Multiply those measurements to find the number of square feet in the wall.

Repeat Step 1 to find the square footage of each wall that you will paint.

Add the square footage of the walls to determine the total area of the wall surfaces.

Multiply the total area from Step 3 by the number of paint coats you intend to apply to the walls. That will give you the total wall area to which you will apply paint.
Tips & Warnings
 In most cases, do not subtract the square footage of doors and windows from the total painting area. Leaving those areas in your total painting area will ensure that you buy enough paint. You can use any leftover paint for other projects.
 If you have enough doors and windows that make it prudent to subtract those areas from your paint total, subtract 21 square feet for each door and 12 square feet for each window from the total in Step 3. Proceed with Step 4 to reach the total painting area.
References
 Photo Credit room painting image by Stephen Orsillo from Fotolia.com