After you have done the multiplication, subtract about 40 sq feet per window, and about 60 sq feet per door. Divide by 400 (interior paint covers 400 sq feet per gallon). The number that remains, is how many gallons you will need. If doing multiple wall colors, then you should do this process for each room (or sets of rooms) with a particular color. 

Paper/poly drop cloths: $2 to $6 for 9x12-feet. The best of both paper and plastic, these cloths are low-cost and disposable. They absorb paint drips while providing more protection for what's underneath. They also provide some protection from slipping. Plus, you can cut paper/poly drop cloths to fit the space. But, like paper, paper/poly drop cloths can rip more easily than plastic or canvas drop cloths.
Painting is one of the quickest and easiest ways to give your home's interior a facelift with dramatic results. Fortunately, it doesn't have to be that difficult. Home improvement stores offer samples that you can take home. With these samples, you can try a few colors in large swaths on your wall to see the how the paint interacts with the room's natural light. Compare it against design elements like pillows or furniture to see whether it will work with your overall décor.

However, some water-based paints are easier to work with than others. The first coat of paint may not provide complete wall coverage, or the color may not be quite as saturated. Some of these problems can be addressed with a separate primer coat, but this is a time-consuming project. The ideal solution is using a higher-quality paint that combines a primer and a finish paint in a “one coat” product.
Each type of siding will need to be treated differently when it comes to primer and paint at different price points. Exterior paint costs between $35–$80 per gallon depending on the quality you choose. It’s not always true that the more you pay for your paint, the longer-lasting your paint job will be, but you do need to be careful when researching exterior paints to make sure it can weather the local elements well.
Homeowners report that painting a home’s exterior costs an average of $2,923 with a typical range between $1,734 and $4,119. The average price per square foot ranges from $0.50 to $3.50 depending on your location, condition of your exterior and accessibility. Stucco and brick cost an average of $1 per square foot more than vinyl or wood. The average 2,500 square feet home costs an average of $4,000, though it can range from $1,250 to $8,750.