For walls, measure the linear feet of wall space (measuring along the baseboards) for the areas to be painted (using a tape measure, laser, or both). Then multiply this by the ceiling height (usually it is 7.5 or 8). If there are 2 story areas, measure them separately, and multiply them by double the regular wall height. Then multiply the total number by 2 (for 2 coats).
A more user-friendly option is a water-based “latex” paint, which blends acrylic pigments with a thin base for easy application with a brush or paint sprayer. Although many water-based interior paints are promoted as one-coat wonders, there can still be a need for an initial primer coat followed by one or two coats of latex paint. Water-based paints are very low in VOCs, which means minimal odors during the drying period. Most spills can be wiped away without the need for chemical cleaners, and clothing containing latex paint can be laundered in water and detergent. Water-based paint is a good choice for first-time DIYers and their younger assistants.

Priming is compulsory if you're painting over a darker color, or on a new drywall, but it's a good idea to include this step before any paint job. A primer is necessary because it blocks any stains from bleeding through. It is also important because it prevents any blisters and paint-peeling by improving paint adhesion. Lastly, primer is a good idea as it allows complete single-coat coverage of the walls. If you want a better appearance, you can tint your primer with the final color you intend on using on the walls. Most paints today come with inbuilt primers, but an old school primer is still a better option. Before you start painting, remember to use painter's tape to cover your door frames, window sills, and any switches on the wall.
Oil-based paint is commonly used for high-moisture areas, such as bathrooms and kitchens, and floors, trims, and moldings, since these areas take more abuse over time than walls do. Typically oil-based paints are less expensive than latex paints, take longer to dry, can create bad odor while drying, and contain more volatile organic compounds (VOCs), which essentially means they're less environmentally friendly.
Of course, paint brushes and rollers are also important line items on your budget sheet. A 3-inch-wide paint brush costs, on average, $10 to $20—though you'll likely need an assortment of paint brush sizes for trim, corners, and narrow surface areas. Brushes come in a variety of bristle types and qualities, and each has a different purpose. Some are intended for specific paint types, finishes, and surfaces, so be sure to buy the right brushes for your painting job.
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. 
Stripping – $54–$151/100 sq. ft.. “The principle of paint strippers is penetration of the paint film by the molecules of the active ingredient, causing its swelling; this volume increase causes internal strains, which, together with the weakening of the layer's adhesion to the underlying surface, leads to separation of the layer of the paint from the substrate.”— Wikipedia. All the stripper will do is soften the paint. You’ll still have to scrub or scrape it off.
When determining the total cost of a home’s exterior project, you’ll want to include things like the trim, a garage, foundation, windows and doors. All work is calculated by square footage and sometimes by the linear foot, such as in the case of trim. Your contractor will give you a price for the entire project unless you specifically ask for each item separately.