Calculating the amount of natural thin stone veneer needed is relatively simple. For the flats which are sold by the square foot you would measure the length times the width of all the areas you wish to cover in stone. On something like a home exterior it is easiest to take a total measurement and then subtract the spaces taken up by the windows and doors.
The exterior corners are sold by the linear foot. On an exterior this would mean measuring from base to soffit for each corner you wish to cover. On an interior fireplace you would measure floor to ceiling. The corners are 90 degrees and are typically four inches on one side with a six to ten inch return on the other. There are no interior corners, you take the flats to the edge.
Working with natural stone veneer requires cutting, trimming, and sorting the individual stones. Making those modifications will result in some of the material being unusable in the project. All of this unusable material is collectively called the waste factor. When starting to calculate the waste factor, you need to consider the type of joint that will be used during installation. The smaller and tighter the joint, the more modifications are needed. Each time you modify a stone, you generate waste, thus more modifications equals more waste.
For example, suppose you have four walls that are 50 ft. x 10 ft. and have calculated a total square footage (4 walls x 50 ft. x 10 ft.) of 2,000 square feet. Using a standard half inch raked mortar joint averages a waste factor of 10%. That results in (2,000 x 10% = 200 sq. ft.) 200 square feet of extra stone that will need to be ordered making the total 2,200 square feet of material. Your contractor will also be a great resource in assisting with these calculations.