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 an eight to ten inch return on the other. There are no interior corners, you take the flats to the edge.
After you have the final measurement you need to consider the waste factor for the cuts required to achieve your desired pattern. This is typically 10% over the actual amount needed.