Question: how to calculate how much spray foam insulation?
Answer: Measure Length x Height of the wall to be filled.
Subtract the square feet of doors and windows in that wall.
Subtract 6% for the studs (which you won't be spraying)
Take that total and multiply by 3.5″ for a 2″x 4″ wall, or 5.5″ if it is a 2″x 6″ wall.