Free Space (or freespace) is a measure of how much allowance there is in the seam for the compound. It represents the amount of space left in the seam after the 5 folds of metal in the seam have formed (3 layers of the end and 2 layers of the can). Ideally, we want a very small amount of free space, so as to prevent the seam from being left open but not too little, so as not to allow any room for the compund.
The formula for calculating Free Space is:
FS = ST – (3XCT + 2XBT) where: FS = Free Space, ST = Seam Thickness, CT = Cover Thickness and BT = Body Thickness.
Visually, a seam with high free space would have gaps between the different layers of can and end material and would appear “open”.
A seam with very low free space would appear overly tight