Measuring for a side zipper

When cutting a side that will have a zipper in it, the question is how tall to make each cut.

Our foam required 4" sides.  With batting, the sides are actually a little larger when inserted, but the compression makes the cushion firm.

So, a side without a zipper would have 2x 1/2" allowances on the top and bottom edge and would be cut 5" wide.

However, when you insert a zipper into the side (obviously the back side) of a cushion, you have to cut 2 strips that when zippered together is 5 inches.  So, how much to you allow for the addition of a zipper?

You cut 2 strips 3 3/16"  Basically, you give 11/16" to the zipper (at least with our material).

We experimented with a small piece of fabric by adding a small piece of zipper tape and seeing the end resulting width.  You can do the same.  Cut two strips of a known width and add zipper tape to both and zipper together.  Then, see how wide it becomes.  Adjust your measurements accordingly...

For us, and our material, two strips at 3 3/6" zippered together created a 5" back side.