How we calculate the Zipper Length?

How we calculate the Zipper Length?
The rules of zipper measurement is very simple but some time it becomes a matter of big trouble for us. Actually it depends on the fabric & zipper properties. So, at first we must need an idea about the shrinkage of body fabric & zipper tape. Some zipper measurements are not very complicated such as pocket, leg opening etc. Here, we only discuss about the front long zipper of sweat shirt...