The zipper we use in clothing today was invented by Whitcomb L. Judson in 1890. The zipper size is quite different from the zipper length. The length of the zipper is measured in inches and centimetres by measuring the area of zipper teeth between the zipper top and zipper stop. Zippers come from the manufacturer in various sizes such from size 3 to 9 and beyond. But these sizes don't reflect the exact zipper dimensions. These numbers reflect industry standards for zipper strengths; the larger the number the stronger the zipper. The method for measuring zipper size is different, but still easy.
Lay the zipper flat with teeth facing up.
Search on the zipper slider for a number or letter, such as 10, for example. The inscription #10 means the zipper is 10mm, while #5 means the zipper is 5mm.
Measure the length from the metal at the top of the zipper to the metal at the bottom of the zipper. Use this same method for all types of zippers. Don't include any extra zipper tape in your measurement.
Write down the zipper number and length for future reference.