Q: What zipper size should I use for denim jeans?
For classic denim, a #5 metal zipper in brass or antique finish is the benchmark. It matches the fabric's visual weight, takes the abrasion of a fly opening and holds its shape through stone washing and repeated laundering. Most 12 to 14 oz denim uses this combination with a 32 to 36 inch chain, depending on rise and size grading.
Above roughly 14 oz, or on utility and carpenter styles where the fly sees extra strain, #8 gives a wider margin and a chunkier look. Below 10 oz, a #4.5 metal or a #5 molded resin chain can be used to reduce bulk without losing the metal appearance.
Metal is preferred for garment-dyed and stone-washed denim because the teeth tolerate mechanical action better than coated coil, but choose a corrosion-resistant finish and specify nickel release compliance if the zipper contacts skin. Avoid lightweight coil on a fly: it flexes under load and the slider can rotate out of alignment. Confirm chain length after wash shrinkage testing, since denim can shrink 3 to 7 percent.