Q: What zipper size is used for jackets and outerwear?
Lightweight shells and windbreakers usually take a #5, which balances flexibility with enough strength for a center-front opening. Insulated and puffer jackets generally move to #8, because the chain must resist the outward pressure of lofty fill and the slider must be operable with gloves. Heavy parkas, expedition wear and work jackets often use #10.
Construction matters as much as size. An open-end chain with a zipper garage protects the chin and reduces water ingress; a two-way slider helps when sitting or driving; a reverse-coil chain hides the teeth and supports a cleaner waterproof face. On waterproof shells, the zipper rating is separate from the size and must be specified independently.
For lightweight fashion outerwear, a #3 or #4.5 molded resin chain can be adequate and looks cleaner. Whatever the size, test the assembled jacket on a mannequin with the fill in place, then pull the slider at an angle to simulate real use. A chain that engages smoothly on a flat bench can still skip once the front edge curves over a chest.