Q: What is the difference between a #3 and #5 zipper?
The number in a zipper size is a nominal gauge tied to the meshed chain, not a millimeter figure printed on the tape. A #3 nylon coil measures roughly 4.5 mm across the closed teeth, while a #5 measures about 6 mm. The same numbering carries across metal and molded resin chains, so a #5 metal zipper is also close to 6 mm wide.
Width is only the visible part. Filament diameter, tooth volume, slider body size and tape width all scale with the number. A #5 therefore delivers noticeably higher pull and burst strength than a #3, and its slider is easier to grip.
In practice, #3 suits dresses, blouses, linings and small bags where the chain is hidden; #5 is the general-purpose choice for trousers, jackets and mid-weight bags; #8 and #10 are reserved for denim, workwear, luggage and upholstery. Going one size up adds durability but also stiffness, weight and cost, so match the size to fabric weight rather than to habit.