Q: Which zipper size should a backpack use?
A backpack uses a size 8 or size 10 coil for the main compartment and a size 5 for internal pockets and small lid compartments. The main body is the part that is overfilled, dropped and dragged, so the heavier chain is not optional; the size 10 is worth it on a travel pack above about 45 litres. Use a large-pull slider that can be gripped with gloves, and sew the curved sections with a relief notch so the tape is not under tension along the bend.
- Size 8 coil for a daypack main body; size 10 for travel packs.
- Size 5 for internal and lid pockets.
- Large-pull sliders and reinforced ends.
The common pitfall is a size 5 on the main compartment: it looks adequate on a flat pattern, then splits at the corner where the zip turns and the pack is stuffed. The second is abrasion. Coil wears through where the pack is set down on concrete, so exposed panels often take a vislon or a covered coil instead. Always test with the bag packed to its stated capacity, not empty.