A torso measurement that lands between two listed frame sizes doesn't mean either size is wrong — it means the choice depends on whether the pack has a fixed or adjustable suspension.
On a fixed-frame pack
Fixed-size packs build one specific frame length into each size label, with no way to lengthen or shorten it. When a measurement sits between two sizes, sizing down keeps the hipbelt riding correctly over the hip bones — the more important fit point for comfort — at the cost of the shoulder straps sitting slightly higher on the back than perfectly ideal. Sizing up risks a hipbelt that can't reach the hip bones at all, which shifts load onto the shoulders instead of the hips.
On an adjustable-harness pack
Many packs use a sliding yoke, held in place with a buckle or ladder-lock strap, that lets the shoulder-strap attachment point move up or down the frame. This typically covers a range wide enough to span what would otherwise be two or three fixed sizes, which makes an adjustable-harness model the more forgiving option for a hiker whose measurement falls on a border.
Confirming the choice
| Situation | Better Choice |
|---|---|
| Between two fixed sizes | Size down, or try both loaded |
| Frequent lending or multiple wearers | Adjustable harness |
| Measurement matches one size closely | Fixed size in that band |
Whichever route is chosen, load the pack with realistic weight and check that the hipbelt sits over the hip bones and the shoulder straps meet the collarbone before finalizing the size — a size chart is a starting point, not a guarantee.