Measuring for backpack size means taking two body measurements — torso length and hip circumference — and comparing both against the specific pack's own size chart, since size labels aren't standardized across brands.
Torso length sets the frame size
Torso length is the distance from the C7 vertebra at the base of the neck to the iliac-crest line at the top of the hip bones, measured with a flexible tape run down the curve of the spine. This number, not height, determines which frame-size label — XS, S, M, L, or a numeric size — fits correctly.
Hip circumference sets the hipbelt size
Hip circumference is measured by wrapping a tape measure around the body at the iliac crest, the same landmark used for torso length, and reading the number in inches. Packs with interchangeable hipbelts size this independently of the frame, since torso length and hip width don't always correlate.
Comparing against a size chart
| Measurement | What It Sizes |
|---|---|
| Torso length (C7 to iliac crest) | Frame size / back length |
| Hip circumference (around iliac crest) | Hipbelt size |
Neither measurement should be estimated from height, weight, or clothing size — clothing sizing and backpack sizing use different reference points and aren't interchangeable. Once both numbers are known, check them against the exact pack model's published chart, since the same torso length can land in different size bands on different brands.