By Marcus Hale, Lead Pack Research Editor · Published September 14, 2026, 6:03 PM ET · Updated September 29, 2026, 6:16 PM ET
Direct Answer
Backpack liters and backpack size aren't directly convertible: liters measure volume, while size (small, medium, large) measures torso length, a separate fit measurement taken from the C7 vertebra to the iliac crest. A 40L pack and a 65L pack from the same brand can both come in identical small, medium, and large torso-length options.
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In plain English
Backpacks have two different numbers, and people often mix them up. The liter number tells you how much fits inside, like the size of a suitcase. The small, medium or large label tells you which body the pack is built for, like a shirt size.
The body size depends on the length of your back, not your height. Two people of the same height can have different back lengths, because some people have longer legs and some have longer bodies. You measure from the bony bump at the base of your neck, where it meets your shoulders, down to the top of your hip bones. A friend with a soft tape measure makes this easy.
Once you know your back length, you can buy almost any liter size in the right body size. A small daypack and a big camping pack from the same brand usually use the same size chart.
There's no formula that converts liters to a torso size, because the two numbers measure different things. Liters describe how much gear a pack holds. Size labels like small, medium, and large describe torso length, the distance the frame is built to span on a hiker's back.
How to measure torso length
Torso length is measured from the C7 vertebra, the bony bump at the base of the neck, down the spine to the iliac crest, the top of the hip bones. Torso lengths vary widely, and each manufacturer maps its own range to small, medium, and large frame sizes, with some brands offering adjustable or women's-specific frames within the same range.
Why the two numbers are independent
Measurement
What It Describes
Typical Range
Liters
Pack volume/capacity
20L–70L+
Torso size
Frame length for fit
Small–Large (varies by brand chart)
A hiker with a short torso and a hiker with a long torso can both need the same 40L pack for a weekend trip, but they'll need different frame sizes to carry it comfortably. Buying by liters without checking torso length against the manufacturer's size chart is a common fitting mistake, since a pack in the wrong torso size won't position the hipbelt correctly no matter how well the volume matches the gear list.
The practical takeaway
Choose liter capacity based on gear volume and trip length, then choose torso size based on a body measurement, and treat the two decisions separately when shopping. Most manufacturers list torso-size ranges directly on the product page next to the liter rating, so both numbers are available at the same time even though they aren't mathematically related.
Size labels are brand-specific
A medium from one brand is not necessarily a medium from another. Granite Gear offers Short (15 to 18 inches), Regular (18 to 21 inches) and Long (21 to 24 inches) torso sizes.1 Deuter's fits are defined by back length: about 42 cm for its SL fit, 46 cm for its regular fit and 50 cm for its EL fit.2 Hyperlite sells its Southwest in five torso sizes running from under 15 inches to 21-plus inches.3
That spread matters for the page's 16 to 22 inch range, which is a general guide rather than a published standard: brands' own charts extend below 15 inches and up to 24. Gender labels are not a shortcut either. Gregory sizes packs identically regardless of gender, so a medium is the same torso size in its men's, women's and unisex packs.4
The third number: hip-belt size
Many packs size the hip belt separately from the torso. Granite Gear bases hip-belt size on the circumference at the hip crest, roughly your regular pant waist size.1 ULA sizes men's hip belts at pant waist size plus 2 inches, and for women adds 5 inches to the smallest waist measurement for curvier builds or 3 inches for straighter builds.5 Osprey says a correctly sized hip belt leaves a gap of 3 to 6 inches between the pad tips when tightened.6
Brands also disagree on what to do between sizes. Mystery Ranch recommends sizing down so the waist belt can tighten securely on the hips,7 while other brands leave the call to fitting advice. Check the specific brand's guidance before ordering.
Where liters and size do connect
The two numbers are independent, but spec sheets tie them together in one way: published figures usually describe one size. REI, for example, lists the Flash 55's weight and 55-liter volume for the medium.8 Other sizes of the same model can differ slightly, so check the listing for the size you actually need. The liter figure also often includes outside pockets, as with the Mariposa 60's 36-liter main body plus 24 liters of exterior pockets.9
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