What does a backpack's liter rating actually measure?

Liters look like a unit. They behave like a label. Two packs on the same rack, both marked 40, can differ by a good fraction of that in usable space, and neither maker has broken a rule doing it.

There is a written test method for this. The problem is that almost nobody says whether they used it.

Is there a standard for measuring backpack volume?

Yes — and running it is entirely voluntary. ASTM F2153-07(2018), Standard Test Method for Measurement of Backpack Capacity, is an active ASTM standard. It “determines and standardizes an unextended and extended capacity for backpacks and related bags,” reports in liters, excludes unenclosed areas such as mesh pockets and bottle holders, and cannot measure anything under 4 liters.

Nothing obliges a maker to run it, or to say whether it did. Treeline Review, which measured a set of packs against their published figures, puts it plainly: “there is no regulatory requirement that any brand uses the measurement in their reporting.”

Which makers say how they measure?

One of the seven checked here publishes a method, one publishes a closure convention, and five publish a number with nothing attached to it.

Osprey is the one with a method. Its Volume & Packing page states that “20mm plastic balls are used to fill the pack,” that “All components are filled with the plastic balls to their extended capacity,” and that “All balls are then placed into a Volume Measurement Cylinder.” Osprey calls this “a much more accurate industry standard” than calculating from outside dimensions. The 20 mm sphere and the extended-capacity wording both match what ASTM F2153 describes, though the page does not name the standard.

SealLine publishes a different kind of statement — not how volume is derived, but what state the bag is in when it is measured. Its FAQ reads: “All of the dimensions we list for our roll-top dry bags are the dimensions when the dry bag has been properly sealed (tops rolled down three times and secured).”

Black Diamond, Gregory, Mammut, CamelBak and Silva publish no measuring method on any product or help page we opened. Black Diamond’s Mission 75 page calls it “a true 75-liter volume” without saying what makes it true. Silva’s Strive 10 is sold on “10-liter packing volume” with no breakdown of what is counted.

Maker and model	Published volume	Same figure across sizes?	Method stated	Torso / back length published	Max load published
Osprey Atmos AG 65	65 L (S/M), 68 L (L/XL)	no — two figures	yes: 20 mm ball fill	yes, on the fit page	yes: 30–40 lb
Gregory Baltoro 75	75 L	yes, across SM/MD/LG	no	yes: torso and hipbelt per size	yes: 65 lb
Black Diamond Mission 75	75 L	yes, across S/M and M/L	no	yes: torso range per size	no
Mammut Ducan Spine 50-60	50–60 L	yes, across 3 back lengths	no	yes: 43.5 / 46 / 48.5 cm	no
CamelBak H.A.W.G. Pro 20	none published	n/a	no	no	no
Silva Strive 10	10 L	yes, across XS–L	no	no	no
SealLine Black Canyon Dry Pack	65 L / 115 L	one size each	closure convention only	no	no

“Method stated” means a published description of how the number was arrived at, not a claim that the number is accurate.

Does the liter figure change with torso size?

On one maker here it does, and on the rest it does not — even where the pack is demonstrably a different size. Osprey’s Atmos AG 65 is published as 65 liters in S/M and 68 liters in L/XL. Two numbers, one model.

Gregory does the opposite, and its own dimensions give it away. The Baltoro 75 is 75 liters in SM and 75 liters in LG, but the published height goes from 31.9 to 33.7 inches — nearly two inches more pack, same figure. Black Diamond publishes 75 liters for both sizes of the Mission 75 and 55 liters for both sizes of the Mission 55. Mammut’s Ducan Spine 50-60 is offered in back lengths of 43.5, 46 and 48.5 cm with one volume range across all three; the women’s Ducan Spine 28-35 carries a single 28–35 L range as well. Silva’s Strive vests come in four sizes, XS to L, at 5 liters or 10 liters flat.

Buy the long size and you are probably getting more pack than the number admits. That is not a bonus. It is a reason the number cannot be compared across brands.

What fit dimension actually matters?

Torso length — the distance from the C7 vertebra to the top of the iliac crest — not height, and not the liter figure. Osprey’s size and fit page states that “Pack size is based on torso length” and gives the measurement as: “Find iliac crest. On back of neck, find C7 vertebra. Measure from C7 vertebra to the iliac crest to determine torso length.” Deuter defines its back lengths from the same two landmarks: “the distance between the seventh cervical vertebra and the upper edge of the iliac crest.”

Most makers publish this, and it is the more useful number of the two. Black Diamond lists 16–19 in (41–48 cm) for the Mission’s S/M and 18.5–21.5 in (47–55 cm) for the M/L. Gregory publishes a torso range and a hipbelt range per size — 16–19 in torso with a 26–48 in hipbelt for the Baltoro 75 SM, 18–22 in with 30–52 in for the LG — plus a size chart covering the line. Mammut publishes back length in centimeters per size. Silva’s vests are sold XS to L with no torso or chest figure on the product pages we opened.

Do makers publish a maximum load?

Two of the seven do. Gregory publishes a max carry weight of 65 lb for the Baltoro 75. Osprey publishes a load range of 30–40 lb (14–18 kg) for the Atmos AG 65. Black Diamond, Mammut, CamelBak, Silva and SealLine publish no load figure on the pages checked. What they publish instead is the pack’s own empty weight — 1,775 g for the Mission 75 in S/M, 1,867 g in M/L — which answers a different question.

Does a hydration pack’s number include the water?

Not consistently, and CamelBak’s pages differ from model to model. The Mil Spec M.U.L.E. lists “8L of cargo storage in top and bottom front pockets” and, separately, a 3-liter reservoir, with no combined total. The H.A.W.G. Pro 20 publishes external dimensions of 48 × 21 × 25 cm and a weight of 1,290 g, plus the same 3-liter reservoir — and no total volume anywhere, despite the 20 in the name. On a hydration pack the cargo figure is the one to read, and it is not always the one on the label.

Why does a dry bag’s liter figure behave differently?

Because it appears to be computed from dimensions rather than measured by filling — and on a roll-top those dimensions already assume the closure is rolled three times. SealLine’s Black Canyon Dry Pack is published at 65 liters with sealed dimensions of 44 × 23 × 64 cm. Multiply those three and you get 64.8 liters. The 115-liter version is 47 × 32 × 76 cm, which multiplies to 114.3. Both land within about 1% of the stated figure.

Run that arithmetic on a ball-measured pack and it falls apart. Osprey’s published dimensions for the Atmos AG 65 in S/M, 36 × 39 × 83 cm, multiply to roughly 117 liters against a stated 65. Gregory’s Baltoro 75 in LG, 33.7 × 13.4 × 13.8 in, multiplies to about 102 liters against a stated 75. A sealed dry bag is close to its own bounding box; a suspended pack is not, because harness, frame and taper eat the difference. Mammut’s Ducan Spine 50-60 goes the other way — 27 × 63 × 29 cm multiplies to 49.3 liters, below the bottom of its own stated range, which is what happens when an extension collar counts toward volume but not toward height.

Why doesn’t a liter figure tell you what will fit?

Because a liter has no shape and the things that fill a pack do. A tall narrow 40 and a wide flat 40 swallow the same volume of small objects and completely different single items — a rope, a rack, a tripod, a folded pad.

The gear that decides the question mostly is not sold in liters either. Therm-a-Rest publishes the packed size of the NeoAir XLite NXT as 9 × 4.1 in (23 × 10 cm) for the Regular and 11 × 4.6 in (28 × 12 cm) for the Regular Wide — a cylinder, given as dimensions, because whether it drops in upright or has to lie across the bottom is a dimension question. Its warmth is a separate spec entirely, which is what an R-value measures. The same logic runs through any fixed kit list: what belongs in a 72-hour kit is a set of shapes before it is a number of liters.

The short version

A liter rating is a manufacturer’s claim, not a measurement you can check, and ASTM F2153 exists precisely because the claims were not comparable. Of the seven makers checked here, only Osprey publishes how it arrives at the figure, and only SealLine states the condition the bag is in when measured — the other five publish a bare number. Volume usually does change with torso size, but only Osprey publishes that change; Gregory, Black Diamond, Mammut and Silva carry one figure across every size they sell. The dimension worth reading is torso length, which most of them do publish, and the load limit is worth reading where it exists, which is rarely. Compare packs on dimensions, torso range and published load — treat the liters as a rough shelf label.