Backpack Volume Claim Comparator

Last updated 2026-08-22

Every ultralight pack's product page states a main-body volume in liters. Whether that number means what it says is a separate question. Treeline Review independently measured six packs by filling each main compartment with pinto beans — "the same person measured all the packs on the same day" — and found the stated figures off by "10% less to 39% more" than what they actually held. We checked each brand's own current page against that 2021 test before using it here.

Two packs, two opposite directions. Zpacks' own current page claims "42L main body" for the Arc Blast. Treeline's own measurement: 34L — 8L, or 19%, less than claimed. Gossamer Gear's own current page claims 36L main body for the Mariposa 60 (unchanged by its recent frame redesign). Treeline's own measurement: 42L — 6L, or 17%, more than claimed. The same test found brands overstating and understating their own number, not one consistent direction.

When the claim and the measurement disagree about whether it fits

A pass/fail based on the brand's own claimed number and a pass/fail based on Treeline's measured number don't always agree. When they don't, this tool flags it — that's the volume at which trusting the spec sheet alone would have given you the wrong answer.

Two packs from the same test we didn't use

Treeline's 2021 article also tested a Hyperlite Mountain Gear pack sold at the time as the "2400 Junction." Hyperlite's current lineup (Southwest, fetched directly) doesn't sell that name any more, and states only one volume figure per size with no separate main-body claim to check the old measurement against. Six Moon Designs' current Swift X page (fetched directly) states a 41L main body — Treeline's 2021 test recorded a 36L claim for the same model name, five liters lower, suggesting the pack itself has changed since the test. Both are named here rather than shown as a row you could mistake for a current comparison.

Every figure, unfiltered

This table doesn't depend on what you typed above — every figure this page cites, straight from each brand's own current page and Treeline's 2021 measurement.

Brand	Model	Their own claimed main body	Treeline's measured main body	Measured, collar fully extended
Zpacks	Arc Blast Backpack	42 L	34 L	34 L
ULA Equipment	Circuit	39.3 L	38 L	44 L
Gossamer Gear	Mariposa 60	36 L	42 L	50 L
Gossamer Gear	Kumo 36	28 L	29 L	32 L

What this does not do

It judges pass/fail on Treeline's 2021 measurement, a single independent test, not a repeated or brand-audited standard — there is no ASTM-style volume standard every maker follows (Treeline's own methodology note says as much).
It compares one model per brand, not every pack each brand sells, and only the four of Treeline's original six packs whose brand-stated claim still matches what that brand states today.
The "collar fully extended" column is shown for context — it is real usable space on a roll-top pack, but it is not part of either brand's stated main-body number, so it never enters the pass/fail check.
It is not a recommendation on which pack to buy — only whether the volume you need is covered by the claim, by the measurement, or by neither.

Runs entirely in your browser. Nothing you type is sent anywhere. Source: Treeline Review, "How Accurate are Stated Volumes of Backpacking Backpacks?", Mike Unger & Naomi Hudetz, 26 April 2021. The knife sharpening angle comparator found the same "a brand's own stated figure disagrees with an independent, directly-checkable source" shape in a different market. The suitcase capacity claim comparator checks a related claim a different way — geometry instead of a physical test — and finds two brands whose own capacity claim already exceeds their own stated box.