Best Carry-On Bags With the Most Realistic Capacity (Not Just Marketing Claims)
August 22, 2026 · 4 min read
Carry-on liter ratings look like precise measurements, but nothing requires them to be. A voluntary standard exists for measuring bag capacity, most brands don't say whether they follow it, and that gap is why two bags labeled 40L can hold visibly different amounts of stuff.
Quick Answer
- Liter ratings are unregulated; ASTM F2153 defines a method, but using it is optional
- The standard fills a bag with 20mm plastic balls and excludes anything not fully enclosed by fabric
- Minaal states it uses ASTM F2153; Tom Bihn uses its own refined version of it, implemented in 2018
- Publishing a measurement method is the best available signal, since no independent body audits these numbers
- Buy on external dimensions for airline compliance, and treat liters as a rough comparison only
The Standard Most Brands Don't Mention
There is an actual standard for this. ASTM F2153-07(2018), titled Standard Test Method for Measurement of Backpack Capacity, was developed by ASTM subcommittee F08.22 and explicitly covers duffel bags and travel packs alongside backpacks. Its stated purpose is to give manufacturers and consumers a consistent way to compare pack volumes.
The method is more physical than you might expect. The bag is filled with 20mm hard polypropylene balls to its fullest usable volume, and those balls are then emptied into a graduated cylinder to read the capacity. The standard draws a firm line about what counts: section 1.3 excludes any area not completely enclosed by fabric, which rules out mesh pockets, water bottle holders, and compression pockets. It also distinguishes extended from unextended capacity, which matters for expandable bags, and notes it can't reliably measure anything under 4 liters.
Here's the catch: following it is entirely voluntary. Nothing stops a brand from measuring differently, counting open pockets, or estimating from external dimensions. That's how you get the anomaly one traveler flagged on FlyerTalk, comparing a bag whose external volume was about 2% larger than a competitor's while claiming roughly 30% more liters. Both numbers can be published in good faith under different methods, and the buyer has no way to tell which method produced either one.
Common Mistakes
People assume a liter is a liter, but in practice the unit is standardized while the measurement isn't, so the number describes the brand's chosen method as much as the bag. Two bags rated identically can differ meaningfully in what they actually swallow.
People assume a higher liter rating means the bag will fit more into an overhead bin, but in practice airlines measure the outside of your bag and never its internal volume. A soft bag measured by the ball-fill method is measured while bulging, which is exactly the state that causes it to fail a gate sizer. For what airlines actually enforce, see our breakdown of strict carry-on size rules.
People assume review sites have independently verified these capacity claims, but in practice almost nobody has. There's no public dataset of measured-versus-claimed capacity across carry-ons, and any site presenting a precise universal figure for how much capacity you "really" get is estimating, not measuring.
Which Bags Have the Most Credible Numbers
Given that no independent body audits capacity claims, the most defensible way to answer "most realistic capacity" is to look at which brands disclose their method. Two stand out.
Minaal states plainly on its own site that many bag makers don't use ASTM F2153 and that it does, listing the Carry-on 3.0 at 35L and the Daily 3.0 at 21L. It also publishes packing videos of the bags, which is a second useful form of evidence: seeing a bag loaded tells you more about real capacity than any number.
Tom Bihn went a step further, developing a refined version of the ASTM method and implementing it in 2018. Its reasoning for keeping the 20mm balls is worth knowing, since it explains why the standard specifies them: the balls leave less space between them than ping pong balls, making small compartments more accurate, and their heavier weight better mimics the density of a real load, letting fabric and seams give the way they would in use.
Neither of these is a claim that these bags hold more than competitors. It's a narrower and more useful claim: their published numbers were produced by a stated, repeatable method, so the figures are comparable to each other and to any other brand using the same standard. Absent independent testing, that's the strongest available signal, and it's a reasonable thing to ask any brand before you buy.
JetKit works from what you're actually packing rather than a number on a hangtag, so you can see whether your list fits the bag you own before you're standing at the gate.
Frequently Asked Questions
Are luggage liter ratings regulated?
No. There's a voluntary standard, ASTM F2153, that defines how to measure bag capacity, but no rule requires manufacturers to use it. Brands are free to calculate and report capacity however they like, which is why two bags labeled 40L can hold noticeably different amounts.
How is bag capacity supposed to be measured?
Under ASTM F2153, the bag is filled with 20mm hard polypropylene balls, which are then emptied into a graduated cylinder to read off the volume. The standard specifically excludes any area not completely enclosed by fabric, so mesh pockets, bottle holders, and compression pockets don't count toward the number.
Which carry-on brands actually publish how they measure capacity?
Minaal states directly that it uses ASTM F2153 and notes that many bag makers don't. Tom Bihn developed its own refined version of the standard, keeping the 20mm balls, and implemented it in 2018. Publishing a method is currently the most reliable signal available, since no independent body audits these numbers.
Why can a bag claim more liters than its outside dimensions suggest?
Because the ball-fill method measures the bag when it's packed and bulging, while external dimensions describe it flat and empty. A soft-sided bag that measures within carry-on limits empty can expand well past them once loaded, which is a common reason bags fail gate sizers.
Should I choose a carry-on by liters or by dimensions?
Use dimensions for airline compliance and liters only for rough comparison. Airlines measure the outside of your bag at the gate and never check its internal volume, so the liter rating tells you something about packing space but nothing about whether you'll be allowed to board with it.