Jelkin 25 vs a personal-item backpack: volume per gram, and the sizer it costs you

Short answer: a typical 18 L personal-item backpack weighs about 800 g, giving ~22 mL per gram of capacity. The Jelkin 25 gives ~7 mL per gram of usable volume. The backpack is three times more volume-efficient — but it is also the bag that gets size-checked at the gate, while the 600 g Jelkin clears every sizer as a free personal item. You are not comparing capacity; you are comparing a free pass against a bigger bag you may have to justify.

Capacity per gram

If raw volume-per-gram is the metric, the backpack wins. That is the wrong metric for a personal item.

The metric that actually matters

The backpack's extra volume comes with a cost the table does not show: it is the bag that gets pulled at the sizer, weighed, and potentially reclassified as a paid cabin bag on low-cost fares. The Jelkin 25's lower volume-per-gram buys something the backpack cannot — guaranteed personal-item status on all seventeen carriers we track, for free, no priority purchase.

They are not rivals

Use both. The backpack is your stowed carry-on; the Jelkin 25 is your under-seat personal item. Together they are ~22 L of free cabin space. The mistake is treating the backpack as your only bag and then discovering at the gate that "free" depended on a size you exceeded.

Should you buy the Jelkin 25 for this?

A backpack wins on volume per gram — ~22 mL/g against ~7 mL/g — and loses on the thing that actually matters at the gate: it is the bag that gets pulled, weighed and reclassified on low-cost fares. These are not rivals. Buy the Jelkin 25 as your guaranteed-free under-seat item and keep the backpack as your stowed carry-on; together that is about 22 L of free cabin space.

How to check this yourself before you fly

1. Decide which bag is stowed and which stays with you before you pack. Mixing them up is what causes gate problems.
2. Check the backpack against the sizer if it is your only bag. Most 18 L personal-item backpacks exceed 20 cm depth when full.
3. Use the smaller bag for what you need mid-flight. Access, not capacity, is its job.
4. On low-cost fares, assume the backpack is the paid item. The under-seat bag is the free one.

Keep reading:

• airline personal item size checker — all 19 carriers side by side
• one-centimetre depth cliff — why 15 cm decides six airlines
• the 25 cm bag that fits every sizer — length-vs-depth end to end
• why external dimensions lie about capacity — how we measure bags
• Jelkin 25 — the 25 × 17 × 14 cm bag measured on this page