Packing is a volume problem until an airline turns it into a weight problem

Two constraints that are usually confused

Everything you carry is limited in two independent ways. There is volume, which decides whether the bag closes and whether it fits the space allowed, and there is mass, which decides whether you can carry it comfortably and whether an airline will accept it at the gate.

Most packing guidance is really about volume, because volume is the constraint people meet at home while the bag is open on a bed. Weight is the constraint that appears later, at a counter, with a queue behind you and no good options.

The two also pull in different directions. Compression makes a bag smaller and no lighter; leaving things out makes it lighter and smaller. Only one of those actually solves both problems, which is the whole argument for packing less rather than packing cleverly.

Volume limits are about shape as much as capacity

Cabin baggage allowances are expressed as three dimensions, and the sizer at the gate tests all three simultaneously. A bag of the correct stated capacity in the wrong proportions will fail, and a soft bag that has been overstuffed will fail even when an identical bag packed loosely passes.

The dimensions include wheels, handles and external pockets, which is where a great many bags marketed as compliant quietly are not. Measuring your own bag when full, rather than trusting the label, is the only reliable check.

Airlines also increasingly separate a small personal item from a larger cabin bag, with different allowances attached to different fare types. What is included and what costs extra changes frequently enough that it needs checking per booking rather than per airline.

Weight limits are enforced unevenly, which is the trap

Cabin bag weight limits exist on many carriers and are checked inconsistently. That inconsistency is what causes the problem: people learn from experience that nobody weighs the bag, pack accordingly, and then meet the one flight where everything is weighed and the excess charge is calculated at the gate.

Checked baggage is weighed every time and the thresholds are hard. Where an allowance is a single figure for the whole bag, a small overage costs the same as a large one, and the charge levied at an airport is generally far higher than the same weight bought in advance online.

Different rules apply on different legs of the same trip, which catches people out on journeys combining a long-haul flight with a regional one. The tighter allowance governs the leg it applies to regardless of what you were allowed earlier in the day.

Capacity is really a question of laundry

How much clothing you need is not a function of trip length; it is a function of how often you can wash. Beyond about a week, every additional day costs nothing in luggage provided a laundry exists, and a fortnight and a month require nearly the same amount of clothing.

This is the single largest lever available, and it is why experienced long-trip travellers often carry less than people going away for ten days. They have accepted a washing routine as part of the trip rather than treating laundry as a failure of planning.

The counterweight is that some destinations make it hard, whether through climate, the pace of the itinerary or the type of accommodation. Constant movement is the real obstacle, since drying requires being somewhere for a night longer than you planned to be.

What actually drives the size of the bag

Two things dominate: shoes and the range of conditions you are packing for. Footwear is heavy and incompressible, and every additional pair is a meaningful fraction of a cabin bag. A trip spanning a mountain and a coast, or a season boundary, is simply a bigger packing problem than one that does not.

Everything else is small by comparison, which is why the endless optimisation of toiletries and cables produces so little. It is satisfying and it saves grams, while the second jacket saves kilograms.

Outerwear behaves the same way. A coat that covers a wider range of conditions is almost always better value in a bag than two garments that each cover half of it, which is why layering is repeated so often as advice and why it is one of the few pieces of packing advice that survives contact with an actual trip.

It is also worth saying that travelling light is a preference rather than a virtue. Someone with a fixed base, a car, or a medical or professional reason to carry equipment has no obligation to compete at this, and the aim is only to know which of the two constraints you are actually up against.