Carry-on packing is less about fitting in everything and more about packing the few things you cannot afford to lose access to.
Check the airline first
Cabin-bag allowances can differ by airline, ticket type, cabin and aircraft. Confirm the exact size, weight and personal-item rules for every flight in your itinerary before you pack.
Keep the essentials with you
Travel documents, medication, valuables and a change of clothes belong close at hand. Carry-on space can run out at the gate, so pack those essentials in the small item that stays under the seat where possible.
Handle batteries deliberately
Airlines commonly require spare lithium batteries and power banks in carry-on baggage, protected against short circuits. Rules can change by carrier and destination; check the airline’s dangerous-goods guidance before travel.
Make security easier
Place liquids and electronics where they can be reached quickly. Security requirements are set locally, and airport rules can differ even on the same journey. When in doubt, consult the departure airport and airline rather than relying on a generic checklist.
Further reading: IATA’s passenger baggage rules.