Test the full weight and the pocket
Fill a bottle you already own and take it on an ordinary walk. Notice whether you drink the amount you carry and whether the weight makes you leave it behind. The right capacity depends on conditions and personal needs; this is not a reason to restrict water when more is needed, but a way to plan refills realistically.
Put the full bottle in the daypack's intended pocket. Check that it does not slip out when you bend or push uncomfortably against your back. A tall bottle can change the balance of a small bag, while a wide one may fit only in the main compartment. Keep liquids separated from electronics wherever possible.
Look closely at the closure
Choose a lid you can open and close securely with the hands you will have free. A drinking spout may be convenient while moving, but its seals and small parts need cleaning. Test the bottle for leaks before travel, including on its side, rather than trusting a new closure without checking it.
Consider whether a narrow neck makes cleaning difficult with the equipment available during the trip. Follow the manufacturer's cleaning instructions and allow the bottle to dry when not in use. Do not assume a reusable bottle is automatically suitable for hot drinks or other liquids unless its specifications say so.
Know where the next refill comes from
Look for confirmed drinking-water points at stations, museums or accommodation, and check local guidance on tap-water suitability. A decorative fountain is not a refill station unless it is identified as safe for that purpose. On a route with few facilities, plan accordingly rather than choosing a tiny bottle solely to reduce luggage.
Airport screening can require the bottle to be empty, depending on the location and its rules. Make the bottle easy to remove and refill it afterward from a suitable source. At the end of the trip, clean the lid and seals rather than simply rinsing the body. A durable bottle with a manageable shape and a dependable cleaning routine is more useful than one selected only for its insulation claims or appearance.
