Handling shared bills when one roommate travels a lot
Rent, internet, and most household basics are fixed; a traveler usually continues to share them. Variable costs, such as a personal utility overage or groceries, may justify a different rule if everyone agrees in advance.
Set the rule before the trips begin. Define what counts as an absence, whether a subletter changes the split, and how the traveler will pay while away. Keep the system simple enough to use every month.
Fairness is not a precise calculation of every day spent elsewhere. It is a known rule applied consistently.