Utility split calculator: split a bill by days at home
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Someone moved in on the 18th, someone was away for three weeks: enter the bill and the days each person was living there, and everyone pays for the days they were home.
How the math works
Each person pays the bill times their days at home, divided by everyone's days added together. Two people home all 30 days and one home for 12 means 72 person-days: the full-timers each pay 30/72 of the bill and the part-timer pays 12/72. Every share is rounded to the cent and the leftover cents go to the largest remainders, so the shares add up to the bill exactly.
Days work best for usage bills (electric, gas, water). For a flat bill like internet, most houses split evenly between whoever lives there that month, even if someone travels. Agree on which bills are which before the first one arrives, and write it in your roommate agreement.