Someone leaves on the 10th. A new person arrives on the 11th. The rent is due on the 1st and the bills don't care about your calendar. Who pays what?
The cleanest answer is to count days.
The method: prorate by days
- Find the number of days in the month.
- Divide the rent by that number. This is the rent per day.
- Divide the daily rent by the number of people living there that day. That's each person's daily share.
- Multiply by the days each person lived there.
Use the real length of the month: 28, 30 or 31.
Decide the move-day rule up front. Does the day someone arrives count? Does the day they leave? Pick one and keep it for everyone.
Worked example (illustrative numbers)
A 3-bedroom home with three people at a time. Rent is $2,400 for a 30-day month, and the electricity bill is $90.
- Ana and Ben live there all month (30 days).
- Cam moves out after day 10 (10 days).
- Dee moves in on day 11 (20 days).
Rent. $2,400 / 30 = $80 per day. With three people at a time, that's $80 / 3 = about $26.67 per person per day.
- Ana: 30 days = $800.00
- Ben: 30 days = $800.00
- Cam: 10 days = $266.67
- Dee: 20 days = $533.33
Total: $2,400.00.
Electricity. Person-days: 30 + 30 + 10 + 20 = 90. $90 / 90 = $1 per person-day. So Ana $30, Ben $30, Cam $10, Dee $20. Total $90.
When rounding makes the total a cent off, give the extra cent to someone and move on. These numbers are examples. Use your own.
If the rooms aren't equal
Splitting the daily rent equally assumes similar rooms. If Cam's room was the large one, the new person inherits that room and its rent share. Work out each room's share first with the rent splitter, then prorate that share by the days someone was in the room.
Practical points
- Utilities. Bills often cover a period that doesn't match the calendar month. Prorate by the actual billing dates.
- Deposits. Money you paid in as a deposit is a separate matter. Settle it directly between you and check what your lease says. This isn't legal advice.
- Timing. Agree on the numbers before the move, not after.
- Written record. A message in the group chat with the move dates and the final shares is enough.
FAQ
Can the new person just take over the leaving roommate's share? If you want. Prorate the month, then they pay the full share from the next month.
What if someone leaves with no notice? That's a conversation about your lease and your agreement. This guide covers only the split of costs.
Do we prorate internet too? Internet is a fixed bill, so many flats split it equally or prorate it by days. Choose one and be consistent.
Keep the numbers straight
Billmates is a web app, still on the waitlist, that tracks who owes what in a shared home. It never holds or moves your money. You pay each other the way you already do. Roommates join from a browser link. No ads. Leave your email to hear when it's ready.