The old split ends on a date
When a roommate moves out on the 10th, the month has two eras. Era one is the old household, through the last night they were responsible. Era two is the people who remain, through the end of the bill or the lease period you are settling. If you keep charging the old share for the whole month, you are billing someone for a room they no longer have. If you drop their share to zero for the whole month, the people who stayed are giving them a gift they may not have agreed to give. The fair version is a count of days.
Pick the last responsible day in writing. The day the keys were returned is a clean choice. The day they stopped sleeping there is a clean choice if the keys lingered. Two dates create two bills. Choose one and put it in the note. Then count the nights or the days in each era. A 30-day month with a last day on the 10th might be 10 days in the old split and 20 days in the new one. Say which count you are using so nobody brings a different calendar to the argument.
Rent and utilities may use different dates. Rent might follow the lease's proration clause. Utilities might follow the meter period, which often is not a calendar month. Read both. This guide shows the arithmetic. It is not a substitute for the lease or for local rules about notice. If the lease says rent is not prorated, the arithmetic of a courtesy proration is a gift, and you should call it a gift rather than a right you invented.
Prorate, then split
Suppose rent is $2,400 for a 30-day month, three people, equal shares, so $800 each in a normal month. The third person leaves after 10 days. The old era is 10/30 of the rent, which is $800 of rent-days to split three ways, or $266.67 each if you round carefully. 800 / 3 = 266.666..., so cents matter: $266.67, $266.67, and $266.66 add to $800. The new era is 20/30 of the rent, $1,600, now split two ways, $800 each for the people who stayed.
Person A, who stayed, pays 266.67 + 800 = $1,066.67. Person B, who stayed, pays the same if the cents were assigned that way, or one cent different. Person C, who left, pays only the old-era share, about $266.66 or $266.67. Add all three and you must get $2,400. Do that addition before anyone sends money. The Bill Split calculator can run the two eras as two calculations: one with three people on the $800, one with two people on the $1,600. Write both results in the same note.
If the rooms were unequal, prorate the total rent first, then apply the old weights to the old era and the new weights to the new era. Do not prorate each person's old share and also change the weights. That double-applies the change. One order: cut the month into two totals, then split each total with the household that existed during those days.
Deposits, furniture, and the last grocery run
The deposit is not a monthly split. If person C paid a third of the deposit, they do not automatically get it back on the 10th unless the landlord returns it, which landlords usually do not do mid-lease. The people who stay may buy out C's deposit share, or C may wait until the end of the lease, or the landlord may allow a replacement whose deposit replaces C's. Those are three different cash flows. Write which one you are doing. A buyout of $600 should say "$600, paid by the 15th, and C has no further claim on the deposit." Without that sentence, C may ask again at move-out.
Shared furniture and supplies need a similarly dull sentence. If you bought a couch together, either the stayers buy the share, the leaver takes it, or it stays and the leaver is paid an amount you all sign. Pick one that day. A couch that is "we'll figure it out" becomes a group-chat haunting. Groceries already in the fridge are usually not worth a formal split unless someone just bought a large run. If they did, custom amounts for that receipt are enough. Do not let a $40 grocery debate delay the rent math.
Utilities that arrive later should name the dates. "Electric bill covering the 28th through the 27th, C responsible through the 10th." When the PDF arrives, split it with day weights or with two eras if you can see the daily use. If you cannot see daily use, day weights on the total are the honest approximation. Say it is an approximation. An approximation you agreed to is better than a perfect method you cannot measure.
The person whose name is on the bill
If C's name is on the electric account or the lease, the company and the landlord still consider C responsible until the paperwork changes. Your internal split does not rewrite their contract. Start the transfer or the removal on the day you agree, not after the next crisis. The internal note should say who will pay the company during the gap, so a bill does not go unpaid while everyone believes someone else called.
Replacement roommates should see the prorated month before they agree to a "normal" share. Their first month may be the new era only, a partial, or a full month starting on the 1st. Show the dollars. A replacement who is charged a full month for ten days will leave, and you will do this arithmetic again. A replacement who is charged nothing for ten days is a discount the stayers are funding. Both can be chosen. Neither should be accidental.
If nobody replaces C, the stayers' new full-month shares rise. In the equal example, two people on $2,400 pay $1,200 each going forward, not $800. That increase is the real cost of the empty room. If they cannot cover it, the next step is a new roommate, a cheaper place, or a conversation with the landlord. Hoping the empty room will "work out" without a new monthly number is how the following month's rent is late. Put the new $1,200 in the budget calculator as rent for each person who pays it, or as the full rent in the household view, and look at the left-over figure before you promise the landlord you are fine.
Common shortcuts that cost friendships
Charging C the full $800 because "the lease is the lease" may match a no-proration clause, and then you should cite the clause. Charging it because you are angry about the dishes is a different thing, and it will be heard as a penalty. If you want a penalty, it has to be in an agreement you already had, not invented at the door. Anger about chores can be said as chores. It should not be smuggled into the rent cents.
Rounding the proration to the nearest $50 creates a winner and a loser. Use cents. The whole point of a mid-month move is that people are already tense. A $50 shortcut is a new grievance. Another shortcut is deleting C from the chat and guessing. Guessing guarantees a second fight when the utility PDF arrives. Leave C in the note until the last bill that includes their days is settled. Then you can archive the chat like adults.
Do not withhold C's mail or deposit buyout as leverage for a utility share you have not calculated. Calculate, send the rows, and set a date. Leverage feels powerful and makes the buyout late, which makes your own cash short. The order is: dates, two calculations, addition check, payment dates, paperwork with the landlord and the utility.
Leave a record the next household can read
The note for this month should be readable by someone who was not in the room. "C last responsible day Oct 10. Rent $2,400. Old era $800 split three ways. New era $1,600 split two ways. A and B pay $1,066.67. C pays $266.66. Sum $2,400. Deposit buyout separate, $600, due Oct 15. Electric to be split by days when the bill arrives." That paragraph is the whole settlement.
Run the two calculator results and attach them. Future you, if someone disputes the month, will not remember the fractions. You will remember the paragraph if you wrote it the same day. Mid-month move-outs are common. They do not have to be chaotic. They are two small splits and a few sentences about the deposit and the name on the account. Do those while the boxes are still in the hallway, and the next rent day will be a normal bill instead of a sequel.