The easiest way to split bills prorated is to count days: divide the bill by the total number of "person-days" in the billing period, then multiply by the days each person lived there. Someone who was in the flat for half the bill pays half of a full share. It takes five minutes and nobody has to argue about it afterwards.
Picture the 16th of the month. Hana has handed back her keys at noon, and Tomás is wheeling a suitcase up the stairs. Priya looks at the electric bill on the fridge and asks the question every shared flat asks at some point: "So who pays what for this month?"
The day-count method in three steps
First, find the number of days the bill covers. Use the dates printed on the bill, not the calendar month, because many utilities bill from the 8th to the 7th or similar.
Second, count each person's days in that period and add them up. That total is the number of person-days.
Third, divide the bill by the person-days to get a price per day, then multiply by each person's days. Everyone's shares add back up to the bill.
A worked example: one person leaves, one arrives
Here is the 16th-of-the-month scene with real numbers. The electric bill is $186 for a 31-day period. Priya and Marcus live there all month. Hana lives there for days 1 to 15, and Tomás takes her room for days 16 to 31.
The person-days are 31 + 31 + 15 + 16 = 93, which is exactly three people for 31 days. So the price per person-day is $186 / 93 = $2.00.
| Person | Days in the flat | Days counted | Prorated share | Equal split of 4 |
|---|---|---|---|---|
| Priya | Whole month | 31 | $62.00 | $46.50 |
| Marcus | Whole month | 31 | $62.00 | $46.50 |
| Hana | Moves out after the 15th | 15 | $30.00 | $46.50 |
| Tomás | Moves in on the 16th | 16 | $32.00 | $46.50 |
| Total | 93 | $186.00 | $186.00 |
An equal split of four would charge Hana $46.50 for half a month, and charge Priya and Marcus less than they used. The day count is closer to what actually happened.
What if the numbers are not so tidy?
They usually are not. Say the same bill is $190, so the price per person-day is $190 / 93 = $2.043. Priya and Marcus each owe 31 x $2.043 = $63.33, Hana owes $30.65, and Tomás owes $32.69.
Those happen to add up to $190.00, but often they will not. When you are a cent off, pick someone to absorb it, such as the person who paid the bill.
Which day counts as the first day?
This is the part that causes the quiet arguments. Decide on a rule and stick to it. A simple one is: the day you sleep there counts, so the move-in day counts and the move-out day counts only if you slept there that night.
In the example above, Hana's last night was the 15th, so she has 15 days and Tomás has the 16th onwards. Whatever you pick, the two people swapping rooms should never both be charged for the same day.
Bills that do not match the calendar month
If the bill runs from the 8th to the 7th of the next month, that is your period. Count the days from the bill's start date to its end date, and count each person's days inside that window. A 28-day bill and a 33-day bill both work with the same method.
If you pay a fixed amount such as internet, the day count works the same way. Some flats prefer to keep fixed bills equal and only prorate usage bills like gas, electric and water, which is fine as long as everyone knows.
When the day count is not fair enough
A day count assumes everyone used the same amount each day. That is usually close enough, but not always: a winter cold snap in the last week, or a guest who ran the heating all day, can skew things.
If you want something more exact for metered bills, take a photo of the meter on move day and split the used amount between the two readings. A few utilities will also work out a leaver's share for you, so check the account first. Whichever method you use, the outgoing person should settle before they leave, because chasing someone who has moved out is no fun for anyone.
A script to agree it before it matters
Put this in the house chat when a move is coming up, so the rule is agreed before the bill lands.
Hi all, Tomás is moving in on the 16th and Hana is leaving after the 15th. For this month's utilities let's split by days lived, using the dates on each bill. The night you sleep here counts as a day. Hana, can you settle your share before you leave? I'll post the numbers once the bills come in.
Rent too? Same idea, different tool
The same day count works for rent when someone arrives mid-month, though rent usually depends on the room too. If your rooms are different sizes, see how to split rent by room size, then prorate the result by days. For the rest of the house costs, our guide to splitting bills with roommates covers the everyday system.
Doing it without a spreadsheet
In Evens, a flat group can repeat monthly bills on their own, and the "shares" split method lets you enter each person's days (31, 31, 15, 16) so the app works out the rest. Evens is not released yet, but a spreadsheet and the three steps above will get you there in the meantime.
Try the bill proration calculator
Frequently asked questions
How do you prorate utilities for a roommate who moves in mid-month?
Divide the bill by the total person-days in the billing period, then multiply by the days the new roommate lived there. Everyone else is charged for their own days the same way, so the shares add up to the full bill.
Should the day someone moves in count as a full day?
Usually yes, if they sleep there that night. What matters is that your flat agrees on one rule and applies it to both the person arriving and the person leaving.
Do I prorate internet and other fixed bills?
You can, using the same day count, or keep fixed bills equal among whoever is there that month. Neither is wrong, so pick one and tell everyone before the bill arrives.
What if the bill period does not match the calendar month?
Use the dates printed on the bill. Count the days between its start and end dates, and count each person's days inside that window.
Is prorating utilities required by law?
Between roommates it is normally just an agreement, though a lease or the utility account can set its own terms. Rules vary by place, so check local rules and your lease if there is a dispute.