RoomProrateDaily split and person-days: two agreements, two answers
Updated 2026-10-20
When everyone lived there for the whole bill, an equal split is simple. Once someone moves in or out mid-cycle, households usually pick one of two methods. They give different results, and neither is the one correct answer; they describe different ideas of what is fair.
Daily split
The usage charge is divided evenly over the billed days. Each day's amount is then shared by whoever lived there on that day. A day with two people costs each of them half of that day; a day with three costs each a third.
Person-days
Add up everyone's days in the period. Each person pays the same share of the usage charge as their share of those person-days. Someone there for 30 of 45 person-days pays two thirds.
Example 1: one person moves in halfway
April electricity usage is 120.00 over 30 days (fictional). A lives there all month; B moves in on April 16 and is there for 15 days.
| Person | Days | Daily split | Person-days |
|---|---|---|---|
| A | 30 | 90.00 | 80.00 |
| B | 15 | 30.00 | 40.00 |
Daily split: April 1 to 15 cost 60.00 and only A was there; April 16 to 30 cost 60.00, shared 30.00 each. Person-days: 30 + 15 = 45 person-days, so A pays 30/45 and B pays 15/45 of 120.00.
The difference comes from one assumption. The daily split treats each day's cost as belonging to that day. Person-days treats the bill as one pool to be shared by time spent there, so B pays a little towards the days before arriving.
Example 2: three people overlapping
June usage 90.00 over 30 days. Ana is there all month, Ben from June 1 to 20, Cy from June 11 onwards.
| Person | Days | Daily split | Person-days |
|---|---|---|---|
| Ana | 30 | 40.00 | 38.57 |
| Ben | 20 | 25.00 | 25.72 |
| Cy | 20 | 25.00 | 25.71 |
With person-days, 90.00 × 20 / 70 is 25.714... for both Ben and Cy, so one cent is left over after rounding. It goes to one of them by a fixed tie rule; see rounding.
Example 3: empty days change everything
A leaves after April 10 and B arrives on April 21. Usage is 120.00.
- Daily split: A 40.00, B 40.00, and 40.00 unassigned for the ten empty days.
- Person-days: A 60.00 and B 60.00. The empty days are paid by the two of them.
If the empty days still used power, for example a fridge left running, someone has to pay for them. The daily split makes that a visible decision; person-days decides it for you.
How to choose
- Pick daily split if the cost of a day should follow the people who were there that day, and you want empty days settled openly.
- Pick person-days if you see the bill as one shared pool and want a simple rule that always assigns the whole amount.
- Whichever you choose, apply the same method to every bill in the period, and decide before anyone sees the totals.
Neither method measures who actually used more. If one person runs a heater all day, that is a separate conversation.
Open the calculator or see all guides.