KokoroFlow · Household calculator
Partial-Month Utilities Split Calculator
Someone moved in on the 12th. Someone else left before the bill arrived. This works out each person's fair share based on the days they actually lived there — and handles fixed service charges separately, since those don't depend on who was home.
Bill details
Who lived there
A worked example
An electricity bill for a 30-day cycle comes to $184.60, of which $24.00 is a fixed service charge. Three roommates share the flat:
- Alex — there all 30 days
- Sam — there all 30 days
- Jordan — moved in on the 19th, so 12 days
First, separate the usage pool: $184.60 − $24.00 = $160.60. Total person-days: 30 + 30 + 12 = 72.
Sam = $160.60 × (30 ÷ 72) = $66.92
Jordan = $160.60 × (12 ÷ 72) = $26.77
Then add the fixed charge, split three ways at $8.00 each:
- Alex: $66.92 + $8.00 = $74.92
- Sam: $66.92 + $8.00 = $74.92
- Jordan: $26.77 + $8.00 = $34.77
Compare that to a naive three-way split of $61.53 each — Jordan would have overpaid by nearly $27 for a flat they lived in for less than half the cycle.
Limitations
- It assumes everyone uses roughly the same amount per day. Days lived is a proxy for usage, not a measurement of it. Someone working from home all winter genuinely uses more heating than a housemate who's out twelve hours a day.
- Seasonal bills aren't evenly distributed within the month. A cold snap in the first week means those days cost more than days at the end — day-count splitting can't see that.
- Whether the fixed charge should be split evenly is a judgement call. Splitting it evenly among current occupants is the common convention, but if someone was on the lease for only part of the cycle, you may prefer to prorate it too.
- It doesn't decide who's legally liable. Whoever's name is on the account owes the utility company in full regardless of what this says; this is for settling up between housemates.
- Guests, subletting, and empty rooms aren't modelled. Add them as their own line if you want them counted.
- Rounding means individual shares may differ from the total by a cent or two.
Frequently asked questions
Should the move-in day count as a full day?
Pick a convention and apply it to everyone. Counting the move-in day and not the move-out day (or the reverse) is simple and avoids double-counting the changeover day when one person replaces another.
Why separate the fixed charge instead of splitting everything by days?
Because the standing charge is paid for the connection existing, not for anything being used. If you prorate it by days, a housemate who was away for three weeks pays almost nothing toward keeping the account open — which most people find unfair. If you disagree, set the fixed field to 0 and everything splits by days.
What if the bill period doesn't match the calendar month?
That's exactly what the "days in billing period" field is for. Use the dates printed on the bill, not the month, and count each person's days within that window.
Someone used far more than everyone else. Can I adjust for that?
Not directly — this splits by time, not by consumption. The practical workaround is to agree a flat surcharge for the heavy user and enter the remainder here, or to give them more "days" as a rough weighting. Both are negotiations rather than calculations.
Can I use this for internet, water, or gas as well?
Yes. Any bill with a usage component and an optional standing charge works the same way. For internet, where nearly the whole bill is fixed, you may find putting the entire amount in the fixed field and splitting evenly is closer to fair.