About RoomProrate

RoomProrate is a free calculator for splitting shared utility bills when people did not all live in the home for the whole billing period. It is made and run by Eddy Hwang, an independent developer. It has no connection to any utility, landlord or payment service.

Why it was made

Someone moves out on the 12th, a new roommate arrives on the 20th, and the electricity bill covers the 3rd to the 2nd. Several calculators already split a total by "days present", but you have to count those days by hand from the dates, decide what to do with the fixed fee yourself, and accept whatever happens to the odd cents. In shared homes, those are exactly the details that start arguments.

RoomProrate was made to take dates as input and give working as output: it counts the days from the move-in, move-out and billing dates, keeps fixed charges apart from usage, shows both common methods, leaves the cost of empty days visible instead of hiding it, and proves that the shares add up to the bill.

How it calculates

• All dates are calendar days counting both ends, so daylight saving changes cannot shift a day.
• Amounts are converted to whole minor units (cents, or yen and won) and calculated with exact integer fractions, not floating point.
• Rounding happens once, by the largest remainder method, so people plus unassigned always equals the bill.

How it was tested

Automated tests cover leap years, month ends, daylight saving dates, partial overlaps, empty days, a single resident, zero amounts, one cent shared by three people, currencies with and without decimals, rounding ties, one thousand random bills checked for exact totals, backup files that are damaged or hostile, and spreadsheet formula injection in exported names. Examples on this site are fictional.

Sources

Explanations of billing periods, standing charges and unit rates quote Octopus Energy's public help pages (UK). Billing practices vary by supplier and country.

Questions or a case the calculator handles badly? See contact.