Daily Proration Calculator for Rent, Salary and Subscriptions
Enter a monthly amount and either a number of days or a start and end date to get the prorated amount. It handles leap years and periods that cross month ends, and lets you choose the day-count basis (days in the month, 30 days, or annualized over 365 days) and how to round. For move-in costs in Japan, see the Rental Initial Cost Calculator; to count days between two dates, use the Date Calculator.
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Proration settings
How to use
- Choose a "Use case" and enter the "Monthly amount".
- In "Period input", choose whether to enter a number of days or pick start and end dates.
- Set the "Proration basis" and "Rounding" to match your contract or payroll rules.
- The prorated amount and its breakdown appear. Use "Copy result" to copy the amount.
Notes
- Proration bases and rounding differ by lease, employment contract and service terms. This result uses common methods as a guide and does not guarantee any billed amount.
- Both the start date and the end date count as full days (for example, Jan 16 to Jan 31 is 16 days).
- With the calendar basis, a period that spans several months is split by month (days / days in that month), summed, and rounded once at the end.
- The annualized basis uses monthly x 12 / 365 per day, and divides by 365 even in leap years.
- Payroll proration varies by employer: some divide by scheduled working days, others by calendar days. Check your employment rules for absence deductions.
- Taxes and social insurance are not included. Amounts are rounded to whole units, which suits yen; results for currencies with decimals are a rough guide.
Glossary
Proration
Splitting a monthly amount so you pay or receive only for the days used. It applies when you move in or out, start or leave a job, or cancel partway through a month.
Calendar-day basis
Dividing the monthly amount by the actual number of days in that month (28 to 31), so the daily amount changes from month to month.
30-day basis
Treating every month as 30 days. It is common for prorated rent in Japan and keeps the daily amount the same each month.
Annualized basis
Multiplying the monthly amount by 12 and dividing by 365 to get a daily amount that stays constant all year.
FAQ
Should I divide by 30 days or the days in the month?
No law fixes one method; it depends on your contract. Prorated rent in Japan often uses 30 days or the actual days in the month, and some employers divide salary by scheduled working days. Check your agreement and pick the same basis.
Are the start and end dates counted?
Yes, both are counted as full days. January 16 to January 31 is 16 days. If your contract excludes the move-in or cancellation day, shift the date by one day before entering it.
How is a period across several months calculated?
With the calendar basis, each month is calculated as days / days in that month and the results are added up. February in a leap year uses 29 days. The 30-day and annualized bases use the total day count.
When is the rounding applied?
Once, after all the monthly parts are added together. Nothing is rounded along the way, so errors do not build up across months. The amount before rounding is shown in the result too.
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