Prorate Calculator

Compare two prorated estimates for the same monthly amount and inclusive date range.

Days used: 16

Days by month: 2026-10: 16 of 31 days

Actual calendar-month basis: 516.13 USD

Fixed 365-day annual basis: 526.03 USD

Difference (365-day minus actual-month): 9.90 USD

Estimates only. Your agreement and local requirements may use a different method; these results do not determine what you owe.

Sources

Zillow — Prorated rent calculator

Reference for prorating by the actual number of days in the month.

Calculation rules

Actual calendar-month basis = the sum, for each calendar month, of monthly amount × days used in that month ÷ days in that month.

Fixed 365-day annual basis = monthly amount × 12 × days used ÷ 365. The denominator stays 365 in leap years.

Days used counts both the start date and the end date. Dates from 1900 to 2099 are supported.

Amounts are calculated exactly and rounded once to cents at the end, with halves rounded away from zero.

Frequently asked questions

Are both dates included?: Yes. A single date counts as one day.
What happens across months?: The actual-month calculation allocates each day to its calendar month and uses that month’s length.
Does the annual basis change in a leap year?: No. This comparison deliberately uses a fixed 365-day denominator.
Why do the methods differ?: Calendar months have different lengths. The annual comparison spreads twelve monthly payments over 365 days.

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Calculations run in your browser without sending your calculation inputs or results to the server. See “Privacy on this page” for information about access data.