What a prorated-rent comparison measures
Prorated rent is a partial-period amount derived from a recurring rent amount, a count of chargeable days, and a daily-rate convention. The Prorated Rent Calculator compares one US-style, USD monthly billing period. It does not decide whether rent should be prorated, which convention controls, or whether a tenant or landlord is legally entitled to a charge, credit, or refund.
Formula version prorated-rent@1.0.0 supports two operational questions:
- Move-in: what does each method produce when the entered move-in date through month end is chargeable?
- Move-out: what does each method produce when the first of the month through the entered move-out date is chargeable?
Both modes count the entered boundary date. A separate mid-month-adjustment mode is unnecessary because version 1 models one continuous edge of one month, not a rent increase, concession, possession dispute, or ledger adjustment.
Start with the governing source, not a preferred formula
The four calculator methods are mathematical conventions. None is labeled universally correct, standard, fairest, or legally required. Before calculating, check the lease and amendments, written landlord policy, billing record, applicable state and local law, and any advice from a qualified professional.
Primary law illustrates why that boundary matters. Oregon's official residential-tenancy chapter permits rental-agreement terms that are not otherwise prohibited and also uses day-to-day apportionment in a defined termination provision. California Civil Code section 827 specifies a particular rate treatment for a particular mid-term change. Those are context-specific legal rules, not evidence for a nationwide divisor or day-count rule.
The Oregon Residential Landlord and Tenant Act and California Civil Code section 827 were reviewed from official legislative sources on August 10, 2026. Recheck current primary law and the actual lease for the property and event being modeled.
Calendar dates and chargeable days
The calculator accepts a billing month in YYYY-MM format and a boundary date in YYYY-MM-DD format. The boundary date must be a valid Gregorian date inside the selected billing month.
Move-in mode uses:
Chargeable days = Days in billing month − Move-in day + 1
Move-out mode uses:
Chargeable days = Move-out day of month
Therefore:
- A move-in on the first is a full month.
- A move-in on the last day is one chargeable day.
- A move-out on the first is one chargeable day.
- A move-out on the last day is a full month.
- February uses 28 days in a common year and 29 in a Gregorian leap year.
- April, June, September, and November use 30 days; the other months use 31.
Dates are calendar identifiers, not timestamps. DealYield parses the entered year, month, and day directly, so a browser timezone or daylight-saving transition cannot shift a boundary into another date.
Version 1 does not accept a range that crosses a month boundary. Split a cross-month occupancy period into one calculation per billing month. This prevents a daily convention from silently mixing two month lengths and keeps each result traceable to one monthly rent amount.
Actual days in month
The actual-days convention derives the daily rate from the selected calendar month:
Actual-days daily rate = Monthly scheduled rent ÷ Calendar days in billing month
Actual-days charge = Actual-days daily rate × Chargeable days
The divisor can be 28, 29, 30, or 31. A February daily rate is therefore different from a July daily rate even when monthly scheduled rent is unchanged.
DealYield uses actual-days as the neutral comparison baseline only. It is not presented as the recommended or legally controlling method.
Fixed 30-day convention
The 30-day convention uses:
30-day daily rate = Monthly scheduled rent ÷ 30
30-day charge = 30-day daily rate × Chargeable days
The divisor remains 30 in every selected month, but chargeable days still follow the real inclusive calendar count. That means a partial 31-day month can have as many as 30 chargeable days without being a full month.
For a full chargeable month, DealYield returns the entered monthly rent directly. It does not multiply a 30-day rate by 31 or reduce February's full rent to 28 or 29 daily units.
Annual/365 convention
The annual/365 convention annualizes monthly rent and divides by a fixed 365:
Annual/365 daily rate = (Monthly scheduled rent × 12) ÷ 365
Annual/365 charge = Annual/365 daily rate × Chargeable days
The divisor stays 365 during a leap year. The calculator warns when the selected year has 366 calendar days so that the fixed convention remains visible. As with the other methods, a full chargeable month returns monthly scheduled rent.
Custom lease convention
The custom method accepts an explicit USD-per-day rate:
Custom charge = Entered lease-defined daily rate × Chargeable days
It does not accept a divisor, percentage, free-form formula, or amount for the entire partial period. A direct daily rate is finite and auditable: the calculator can use exactly what a lease, addendum, written policy, or other governing source states without trying to interpret that source.
The custom rate is optional. Leave it blank when no supported daily amount exists. DealYield does not infer one from another field, decide whether the source applies, or validate enforceability.
Worked move-in example
The default example uses:
- Monthly scheduled rent: $1,800.00
- Billing month: February 2028
- Inclusive move-in date: February 12
- Custom lease daily rate: $58.50/day
February 2028 has 29 days. Counting February 12 through February 29 gives 18 chargeable days.
Method | Daily rate before charge rounding | Prorated charge | Difference from actual-days |
|---|---|---|---|
Actual days in month | $62.068966/day | $1,117.24 | $0.00 |
30-day convention | $60.000000/day | $1,080.00 | −$37.24 |
Annual/365 | $59.178082/day | $1,065.21 | −$52.03 |
Custom lease rate | $58.500000/day | $1,053.00 | −$64.24 |
The lowest modeled charge is $1,053.00, the highest is $1,117.24, and the method spread is $64.24. That spread measures the consequence of changing conventions while holding rent and days constant; it does not rank the methods.
The same date in move-out mode
If February 12, 2028 is instead an inclusive move-out date, the calculator counts February 1 through February 12: 12 chargeable days. With the same rent and custom rate, the charges are:
- Actual days in month: $744.83
- 30-day convention: $720.00
- Annual/365: $710.14
- Custom lease rate: $702.00
The calculator also shows Full monthly rent − modeled charge. This is a remaining-period arithmetic difference. In move-out mode it may help reconcile a prepaid full month, but it is not labeled or treated as an owed credit or refund. Payment status, possession, lease language, notices, and applicable law remain outside the calculator.
Rounding and intermediate precision
Each derived daily rate is calculated without intermediate rounding. DealYield multiplies that exact rate by chargeable days and then rounds the charge half-up to the nearest cent. Daily rates are displayed to six decimal places for review.
Method differences use the already rounded cent charges, which keeps the table internally reconcilable. The full-month rule returns monthly scheduled rent exactly. Inputs must be finite, non-negative amounts with no more than two decimal places; invalid values pause the result and are not saved, shared, exported, or placed in a report.
Full-month, single-day, zero, and month-end cases
Check boundary cases before relying on a scenario:
- Full month: move-in on day 1 or move-out on the last day returns monthly rent for every included method, with no proration.
- Single day: move-in on the last day or move-out on day 1 multiplies each method's daily rate by one.
- Zero rent: $0 monthly rent produces $0 for the three derived methods. A positive custom daily rate cannot be combined with zero monthly rent.
- Invalid range: a boundary outside the billing month is rejected rather than clipped or moved.
- Leap day: February 29 is accepted only in a Gregorian leap year; annual/365 still uses 365.
- Month end: the calendar's actual final day controls the full-month and single-day boundary tests.
- Cross boundary: a single calculation cannot span two months; calculate each month separately with the rent and governing terms that apply to that month.
Use the comparison as an operational check
A method-comparison table is more useful here than a generic sensitivity grid. Each row preserves the same rent, mode, month, date, and chargeable-day count, then changes only the daily-rate convention. Review:
- The selected move-in or move-out mode.
- The inclusive boundary date and chargeable-day count.
- The daily rate under each supported convention.
- Each prorated charge and its difference from actual-days.
- The full-rent difference and total spread across methods.
- The lease, current legal sources, and billing evidence that determine which—if any—modeled amount applies.
Do not select the lowest or highest amount merely because it favors one party. Keep a record of the governing source and any required rounding instructions outside DealYield.
What version 1 excludes
The calculator does not provide or manage:
- State or local legal determinations or a dataset of jurisdiction-specific rules
- Lease interpretation or a recommendation of the “correct” proration method
- Rent collection, payment posting, tenant ledgers, notices, or property-management workflows
- Late fees, security deposits, concessions, utilities, tax handling, or other charges
- Possession-time disputes, holdover rent, lease breaks, or damages
- Automatic credits, refunds, balances due, or legal entitlements
- Multi-month ranges, changing monthly rent within a calculation, or partial-day occupancy
- Property labels, scenario names, workspace identifiers, rent amounts, or calendar dates in analytics payloads
Use the Rental Property Cash Flow & ROI guide for a broader rental investment model, the NOI guide for property operating income and expenses, and the Break-Even Rent and Occupancy guide for required rent or collection assumptions. Those published tools answer different questions and do not decide the proration method for a lease.
Prorated-rent results are educational arithmetic comparisons. Verify the lease, billing history, possession dates, current state and local law, and any charge, credit, or refund with reliable primary sources and appropriately qualified professionals.