Methodology & sources

What these tools calculate

Rentcalc performs arithmetic on the values you enter. The tools estimate partial-period charges, rent changes, concessions, selected fee structures, property cash flows and a three-times-rent income ratio. Editable letters organize the facts you enter. They do not look up a lease, apply local laws, check income evidence or determine an entitlement.

The calculator definitions and examples below are versioned as of October 25, 2026. External references provide context, while these explicitly stated conventions determine the outputs.

Proration: count calendar days first

Let M be monthly rent, D be the number of days in the selected month, and d be the selected day of the month. With the move date included, move-in counts D − d + 1 days and move-out counts d days. Excluding that date changes the counts to D − d and d − 1.

The calculation stays within one calendar month. An excluded last-day move-in or an excluded first-day move-out produces zero days. Dates are parsed as Gregorian year, month and day values without local time zones or daylight-saving time conversion. Years from 1900 through 9999 are accepted.

Three different daily-rate methods

• Actual month: daily rate = M ÷ D. D can be 28, 29, 30 or 31.
• 365-day year: daily rate = M × 12 ÷ 365. The denominator remains 365 in a leap year. There is no monthly cap.
• 30-day month, capped: daily rate = M ÷ 30. Count actual occupied calendar days, multiply by that rate, then cap the total at M.

The third option is our specific fixed-denominator convention. The name “30-day” is used for other conventions too; this tool does not use a financial 30/360 date-adjustment algorithm or automatically replace February's occupied days with 30. Verify that the chosen convention matches the calculation you need.

Final proration = daily rate × counted days; apply the 30-day cap only when selected

Displayed daily rates have four decimal places. The calculation uses the unrounded rate and rounds the final amount to cents. Rounding a daily rate first may yield a different total.

Rent-change formulas

For current rent C, new rent N and percentage change p:

N = C × (1 + p ÷ 100)
p = (N − C) ÷ C × 100
Annual difference = (N − C) × 12

Current rent must exceed zero because the reverse percentage calculation divides by it. Negative changes are accepted if new rent remains at least zero. The annual difference assumes 12 complete months at the changed rate. No midyear start date, compounding, discount or other charge is included. Amounts are calculated before display rounding; sub-cent changes can therefore produce an annual figure different from twelve times the displayed monthly figure.

Three-times-rent formulas

The income calculator chooses gross income, before deductions, as its input definition. It compares monthly rent with gross monthly income at a ratio of exactly three.

Monthly income = monthly rent × 3
Annual income = monthly income × 12
Maximum monthly rent = monthly income ÷ 3

Maximum rent is rounded down to cents so the displayed maximum does not exceed the exact quotient. This is only a ratio test. Tax deductions, debts, utilities, variable income, household needs, other screening requirements and local rules are outside the model.

General date-range proration

The general prorate calculator divides any full-period amount by the counted days in that period, then multiplies by the counted service days. Both ranges use the same selected end-date convention: include both endpoints, or include the start and exclude the end. Service dates must be ordered and inside the full period. Calendar days count equally, including weekends; the tool does not implement payroll workday rules.

Partial amount = full amount × service days ÷ full-period days

The final result is rounded to cents. A full included period returns the full amount; a zero-day service interval is possible only with an excluded end date. See the separate move-out rent tool when the agreement instead uses a monthly rent denominator.

Concessions and entered late fees

The net effective rent tool subtracts monthly rent multiplied by free-rent months, plus any additional rent credit, from the gross lease rent. It divides that net cost by the complete lease-month count. Credits cannot exceed gross rent. This average does not change the payment schedule or spread a concession automatically.

The late fee tool counts calendar days after the due date, subtracts the grace days you enter and floors the chargeable count at zero. The selected fee is charged once as a flat amount, once as a percentage of overdue principal, or per chargeable day. An optional entered cap limits that calculated fee. It does not compound, combine structures or charge against a zero principal.

These are calculator conventions, not a recommended fee or a determination of enforceability. Read the fee comparison guide and annual increase explanation before treating an illustrative percentage as market evidence.

Rent versus sell: one consistent horizon

The property comparison starts both options from the same entered home value and loan balance. Selling now deducts the chosen selling-cost percentage and mortgage balance; the resulting proceeds compound at the entered investment return. The rental option projects the home value, amortizes the mortgage using APR divided by twelve, and collects entered rent less a vacancy allowance, operating costs and the actual mortgage payment.

At each month end, rental cash flow enters a comparison account earning the same investment return. A negative account represents cumulative contributions and their opportunity cost, not an actual loan. At the selected final year, the model subtracts the same percentage selling costs and remaining mortgage from the projected property value, then adds the rental account. It compares that total with the sell-now account.

Annual home-value and investment-return assumptions become effective monthly rates, (1 + annual rate)^(1/12) − 1. Rent and operating costs stay fixed; the payment stops after the loan is repaid. Negative amortization and selling now with insufficient mortgage payoff proceeds are not modeled. Taxes, depreciation, capital gains, inflation and unentered repairs are excluded. No result is a forecast or recommendation.

Letters and state guidance

The notice draft subtracts current rent from proposed rent; it does not calculate a lawful effective date. The return accounting uses integer cents: deposit + entered credits − prior refunds − listed deductions. If deductions exceed the funds, it reports zero remaining refund and the excess separately, without establishing a debt. The request letter distinguishes the unrefunded amount from the amount legally owed.

Text is generated locally and remains editable. A downloaded or printed letter is not sent. Check the parties, figures, supporting documents and local requirements. The California increase guide and California deposit guide cite the DRE 2026 handbook. General tools do not silently apply those state rules to other users.

Validation and reproducibility

The forms reject missing inputs, impossible dates, negative rent or income, and nonfinite numbers. Rent and income fields accept up to $1 billion as a technical input limit, not a market statistic or legal threshold. Money inputs use cents; the rent-change input accepts finer precision so you can test percentage changes.

The formulas live in pure TypeScript modules and have automated tests for ordinary examples, calendar boundaries, leap years, zero-day periods, invalid inputs, reductions and reverse calculations. Tests verify the implemented conventions; they do not prove that a particular lease uses those conventions.

To reproduce an output, record the exact inputs, mode, date-inclusion setting and method. The worked examples provide independent arithmetic you can follow with a basic calculator.

Corrections and updates

If an output disagrees with these published formulas, send a correction request with hypothetical inputs and the expected result. State guidance is published only where dated official text has been verified. It is not automatically current for every locality. Review dates record a source check, not a promise of a continuous legal-data feed.