A tenant takes possession on May 18. Rent is $2,200 a month. What do they owe for May?
Open three different online prorated-rent calculators, plug in the same numbers, and you will get three different answers. The spread is usually $30 to $100 on a single move-in, sometimes more when February is involved. None of the calculators is wrong. They are each using a different method, and not saying so.
That mismatch is also the reason most prorate disputes happen. The tenant pulled out their phone, calculated one number. The landlord ran payroll-style software that produced another. The lease said nothing about how to settle the gap. Now there is a $94 argument that has to be resolved by a small-claims judge or, more often, by the side that gives up first.
This article is the resolution. It explains the three methods, gives you a calculator that runs all of them on the same scenario, and lays out the cheat sheet for which one applies when.
Why “prorated rent” is harder than it sounds
There are three legitimate ways to convert a monthly rent into a partial-month charge:
- Banker’s 30-day: treat every month as if it has 30 days.
- Actual days in that month: divide the monthly rent by the number of days that month actually has.
- Annualized 365 (or 366): annualize the rent and divide by the number of days in the year.
All three are in active use. None is universally “correct.” The right one depends on:
- Whether your lease specifies a method (it usually doesn’t).
- Whether your state has a default rule by statute or case law (a handful do).
- Whether the property is in a federally subsidized program with its own rule (HUD HAP contracts are explicit).
- The convention of the market you operate in.
Most property management software defaults to Method A. Most state statutes that address the question default to Method B. Most commercial leases use Method C. Most residential leases say nothing at all.
So three landlords using three different tools, on the same move-in, will produce three different first-month rent invoices, all defensible. That is the trap.
The three methods, explained
Method A: Banker’s 30-day
The formula:
Prorated rent = (Monthly rent ÷ 30) × days occupiedEvery month gets 30 days, regardless of whether it actually has 28, 29, 30, or 31. The per-diem is the same every month, year-round.
For a $2,200 rent, the per-diem is $73.33. Move in on the 18th of a 31-day month, charge for 14 days (the 18th through the 31st): $1,026.67.
Why it’s popular: simplicity. The per-diem is a single number you can memorize for a property. It does not change month to month. Most national property management platforms (Buildium, AppFolio, Yardi defaults) use this method out of the box. Federal HUD Housing Assistance Payment (HAP) contracts also use a 30-day month for partial-month proration.
The hidden tradeoff: in a 31-day month, the tenant gets one day “free” (you charge 14 days of a 14-day stay but at the 30-day rate). In February, the tenant overpays slightly. A 28-day February at 30-day per-diem means the per-diem is higher than the actual daily share. This evens out across a year, but landlords and tenants don’t think in annualized averages. They think in next month’s rent invoice.
Method B: Actual days in month
The formula:
Prorated rent = (Monthly rent ÷ days in that month) × days occupiedFor the same $2,200 rent in May (31 days), the per-diem is $70.97. Move in on the 18th, charge for 14 days: $993.55.
The per-diem changes each month. In February (28 days), the same $2,200 rent has a $78.57 per-diem. In June (30 days), it’s $73.33. The tenant always pays exactly proportional value: half a month of occupancy equals half a month of rent.
This is the most arithmetically “neutral” method. It’s also the method most state statutes that address proration point at, and the method most reasonable people (the kind that end up on small-claims juries) assume is correct when no one has told them otherwise. California Civil Code §1947 has been interpreted along these lines; Massachusetts and a handful of other states reach similar results through case law.
The tradeoff: the per-diem moves around. For an owner-operator with one or two properties, that’s a non-issue. For a larger portfolio running automated billing, it adds a small amount of operational complexity if the software isn’t set up for it.
Method C: Annualized (365 or 366)
The formula:
Prorated rent = (Monthly rent × 12 ÷ days in year) × days occupiedFor the same $2,200 rent (× 12 = $26,400 annual), the per-diem in a 365-day year is $72.33. Move in on the 18th of May, charge for 14 days: $1,012.60. In a 366-day leap year, the per-diem drops slightly to $72.13.
This is the most precise method over a full year: you charge exactly $26,400 to a tenant who pays rent for all 365 days, no matter how the days fall across months. Annualization is standard in commercial leasing, where lease terms often start mid-month and run for an odd number of years.
In residential, it’s rare. It produces an in-between number that doesn’t match either the tenant’s intuition (Method B) or the software’s default (Method A), so it tends to be the worst of both worlds for residential disputes: defensible, but no one’s expecting it.
Run the numbers on your scenario
The fastest way to see the spread is to plug your numbers in. Change the date to a 28-day or 31-day month and watch what happens.
For a $2,200 May 18 move-in, the three methods land at roughly $993, $1,013, and $1,027. That’s a $34 spread. Push the same scenario into February (28 days, 11 days occupied from the 18th) and the spread between Method A and Method B widens, because the per-diem on a short month under Method B is much higher than the 30-day banker’s per-diem under Method A. February is where the choice matters most.
How to choose the right method
The decision flow is short:
- Does the lease specify a method? If yes, that controls. Use it. End of analysis.
- Is this a federally subsidized unit (Section 8 HAP, project-based subsidy, LIHTC compliance file)? The program’s contract or compliance manual will tell you which method applies. HUD HAP contracts typically use Method A (30-day month).
- Does your state have a default proration rule? A small number of states do, either by statute or settled case law. California, Massachusetts, and Oregon all have authority pointing at Method B as the default. If you’re in one of these states and your lease is silent, Method B is the safe default.
- None of the above? Pick one, write it into the lease going forward, and apply it consistently across move-in and move-out. Method A is the simplest. Method B is the most defensible to a small-claims judge. Method C is rarely the right answer in residential.
The single most expensive mistake is inconsistency: proration by Method A on the way in, Method B on the way out, because two different people did the math with two different tools. The tenant will spot it. The judge will not be impressed.
For the full state-by-state default landscape on related security deposit and notice questions, see the security deposit laws US overview. Many of the same statutes that govern deposits also govern partial-month rent.
Move-in vs move-out: a small but consequential difference
The math is the same; the day count is different.
Move-in: you charge from the move-in date through the last day of the month. If a tenant moves in on May 18 in a 31-day month, that is 14 days (the 18th, 19th, 20th, … the 31st, inclusive).
Move-out: you charge from the first day of the month through the move-out date. If a tenant moves out on May 18, that is 18 days (the 1st through the 18th, inclusive).
Two boundary-day pitfalls:
- Don’t double-count the transition day. If a tenant moves out on May 18 and the new tenant moves in on May 18 (same day turnover, common for back-to-back leases), only one of them pays for the 18th. The lease has to be explicit about which.
- Mid-month rent due dates. Not every lease starts rent on the 1st. If rent is due on the 15th and a tenant moves in on the 22nd, the “partial period” runs from the 22nd through the 14th of the following month (about 24 days), and the math has to follow the rent cycle, not the calendar month. This is where most software defaults get it wrong.
A move-in or move-out record that shows the formula in plain English (not just a dollar total) defuses both pitfalls before they become disputes. That’s the entire premise of a structured move-in record and a court-ready move-out inspection.
The five most expensive prorate mistakes
1. Charging a full month and prorating. Some landlords collect a full first month’s rent at lease signing, then “prorate” the second month without clearly explaining the credit. The tenant sees a full month bill, a partial month bill, and assumes they’re being charged twice. If the lease doesn’t spell out the order, the tenant is often right.
2. Different methods at move-in and move-out. The most common version: Method A at move-in (the property manager used the PMS default), Method B at move-out (the new property manager used a calculator). The tenant ends up paying for more days than they occupied, by a small but visible margin. A signed move-out checkout form that shows the formula prevents this.
3. Not prorating ancillary charges. Pet rent, parking, utilities, HOA pass-throughs are usually monthly, and they should usually be prorated too, on the same method as base rent. If the lease bundles them, prorate the bundle. If it itemizes them, itemize the prorate.
4. Wrong day count on the boundary day. Off-by-one errors are the most common math mistake. Use the inclusive count: from the 18th through the 31st is 14 days, not 13. (31 − 18 + 1 = 14.)
5. No formula in writing. The dollar amount alone is unverifiable. The formula (“$2,200 ÷ 31 × 14 = $993.55, for occupancy May 18 through May 31”) is auditable. Every move-in and move-out record should show the formula. That single line resolves most disputes before they escalate.
Writing the prorate into the lease and the record
The lease should say two things:
Sample clause. Rent for any partial calendar month shall be prorated based on the actual number of days in that calendar month, calculated as: (Monthly Rent ÷ Days in Calendar Month) × Days Occupied. The same method shall apply at both move-in and move-out.
That’s it. Two sentences. The first locks in the method. The second prevents the move-in/move-out inconsistency that drives most disputes.
The move-in record (and the move-out record) should then show the calculation, not just the total:
Prorated rent for May 2026: $2,200 ÷ 31 (days in May) × 14 (days occupied, May 18 through May 31) = $993.55.
Tenant signs the record acknowledging the calculation. The signed line is what holds up if the prorate ever comes up later: at lease renewal, at move-out reconciliation, or in front of a judge.
For the broader playbook on what else belongs in a move-in record, see the move-in records complete guide. For the move-out side, see the move-out security deposit complete guide.
Common prorate questions
Can I charge a full month and credit the next month instead of prorating the first month? You can, as long as the lease is explicit about it and the tenant agrees in writing. This is sometimes done so the first invoice is “clean,” but it has to be disclosed clearly or it looks like an overcharge. In states with strict first-month-rent rules (a few have caps tied to security deposit limits), it can also create a compliance issue.
What if the tenant moves in on the last day of the month? They owe one day of rent for that month. For example, $2,200 ÷ 31 × 1 = $70.97 in May under Method B. Some landlords waive the one day as a goodwill gesture and start rent on the 1st. Either is fine; write it down.
Do I have to prorate the security deposit too? No. Security deposit amounts are independent of the prorate calculation. They’re typically tied to a multiple of the monthly rent and don’t change based on the move-in date. See the security deposit laws US overview for caps and timing.
What about the last month’s rent if I collected it at lease signing? If you collected one month’s rent as “last month’s rent” at lease signing, and the tenant moves out mid-month, you prorate the last month’s rent and refund the difference. The proration method should match what you used at move-in.
Does the calculator handle leap years? Yes. The annualized method (Method C) uses 366 days in 2024, 2028, etc., and 365 in non-leap years. The shift is small (about 20 cents on a typical move-in) but real.
Is the per-diem the same as the prorated rent? No. Per-diem is the daily rate (rent ÷ some divisor). Prorated rent is per-diem × number of days occupied. The per-diem is constant within a method; the prorated rent depends on how many days you’re charging for.
The closing thought
Prorating rent isn’t complicated math. It’s a disclosure problem. Three legitimate methods exist; the lease usually doesn’t pick one; the state usually doesn’t either; and the tenant and landlord each show up at the move-in assuming the math they ran is the “real” math.
The fix is procedural, not mathematical. Pick a method. Write it into the lease in one sentence. Apply it the same way at move-in and at move-out. Show the formula on the record the tenant signs, not just the total.
Do that and prorate disputes effectively stop. Skip it and you’re rolling the dice on every move-in for the lifetime of the property.
For the rest of the move-in playbook, work backward from the move-in records complete guide, which covers the walkthrough, the photo standard, the signed 47-item checklist, and the final PDF that pairs with your lease. The prorate is one line of that record. It’s also the line tenants ask about first.
