Pick your method before the tenant asks. There are three in common use, they give different answers, and the one you cannot defend is the one you chose after seeing which produced the bigger number.
Here is each of them on the same facts: rent of $1,800 a month, move-in on the 18th of a 30 day month, so the tenant occupies 13 days including the move-in day.
The three methods
Actual days in the month. Divide the rent by the number of days in that specific month, then multiply by days occupied. In a 30 day month, $1,800 divided by 30 is $60 a day, times 13 days, for $780.
The 365 day year. Multiply the rent by 12, divide by 365, then multiply by days occupied. That is $21,600 divided by 365, or $59.18 a day, times 13, for $769.34. The daily rate is the same in every month of the year, which is the whole point of this method.
The flat 30 day month. Divide the rent by 30 regardless of the actual month, then multiply by days occupied. In our 30 day month that is the same $780. In February it is not: 13 days at $60 is still $780 by this method, where the actual-days method would charge $835.71.
On these facts the spread is about $11. On a February move-in it is closer to $56. Small money, and still worth deciding in advance, because a tenant who checks your arithmetic and finds a method that appeared from nowhere will check everything else too.
Which one to use
For most small portfolios, the actual days in the month is the easiest to explain, because the arithmetic uses the calendar the tenant is looking at. It also charges slightly more in short months and slightly less in long ones, which averages out.
The 365 day method is the fairest across a portfolio and the most common in professional management, since every day of the year costs the same. Use it if you would rather explain the method once than explain February every year.
The flat 30 day method is the simplest and the one most likely to be questioned in a 31 day month, where the tenant occupies 31 days but pays for 30. That is in the tenant’s favor, so it rarely produces a complaint.
Whichever you pick, write it into the lease as a sentence with the arithmetic in it, not as the word “prorated.” Something like: rent for a partial month is calculated as the monthly rent divided by the number of days in that month, multiplied by the number of days of occupancy including the first day.
Count the days the same way every time
Two conventions, and you need to have chosen one.
The move-in day is almost always counted as an occupied day, because the tenant has possession from that morning. The move-out day is usually counted too when a tenancy ends mid-month, though some leases exclude it. Say which in the lease.
Then apply it consistently. Charging one tenant from the day they got keys and another from the day the lease says the term starts is the sort of small inconsistency that is hard to explain later.
When to collect it
Two workable patterns, and one that causes trouble.
Collect the prorated amount at move-in, then start full rent on the first of the next month. Clean, and the tenant’s payment schedule normalizes immediately.
Collect a full month at move-in and prorate the second month. Some landlords prefer this because the largest payment lands before keys change hands. It confuses tenants, and it means the second invoice is the odd one, but it is defensible.
What causes trouble is collecting the prorated amount and letting the tenant think it was a full month. They budget for the same figure the following month, get an invoice for $1,800 instead of $780, and now you are having a conversation about a discount nobody offered. Say the number out loud at signing and put it on the receipt.
The rest of the move-in money
Prorated rent is one of three or four amounts changing hands at move-in, and they need to be itemized separately rather than summed into one figure. The deposit is not rent, and in several states it is subject to a statutory cap and its own handling rules: see security deposit limits by state and, for what happens at the other end, security deposit deductions.
This is general information, not legal advice. Rules change and local ordinances vary. Confirm current requirements with your city, county, or an attorney before you act.
Give the tenant one document with each line named: prorated rent for the partial month, first full month if you collect it, security deposit, pet deposit if any, and any one-time fee. Then match the payment record to those lines rather than to a lump sum.
After the first month
The rest of the schedule is ordinary. Rent is due on the first, the grace period runs as the lease says, and if the tenant enrolls in autopay at move-in you have solved on-time collection for the whole tenancy in the same sitting. Automating rent reminders and late fees covers what runs from there, and rent payments carries the first invoice and every one after it.
One last check. If your prorated month is the tenant’s first experience of paying you, make it the moment you set the pattern: a clear invoice, a named period, a recorded payment. It is much easier to establish that on day one than to introduce it in month seven.