How to prorate rent for a partial month

Divide the month's rent by the number of days in that month, then multiply by the days somebody is actually there. The other common method divides by thirty regardless of the month, and the two disagree by a few dollars in every month that is not thirty days long — which is most of them. Neither is wrong. Picking one before anybody knows who benefits is the entire skill.

The two methods, and where they differ

Actual days is the more defensible: a day in February genuinely costs more than a day in March, because the same rent covers fewer of them. Thirty-day is the more common in practice, because it produces the same daily figure all year and is easier to check.

On a $2,000 rent the gap between them is a few dollars either way. It is small enough not to matter and visible enough to argue about, which is a bad combination when it is decided after the fact.

Count the days honestly

Somebody moving in on the fifteenth of a thirty-day month is usually there for sixteen days, not fifteen, because the day they arrive is a day they have the room. That single off-by-one is a surprisingly frequent source of a disagreement about a much larger sum.

Say which method you are using when you say the number

"Eleven days at the actual-days rate" is checkable in ten seconds. A bare figure is something the other person has to either accept or query, and querying it feels like an accusation even when it is not.