Prorated Rent in California: 30-Day Month vs Calendar Month

Rent is almost always quoted per month, but people rarely move on the first. When a tenancy starts or ends mid-month, the rent for that partial month has to be prorated — reduced to cover only the days the tenant actually has the unit. The math sounds trivial, and it is, until you ask the one question that changes the answer: how many days are in a month?

Two answers are in common use. One treats every month as 30 days, so the daily rate never changes. The other uses the actual calendar days in the specific month, so the daily rate moves with the calendar between 28 and 31 days. Both are lawful in California, both are widely used by property managers and rental software, and for the same move-in date they can produce noticeably different bills.

Divide the monthly rent by 30 to get a daily rate, then multiply by the number of days the tenant occupies the unit. The daily rate is the same in February and in July, which is the whole appeal: one number, no calendar lookup, and a tenant can check the math in their head.

Under a strict 30-day method the count runs from the move-in day through the 30th, so in a 31-day month the 31st is simply not counted — the month is treated as if it had 30 days for the rate and for the days alike. That is how most rental software implements it, and it is the reason a 31-day month is the cheapest month to move in under this method.

Divide the monthly rent by the real number of days in that month — 28, 29, 30 or 31 — then multiply by the days occupied. The result is exact for that month, and it is the method people tend to expect when they have not thought about it, because it matches the calendar on the wall.

The pattern is fixed by the calendar. In a 31-day month the 30-day method gives a lower daily rate than the calendar method and also stops counting at the 30th, so the tenant pays less. In February the calendar method spreads the rent over fewer days, so its daily rate is higher and the tenant pays more. In a 30-day month the two methods agree to the penny.

Over a full year the differences roughly wash out, which is why neither side should feel cheated by either method as long as it is applied consistently. The real problem is a landlord who uses whichever method produces the bigger number that month — that is the practice a clear lease clause prevents.

No California statute prescribes a proration method for residential rent. The Civil Code governs when rent is due, how it may be increased, and what fees may be charged, but it leaves the arithmetic of a partial month to the rental agreement. If the lease states the method, the lease controls. If the lease is silent, the parties should agree in writing before money changes hands, because a dispute over $60 is not worth anyone's time.

Two things are worth keeping straight. First, proration is not a rent increase, so the AB 1482 rent cap and the 30- and 90-day notice rules do not apply to it. Second, a prorated first month is not a fee, so it is not touched by the fee-disclosure rules — it is simply rent for the days occupied, and it should appear on the ledger as rent.

Rentor, the property-management company behind this site, prorates on the 30-day method: the daily rate is the monthly rent divided by 30, the days run from the move-in date through the 30th, and the same rule is applied in reverse at move-out. We chose it because it is the easiest for an owner or a resident to verify without a calendar, it matches what our management software does by default, and it never charges a resident for a 31st day.

Whatever method you use, write it into the lease, apply it the same way at move-in and move-out, and show the arithmetic on the first statement. The number is small; the trust it builds is not.