What’s the exact rent for a partial month?

Divide the full monthly rent by the number of days in the month to get a daily rate, then multiply by the number of days the tenant occupies the space. For $3,000 rent and a March 15 move-in: $3,000 ÷ 31 days = $96.77 a day, times 17 days (March 15 through 31, counting both) = $1,645.16. This calculator does exactly that, to the cent, using whichever day count you choose.

The actual-days method divides by the real number of days in that month (28 to 31), so the daily rate changes from month to month. The 30-day method treats every month as 30 days, so the daily rate is always the monthly rent divided by 30. The 365-day method divides the annual rent by 365 (or 366 in a leap year), giving one daily rate for the whole year. For the same 17 days on $3,000 a month, they give $1,645.16, $1,700.00, and $1,676.71. This calculator shows the first two; for the 365-day method, multiply the annual rent by the days occupied and divide by 365.

There's no single standard. Many leases simply say rent for a partial month is "prorated on a per diem basis," which is often read as the actual days in that month; others specify a 30-day month or a 365-day year. The lease controls, so look in the rent section or the definitions before you bill. If the lease is silent, pick one method, tell the tenant which one in writing, and use it consistently so move-in and move-out are treated the same way.

The first partial month usually runs from the rent commencement date to the end of that month. That date can differ from the lease signing date or the day the tenant got the keys, especially after a free-rent period or a build-out. Some leases collect the partial month with the first full month's rent; others bill it on its own. The last partial month runs through the expiration date or the agreed surrender date. If the tenant stays past expiration, the lease's holdover clause, often at a higher rent, usually governs instead of simple proration.

In this calculator, yes: March 15 through March 31 is 17 days, because the tenant has the space on both of those days. That matches how most leases describe a period "from" one date "through" another. If your lease or agreement says rent starts the day after possession, or ends the day before surrender, adjust the dates you enter.

A 30-day method can produce odd results at the edges. Occupying all 31 days of a 31-day month would come to 31/30 of the rent, more than a full month, while 20 days in February would come to two-thirds of the rent even though the tenant had the space for most of the month. That's why leases normally charge the full monthly rent for any full calendar month and use the per-day calculation only for a partial month.

Usually, yes. In a net lease, the monthly estimates for CAM, property taxes, and insurance are typically prorated for a partial month the same way as base rent, unless the lease says otherwise. At year end, the tenant's share of actual expenses is normally prorated for the part of the year they occupied the space. Enter the base rent here, then run the pass-through charges separately or use the NNN calculator, which prorates for a partial year.

Bill it as its own line, labelled with the dates it covers (for example, "Base rent, March 15 to 31"), so the tenant ledger shows why the amount is less than a full month. Keep the calculation with the lease file in case the tenant asks how it was worked out; you can download this result as a CSV for that.