How to calculate prorated rent, and the one method that quietly overcharges you

Move in on the 18th and you shouldn't owe a full month's rent for it. Prorated rent is the partial amount that covers only the days you actually occupy the place, and working it out is easy once you know which day-count your landlord uses.

The basic formula is the same everywhere, where you find the daily rent and multiply by the days you're there, and what changes your bill is how that daily rent gets calculated.

The three methods

Say rent is $1,500 a month and you move in on September 18, leaving 13 days in the month.

Actual days in the month. Divide by the real number of days, here 30 for September. Daily rent is $50.00, so 13 days costs $650.00. This is the most common and the fairest, since it matches the month you're actually in.

Flat 30-day month. Divide by 30 no matter the month. In September that matches, but in February the daily rate rises because you're dividing 28 days of rent by 30. It's simpler for a landlord's accounting and slightly favors them in short months.

Banker's method, 365 divided by 12. Take the annual rent, divide by 365 for a true daily rate, then multiply by the days. On $1,500 that's $18,000 a year, $49.32 a day, so 13 days costs $641.10. This one usually gives the tenant the lowest number because it spreads rent across every day of the year evenly.

The gaps look small on 13 days of a $1,500 unit, under $10, but they widen with higher rents and in short months. Move into a February at a flat 30-day rate and you're paying for two days you don't live there.

What to check before you pay

Your lease should state the proration method, and if it doesn't, ask which one the landlord uses and get that answer in writing. A landlord isn't required to prorate at all unless your lease or state or local law says so, though most do it as standard practice and many jurisdictions require it for the first month.

Two traps are worth naming. Some landlords prorate the move-in month but quietly charge a full month on the way out even when you leave mid-month, so confirm both ends. And a few try to prorate off a 30-day divisor in a 31-day month, which shorts you a day's worth in your favor but is more often applied the other way, so run the number yourself.

The arithmetic is yours to check: monthly rent divided by the days in the month, times the days you're there. If the figure on your first invoice is higher than that, ask how they got it before you pay it. For the rest of the first-month cash, see how much it takes to move in, and read the whole agreement first with our guide to understanding your lease.

Sources

• Calculations use standard proration conventions: monthly rent divided by actual days, a flat 30-day month, or annual rent divided by 365

• RentDataNow, renter guidance