Prorated Rent Calculator
Rent owed for a part month, under either of the two conventions leases actually use — and it says which one it applied.
Rent owed
548.39
- Days covered
- 17
- Whole months
- 0
- Part months
- 1
- Counted as
- Actual days in the month
Month by month
| Month | Covered | Days | Charged as | Rent |
|---|---|---|---|---|
| 2026-10 | 2026-10-15 → 2026-10-31 | 17 | 17 / 31 | 548.39 |
A tenancy that starts on the 14th owes part of a month, and there are two ways to work out how much. Charging by the days the month actually has makes a day in February dearer than a day in January. Treating every month as thirty days makes a day cost the same all year and does not charge for the 31st.
Both are written into real leases, they give different answers, and neither can be derived from the other. So the convention is yours to pick, and the result says which one produced it.
How it works
The period is cut at the calendar months it crosses. A month covered end to end is charged in full, whatever length it has; only the part months are divided.
Under actual days, a part month is the rent times the days covered over the days that month has. Under the thirty-day convention it is the days covered over thirty, with a day number above thirty counted as the thirtieth — so a stay from the 28th to the 31st is three days, not four.
Both dates are inclusive. A tenancy from the 10th to the 20th is eleven days of rent, because the tenant has the keys on both.
Examples
| Case | Input | Result |
|---|---|---|
| Moving in halfway through a month | Rent 1,000, from the 15th to the 31st of a 31-day month | Actual days: 17/31, so 548.39. A thirty-day month: 16/30, so 533.33. |
| A tenancy over several months | Rent 1,000, from 15 March to 10 June | Two part months and two whole ones, itemised, adding up to the amount owed. |
Frequently asked questions
Which convention should I use?
The one the lease says. If it says nothing, the local custom decides and the two parties have to agree — the difference is small in money and is exactly the sort of small difference that turns into a dispute at the end of a tenancy.
Why is the last day charged?
Because a tenancy that ends on the 20th gives the tenant the property on the 20th. Subtracting one date from the other gives the nights between them, which is one less than the days occupied, and that off-by-one is the most common mistake in a part-month calculation.
Does this include charges and deposits?
No. It divides the rent you enter. Service charges are often prorated the same way and can be run through the tool separately; a deposit is not prorated at all.
Good to know
- Amounts carry no currency: the answer is in whatever you put in.
- Results use double precision floating point arithmetic. That is exact enough to decide with and is not an accounting record.
- A whole month is charged in full under both conventions. Only the part months differ, which is why a long tenancy shows the same total either way apart from its two ends.
- No local rule about rounding or about which day rent falls due is applied. Both are matters for the lease.