Proration Calculator
Work out the prorated (daily) amount from a monthly amount and a period within one month. You can choose the days to divide by, whether the monthly amount is a cap, and the rounding.
How it is calculated
The prorated amount is the monthly amount × the days counted ÷ the base days (the days to divide by). Anything below the smallest unit is rounded once, at the very end.
The period must be within a single month. If it spans more than one month, split it by month and calculate each part (for example, October 20 to November 10 is October 20 to 31 and November 1 to 10), because the number of days and the monthly amount can differ from month to month.
You can choose how the base days are decided:
| Choice | Base days | Typical use |
|---|---|---|
| Calendar days | The days in that month (28 to 31) | Prorated rent, monthly fees |
| Fixed 30 days | Always 30 | When you want no difference between months |
| Your own number | The number you enter | When a contract fixes the days to divide by |
Capping at the monthly amount
The default is "cap on". Near the result, the tool shows the base days, the days counted, whether the cap was applied, and the rounding.
| Cap on (default) | Cap off | |
|---|---|---|
| Days counted ≤ base days | Multiply by days counted ÷ base days | The same |
| Days counted > base days (for example, 31 days with a fixed 30) | The monthly amount is the upper limit (the monthly amount itself) | Days counted ÷ base days as it is (above the monthly amount) |
| The whole month is counted | The monthly amount itself (one month) | Days counted ÷ base days as it is |
For example, when you prorate all 31 days of October with a fixed 30 days, the cap on gives the monthly amount itself, while the cap off gives the monthly amount × 31 ÷ 30. For all 28 days of February with a fixed 30 days, the cap on gives the monthly amount itself and the cap off gives the monthly amount × 28 ÷ 30.
Examples
These examples use a monthly amount of 900.00 in any currency.
- October 10 to October 31, 2026 (22 days), calendar days (October has 31): 900 × 22 ÷ 31 = 638.7096…, which rounds to 638.71.
- The same period with a fixed 30 days: 900 × 22 ÷ 30 = 660.00.
- All of October (31 days), fixed 30 days: the cap on gives 900.00, and the cap off gives 900 × 31 ÷ 30 = 930.00.
- Just February 10, 2026 with a monthly amount of 280.00, calendar days (February has 28): 280 × 1 ÷ 28 = 10.00.
Notes
- The proration method is not set by law. It depends on your contract, lease, payroll rules or billing policy. Common methods divide by the calendar days, by 30 days, or by working days. Follow the method in your own document.
- Prorating by working days (days counted ÷ working days in the month) is not covered. "Your own number" counts the days counted in calendar days and divides by the number you enter.
- Taxes, fees and similar charges are not included in the calculation.
- Dates must be between January 1, 1900 and December 31, 2100. Amounts can have up to 2 decimal places, up to 9,999,999,999.99.
- To convert pay between hourly, daily, monthly and yearly figures, use the Hourly to Salary Calculator. To count the days in a period, use the Date Difference Calculator.
Frequently asked questions
Should I use calendar days or a fixed 30 days?
Follow your contract or rules if they say. If they do not, dividing by the actual number of days in the month (calendar days) is common.
When is the amount rounded?
The tool calculates the monthly amount × the days counted ÷ the base days and rounds once, at the end. It does not round in between.
Can I prorate a period that crosses into the next month?
This tool calculates a period within one month. When a period crosses a month boundary, split it by month and calculate each part.
Can I prorate by working days?
No. This tool counts the days counted in calendar days. You can count working days with the Business Days Calculator.