How it works
How to use this pro rata calculator
- Enter the full amount for the whole period.
- Enter how much of the period applies (days used, months used, or any matching unit).
- Enter the length of the whole period in the same unit.
Formula
The calculation formula
Pro rata = Amount × (Part ÷ Whole)01
Divide the part of the period by the whole period.
02
Multiply that share by the full amount.
Worked examples
Example calculations
Partial month
$3,000 × (10 ÷ 30) = $1,000. The share of the period is 33.33%.
The whole period cannot be zero
The divisor is the length of the full period. A zero period is undefined.
When to use it
Common use cases
- Pay a monthly salary for only some of the days
- Split an annual fee across unused months
- Share a cost in proportion to time used
Limits
Assumptions and limits
- Part and whole must use the same unit. The calculator does not convert months to days.
- A 365-day year or a calendar month length is your choice; enter that number as the whole.
- If the part is larger than the whole, the result is greater than the full amount and a note is shown.
Questions
Frequently asked questions
Should I use 30 days or the real length of the month?
Use the rule in the contract or policy. Some payrolls use calendar days in that month; others use a 30-day convention. Enter that number as the whole period.
Is this the same as a daily rate?
A daily rate is Amount ÷ Whole. Pro rata is that daily rate times the part. This page shows the allocated amount, not a separate daily-rate tool.
Sources
Sources and review
- Investopedia — pro rata
Proportional allocation as amount × (part ÷ whole)
Accessed August 25, 2026