Subscription Proration Calculator
Calculate an explicitly day-based plan-change charge or refund from unused billing-period days.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Apply one entered day-count convention equally to both full-period prices.
Credit=old price×unused/period; new charge=new price×unused/period; net=charge−credit
Worked example
20 to 35 with 10 of 30 days unused produces a net extra charge of 5 before tax/rounding.
Assumptions and input details
Unused days cannot exceed period days. No automatic date/time-zone, month-length, inclusive-day or vendor billing policy inference. No tax or payment action. Monetary results remain unrounded until displayed.
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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