THE NUMBERS, EXPLAINED
How to use this calculator.
Actual-day proration with identical included/excluded day convention for old and new plans. All credit and fee terms are supplied.
Start with your inputs
Enter old plan full-cycle price (currency), new plan full-cycle price (currency), days in billing cycle, unused billable days at migration, one-time migration fee (currency). Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Old plan full-cycle price (currency) | 30 |
| New plan full-cycle price (currency) | 50 |
| Days in billing cycle | 30 |
| Unused billable days at migration | 12 |
| One-time migration fee (currency) | 5 |
A worked example
With the example inputs above, unused old-plan credit (currency) is 12. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Unused old-plan credit (currency) | 12 |
| Remaining-cycle new-plan charge (currency) | 20 |
| Net migration invoice adjustment (currency) | 13 |
How this is calculated
Unused old-plan credit (currency)
Prorate the old price by unused days.
Remaining-cycle new-plan charge (currency)
Prorate the new price over the same remaining days.
Net migration invoice adjustment (currency)
Deduct old credit from new charge and add the supplied fee.
Symbols and units
- a
- Old plan full-cycle price (currency)
- b
- New plan full-cycle price (currency)
- c
- Days in billing cycle
- d
- Unused billable days at migration
- e
- One-time migration fee (currency)
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Actual-day proration with identical included/excluded day convention for old and new plans. All credit and fee terms are supplied.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Unused old-plan credit (currency); Remaining-cycle new-plan charge (currency); Net migration invoice adjustment (currency). The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Actual-day proration with identical included/excluded day convention for old and new plans. All credit and fee terms are supplied.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Further reading
OpenStax: Describe and Identify Cost Drivers ↗
Background reading; this does not imply endorsement or independent review of SumFizz.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
Read the quality report →