Examples
Examples
Try the starting example
- Filters: device, last change date, interval days, units per change
- Air,2026-10-01,30,2 Water,2026-09-01,30,1
- As-of date
- 2026-10-20
- Inclusive planning horizon days
- 30
- Next original due date by device
- Air: 2026-10-31; Water: 2026-10-01
- Already overdue device count
- 1
- Units required by device in horizon
- Air: 2; Water: 2
- Total filter units to obtain
- 4
Try another scenario
- Filters: device, last change date, interval days, units per change
- Air,2026-10-01,30,2 Water,2026-09-01,30,1
- As-of date
- 2026-10-20
- Inclusive planning horizon days
- 10
- Next original due date by device
- Air: 2026-10-31; Water: 2026-10-01
- Already overdue device count
- 1
- Units required by device in horizon
- Air: 0; Water: 1
- Total filter units to obtain
- 1
Calculation method
Calculation method
Next original due date by device
Last change date + supplied interval days.
Already overdue device count
Count next due dates strictly before as-of.
Units required by device in horizon
Replace an overdue filter immediately at as-of, then restart its supplied interval there. Otherwise retain the upcoming due date. Count events through as-of+horizon inclusive.
Total filter units to obtain
Sum changes × units per change across devices.
Symbols and units
- a
- Filters: device, last change date, interval days, units per change
- b
- As-of date
- c
- Inclusive planning horizon days
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions & limits
Assumptions & limits
- Replacement intervals and quantities come from your appliance records, not a universal maintenance policy. An overdue replacement is planned once at as-of and resets its future interval. The final planning date is inclusive.
Are these inputs private?
These calculations run in your browser. A shared link contains the values you choose to share. Only questions submitted to the separate AI finder are sent to OpenAI.
Further reading
OpenStax: arithmetic and geometry ↗
Background reading; this does not imply endorsement or independent review of SumFizz.