Filter Replacement Interval Calculator
Estimate how often you should replace your HVAC filter based on filter type, home size, occupancy, pets, and local air quality conditions.
Formula
Adjusted Interval (days) =
Base Interval
× Size Factor = √(2000 / HomeSizeSqFt) [clamped 0.5–1.5]
× Occupant Factor = 1 / (1 + 0.05 × max(Occupants − 1, 0))
× Pet Factor = 1 / (1 + 0.15 × Pets)
× Allergy Factor = 0.5 if yes, 1.0 if no
× Air Quality Factor = 1.0 (Good) | 0.75 (Moderate) | 0.50 (Poor)
× Runtime Factor = 8 / DailyRuntimeHours [clamped 0.33–3.0]
Base Intervals by MERV Rating:
MERV 1–4: 30 days | MERV 5–8: 90 days | MERV 9–12: 180 days | MERV 13–16: 365 days
Filters per Year = 365 / Adjusted Interval
Final interval is clamped between 7 days (minimum) and 365 days (maximum).
Assumptions & References
- Base intervals follow ASHRAE and EPA guidance for residential HVAC filter maintenance by MERV rating.
- Reference home size is 2,000 sq ft; the square-root scaling reflects that air volume (and filter load) scales sub-linearly with floor area.
- Each additional occupant beyond one is assumed to increase particulate load by 5% (skin cells, activity, cooking).
- Each dog or cat is assumed to increase particulate load by 15% per pet (dander, fur, tracked debris).
- Allergy/asthma households are advised to replace filters 50% more frequently per ACAAI recommendations.
- Outdoor air quality multipliers are based on EPA AQI categories: Good (<50), Moderate (51–100), Poor (>100).
- Standard HVAC runtime is assumed to be 8 hours/day; runtime factor scales linearly with actual usage.
- Results are estimates. Always inspect your filter visually every 30 days — replace immediately if visibly grey/clogged.
- References: ASHRAE Standard 52.2, EPA Indoor Air Quality Guide, ACAAI Allergy & Asthma Network guidelines.