Air Filter Replacement Interval Calculator
Estimate the recommended air filter replacement interval for your HVAC system based on filter MERV rating, household occupancy, pets, smokers, and local air quality.
Higher MERV = finer filtration but may clog faster in dusty conditions.
More people → more skin cells, dust, and activity.
Each pet adds significant dander and hair load.
Smoke particles clog filters rapidly.
Allergy sufferers benefit from cleaner filters.
Poor outdoor air increases indoor particulate load.
More runtime = more air cycled through the filter.
Formula
Interval (days) = BaseInterval × Focc × Fpet × Fsmoke × Fallergy × FAQ × Frun
- BaseInterval – manufacturer-rated service life in days for the selected MERV tier (30 / 90 / 180 / 360 days).
- Focc = max(0.50, 1 − (occupants − 1) × 0.04) — each additional occupant beyond one reduces the interval by 4 %.
- Fpet = max(0.25, 1 − pets × 0.15) — each pet reduces the interval by 15 % (dander and hair).
- Fsmoke = max(0.10, 1 − smokers × 0.25) — each smoker reduces the interval by 25 % (fine particulates).
- Fallergy = 0.75 if allergy/asthma sufferers are present, else 1.0.
- FAQ = 1.00 (good) / 0.85 (moderate) / 0.65 (poor) outdoor air quality.
- Frun = clamp(8 / runtime, 0.33, 2.00) — normalises against an 8 h/day reference; longer daily runtime shortens the interval proportionally.
Assumptions & References
- This calculator provides an estimate. Always follow the filter manufacturer's maximum rated service life and inspect filters visually each month.
- References: ASHRAE Standard 52.2-2017; EPA Indoor Air Quality Guide; American Lung Association Health Effects of Indoor Air Pollution.