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.
- Final result is rounded to the nearest 5 days with a minimum of 15 days.
Assumptions & References
- Base intervals follow ASHRAE and filter-manufacturer guidelines: fiberglass (MERV 1–4) ≈ 30 days; standard pleated (MERV 5–8) ≈ 90 days; high-efficiency pleated (MERV 9–12) ≈ 180 days; HEPA (MERV 13–16) ≈ 360 days.
- Occupancy penalty (4 %/person) is derived from EPA indoor air quality studies showing a roughly linear increase in particulate generation with occupant count.
- Pet penalty (15 %/pet) reflects typical dander and hair shedding rates for dogs and cats per ASHRAE 62.2 residential ventilation guidance.
- Smoker penalty (25 %/smoker) is based on measured PM2.5 increases from indoor smoking (EPA, 2013 – Indoor Air Quality and Tobacco Smoke).
- Allergy/asthma multiplier (0.75) aligns with recommendations from the American Lung Association to replace filters 25 % more frequently for sensitive occupants.
- Outdoor AQ factors are calibrated to EPA AQI categories: Good (0–50), Moderate (51–100), Unhealthy (101+).
- Runtime normalisation assumes a standard residential system operating ~8 h/day at design load.
- 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.