Air Filter Change Interval Calculator
Estimate how frequently you should replace your HVAC air filter based on filter type, household conditions, and environmental factors.
Formula
Load Factor (LF)
LF = F_size × F_occupant × F_pet × F_smoker × F_allergy × F_dust F_size = homeSize / 2,000 F_occupant = max(0.80, 1 + (occupants − 2) × 0.10) F_pet = 1 + pets × 0.20 F_smoker = 1.50 (smoker present) | 1.00 F_allergy = 0.75 (allergy/asthma) | 1.00 F_dust = 1.30 (dusty/construction) | 1.00
Adjusted Filter Life
adjustedHours = baseHours / LF days = adjustedHours / dailyRuntime days = clamp(days, 7, 365)
baseHours is the manufacturer-rated filter life in operating hours: Fiberglass ≈ 1,500 h · Pleated 1″ ≈ 2,160 h · Pleated 4″ ≈ 4,320 h · High-Efficiency ≈ 8,760 h.
Assumptions & References
- Baseline home: 2,000 sq ft, 2 occupants, 0 pets, no smoker, no allergies, no dust, 8 hrs/day runtime.
- Filter base-life hours derived from ASHRAE 52.2 dust-holding capacity ratings and manufacturer guidelines (3M Filtrete, Honeywell, Nordic Pure).
- Each additional occupant beyond 2 increases particulate load by ~10% (EPA Indoor Air Quality guidance).
- Each dog or cat adds ~20% additional dander/hair load (AAFA — Asthma and Allergy Foundation of America).
- Smoking indoors increases particulate matter (PM2.5) by 50–200%; a conservative 50% factor is applied (WHO Indoor Air Quality report).
- Allergy/asthma households should change filters 25% sooner to maintain lower allergen concentrations (AAFA recommendation).
- Dusty or near-construction environments increase filter loading by ~30% (ASHRAE 62.1).
- Results are estimates. Inspect your filter monthly; replace when visibly grey/clogged regardless of schedule.
- MERV ratings per ASHRAE Standard 52.2-2017.