Furnace Filter Calculator

The calculator starts from a baseline change interval set entirely by filter type — 30 days for basic fiberglass, 90 for standard pleated, 180 for HEPA — reflecting how much dust-holding capacity each media actually has. From there it applies a stack of multiplicative penalties rather than picking one dominant factor: each pet shaves 15% off the interval (floored at 30% of baseline so even a house full of pets doesn't imply changing filters absurdly often), each occupant beyond two shaves another 5%, dust level nudges the interval up 10% for a clean newer home or down 30% for a dusty older one or active construction nearby, and any allergy sufferer in the household trims a further 20% since allergists generally recommend more frequent changes to limit recirculated allergens. All of that compounds together and is then floored at 14 days — the calculator won't recommend changing a filter more than roughly every two weeks regardless of how extreme the inputs are.

Annual cost simply multiplies the resulting changes-per-year by the filter type's typical unit price. The recommended MERV rating starts from what's typical for the selected filter type and is bumped up (never down) for allergy sufferers, two or more pets, or high dust — MERV measures how fine a particle the filter captures, so a higher recommended rating doesn't change the interval, just what to buy next time. Keep in mind actual HVAC systems have airflow limits, so jumping to a much higher MERV than your system supports can restrict airflow rather than help.