How to use the filter replacement timing calculator
Air handler filters should be changed on measured pressure drop rather than on a calendar. The same media loads several times faster in a dusty month than in a clean one, so a fixed interval either discards good filters or leaves a blinded one in service. Enter the initial and final resistance, the current reading and the run hours, and this calculator returns the loading percentage, the rise rate, the remaining life and the days left before change-out.
Loading maps the span from clean initial resistance to final resistance onto a scale of one hundred. Past 80% it is time to order spare media and tie the change to the next scheduled maintenance visit rather than waiting for an alarm.
Pressure drop is an energy bill. Fan power rises with static pressure, so running media to the absolute limit is not always the cheapest choice. The added fan energy can exceed the price of the filter, which is why the current-to-initial ratio is shown alongside the remaining hours.
The projection assumes a linear rise. Real loading curves steepen near the end, so the remaining hours are optimistic. Preset values follow common ASHRAE 52.2 MERV practice, and the manufacturer's recommended final resistance together with the fan's spare static pressure should always be checked first.
Frequently asked questions
Loading rate swings by several times with outdoor dust levels and run hours, so a fixed monthly interval either throws away good media or runs a blinded filter for weeks. Pressure drop tracks the actual condition of the filter.
The manufacturer's recommended final resistance comes first. Without it, common US practice is 0.5 to 1.0 in. w.g. for MERV 6-8 pre-filters, 1.0 to 1.5 in. w.g. for MERV 11-13 final filters, and about twice the initial value for HEPA. Check the fan's spare static pressure as well.
Real filters load faster near the end of life, so a linear fit tends to overstate the remaining hours. Treat 80% loading as the point to order spares, and re-run the numbers each time you log a new reading.