HVAC Filtration Efficiency Calculator

MERV ratings are actually three separate efficiency numbers (E1, E2, E3) collapsed into one headline value based on the worst-performing band. This calculator looks up all three bands independently: at MERV 13, the E1 band (0.3–1.0 µm smoke and bacteria) sits at only 50% while E2 and E3 (mold spores, pollen) hit 85–90%. The single "MERV 13" label hides that gap, so enter your actual target particle size rather than assuming uniform performance across the board.

Not by itself — this calculator's pressure drop model depends on face velocity (CFM ÷ filter area), not MERV rating directly, though higher-MERV filters do carry a higher base pressure drop at the same 500 fpm reference velocity (0.25" w.g. at MERV 8 versus 0.45" w.g. at MERV 13 in this model). If you keep the same filter face area when upgrading, expect roughly 80% more resistance and a proportionally larger fan power penalty, which the calculator quantifies as additional watts and annual energy cost.

Because pressure drop and filtration performance both depend on face velocity, not airflow alone — the same 1,200 CFM forced through a small 2 ft² filter creates much higher velocity (and pressure drop) than through a larger 4 ft² filter. The calculator divides your CFM input by filter area to get face velocity in fpm, which is the value actually used to scale the pressure drop estimate. A larger filter area at the same airflow lowers pressure drop and energy cost without changing filtration efficiency.

It's a rough estimate assuming continuous 24/7 fan operation, 65% fan efficiency, and $0.12/kWh electricity — all fixed assumptions in the formula (additional HP = ΔP × CFM / (6356 × 0.65)). If your system cycles rather than running continuously, your local electricity rate differs, or your fan's actual efficiency is different, the real annual cost will scale proportionally from this baseline rather than matching it exactly.