Filtration Efficiency Calculator

MERV ratings are not a single efficiency number — a MERV 11 filter might capture 90% of pollen-sized particles but only 35% of smoke-sized particles — so this calculator's core is a lookup table mapping MERV rating (1–20) and one of three particle size classes (0.3–1 µm, 1–3 µm, 3–10 µm) to an approximate capture efficiency, based on the ranges ASHRAE 52.2 testing typically reports. Filters below MERV 13 show a steep efficiency drop as particle size shrinks, which is why MERV 13+ is commonly recommended for smoke, viral aerosols, and other fine particles, while a lower-rated filter can still perform well against pollen or coarse dust. From the selected efficiency, Clean Air Delivery Rate (CADR) is airflow times efficiency — the volume of effectively clean air actually delivered, often more useful than the efficiency percentage alone, since a high-MERV filter moving too little air can under-perform a lower-MERV filter moving more air. Penetration is just 1 minus efficiency.

Face velocity (airflow divided by filter area) and an estimated pressure drop — which the model scales with the square of face velocity, consistent with turbulent-flow behavior — round out the picture, since pushing air too fast through a small filter wastes fan energy and can reduce true capture efficiency. The pressure-drop and efficiency figures are representative approximations for typical filter media at a given MERV rating, not manufacturer-specific test data — actual filters vary by media type, pleat depth, and manufacturing quality even at the same rating, so treat these as planning estimates rather than a substitute for a specific datasheet. A common mixup: assuming a filter's advertised MERV rating alone tells you how it will handle your specific concern (wildfire smoke, allergens) — always check the efficiency at the particle size range that actually matters, since a single MERV number can hide a large swing in real-world performance across sizes.