Air Purifier Size Calculator

Clean Air Delivery Rate is the product of a purifier's airflow and its single-pass removal efficiency, expressed as the equivalent volume of perfectly clean air it supplies per minute. Sizing therefore reduces to a dilution problem: to achieve a target number of air changes per hour in a room, the required CADR in cubic feet per minute equals room volume in cubic feet times air changes per hour divided by 60. Because CADR bundles flow and efficiency into one number, a high-flow unit with a mediocre filter and a lower-flow unit with a true HEPA element can score alike, which is precisely why the metric exists. AHAM tests to the ANSI/AHAM AC-1 method in a sealed chamber of about 1,008 cubic feet and publishes three separate ratings covering tobacco smoke at roughly 0.09 to 1.0 microns, dust, and pollen at roughly 5 to 11 microns; the smoke figure is the most demanding and the safest one to size against. Common air change targets are about 2 per hour for general maintenance, 4 to 5 for allergy and asthma households, and 5 or more where reducing airborne infection risk matters, a framing formalised in ASHRAE Standard 241, published in 2023, which specifies equivalent clean airflow per occupant. The familiar AHAM shortcut that CADR should be at least two-thirds of the floor area in square feet is derived from these figures but silently assumes 8-foot ceilings, so it undersizes rooms with cathedral ceilings or lofts, where volume rather than area must be used. Several limitations apply. Rated CADR is measured at maximum fan speed, which is often too loud for a bedroom, so the effective overnight rate can be half the label figure. Real rooms are not the well-mixed test chamber; furniture, corners, and open doorways create dead zones and leakage. A true HEPA filter captures 99.97 percent of particles at 0.3 microns, its most penetrating size, but ionising devices can emit ozone, which is itself a regulated pollutant.

The filter media at the heart of modern air purification came out of wartime nuclear work. High-efficiency particulate air filters were developed in the United States during the 1940s under the Manhattan Project to capture radioactive aerosols from laboratory and process exhaust, where the consequence of a leak made near-total particle capture essential. The technology was declassified after the Second World War and moved into hospital, cleanroom, and eventually consumer use, carrying with it the 0.3 micron test point that defines HEPA performance to this day. Consumer comparability took another forty years. Portable air cleaners proliferated through the 1970s with manufacturers advertising incompatible and often meaningless performance claims, and the Association of Home Appliance Manufacturers responded in the early 1980s by creating the CADR rating programme and the standardised chamber test behind it, giving buyers a single comparable number for the first time. The Environmental Protection Agency later published residential guidance on air cleaners, and California's Air Resources Board established an ozone emission certification requirement in response to ionising and ozone-generating devices marketed as purifiers. The COVID-19 pandemic in 2020 and 2021 pushed the field into mainstream awareness and produced a notable low-cost innovation: the Corsi-Rosenthal box, proposed by engineer Richard Corsi and first built by Jim Rosenthal in 2020, which achieves high CADR from a box fan and MERV-13 furnace filters.