Air Purifier CADR Calculator

Step 1 — room volume. 5 × 4 × 2.5 = 50 m³.

Step 2 — required CADR. CADR = volume × target ACH = 50 × 4.9 = 244 m³/h (145 CFM).

Step 3 — what the unit actually delivers. ACH = CADR / volume = 300 / 50 = 6.0 ACH. One unit is sufficient, with headroom.

The relationship is that simple, and it runs both ways: ACH = CADR / volume. Ceiling height matters as much as floor area, which is why a purifier sized for a 20 m² room with a 2.5 m ceiling is undersized for the same floor area in a room with a 3.5 m ceiling.

This is the point the term is most often misread. CADR = airflow × single-pass removal efficiency.

A unit moving 375 m³/h through a filter that captures 80% of the target particles on each pass has a CADR of 300 m³/h. It is not delivering 375 m³/h of clean air; it is delivering the equivalent of 300 m³/h of perfectly clean air. That equivalence is exactly what makes CADR comparable across machines with different filters and fans.

Why a true HEPA unit and a cheaper one can quote similar CADR. The HEPA unit has near-unity single-pass efficiency but higher resistance, so lower airflow. A coarse filter passes more air at lower efficiency. The products can land at the same CADR while behaving quite differently — the HEPA unit performs far better on the smallest particles, which the CADR smoke test only partly captures.

Three CADR numbers, not one. AHAM certifies separately for smoke (0.09–1.0 µm), dust (0.5–3 µm) and pollen (5–11 µm). Pollen CADR is always the highest and the least demanding. Use the smoke figure — it is the proxy for fine particulate and for respiratory aerosol, and it is the one that will be lowest.

CADR is measured at maximum fan speed. That is also the noisiest setting, typically 50–60 dB(A). Most people run a bedroom unit on low overnight, where CADR can fall to a third of the rated value — turning a nominal 6 ACH into 2 ACH. If the unit will be run on low, size on the low-speed CADR, which good manufacturers publish.

Filter loading. CADR is certified with a clean filter. As it loads, airflow falls; on units with constant-speed fans, so does CADR. HEPA media capture efficiency actually improves slightly as it loads, but the airflow loss dominates.

Room mixing is assumed perfect. The AHAM test uses a stirred 28.5 m³ chamber. A real room with the unit in a corner, behind a door or under a shelf will short-circuit — clean air returns to the inlet without mixing. Central placement with clearance on all sides can be worth more than a larger machine.

It is a steady-state single-zone model. Opening a door to an unfiltered space resets much of the benefit. The calculation applies to an enclosed room, not to a house.

Particles only. CADR is a particulate metric. It says nothing about gases, VOCs, formaldehyde or odour, which need sorbent media (activated carbon) and have their own, much shorter, service life. A HEPA-only unit does not remove gases.

Air cleaning is not ventilation. A purifier recirculates; it does not supply outdoor air and does not remove CO&sub2;. A room can sit at 6 ACH of filtered air and still have CO&sub2; above 1500 ppm. If occupants report stuffiness, the answer is ventilation, not a bigger filter.

Source control beats both. Removing the source — extract at the cooker, no indoor smoking, managing damp — is more effective per unit effort than any amount of filtration.

Ozone-generating devices are excluded. Ionisers, plasma and "active" technologies are not covered by this calculation and some generate ozone as a by-product. CARB maintains a list of certified air cleaners for exactly this reason.

CADR itself: ANSI/AHAM AC-1 defines the test — a 28.5 m³ chamber, decay measured over 20 minutes against natural decay, reported separately for smoke, dust and pollen. GB/T 18801 is the Chinese equivalent and uses a similar method, so figures are broadly comparable. Certification marks matter here: an uncertified "CADR" is a manufacturer's own claim.

Filter classification: EN 1822 and ISO 29463 define HEPA classes (H13 captures 99.95% at the most penetrating particle size; H14, 99.995%). ISO 16890 covers general ventilation filters by ePM1/ePM2.5/ePM10.

How much is enough: ASHRAE Standard 241 (2023) sets equivalent clean airflow targets for infection risk management, expressed per person rather than per room. For general indoor air quality, 4–6 ACH of clean-air equivalent is a common design target; 5 ACH is the figure most often quoted in guidance.

Next steps: check the noise rating at the speed you will actually use, not at maximum — a unit you turn down is a unit you have derated. Confirm the smoke CADR rather than the headline figure. Price the replacement filters over three years; they frequently exceed the purchase price. And measure CO&sub2; alongside particulate: the two problems have different solutions, and a purifier only addresses one of them.

Clean Air Delivery Rate is the volume of contaminant-free air delivered by the purifier. Higher CADR means faster particle removal.

Room-size charts often assume an 8 ft ceiling. A taller room contains more air, so the same CADR produces fewer clean-air changes per hour.

No. CADR is a particle-removal performance metric. Gas removal depends on activated carbon or other sorbent media and is not represented by the particle CADR.