Air Purifier CADR Calculator

Particle filtration works only on the air that actually reaches the purifier. A room with a low ceiling, a closed door, and a clear circulation path can be served by much less clean-air flow than an open-plan zone with a stairwell, a hallway, or furniture blocking the intake. Floor area is easy to measure, but room volume and air mixing decide how much air must be cleaned in each hour.

Clean air delivery rate, or CADR, is the clean-air airflow a portable air cleaner delivers for particles. Air changes per hour, or ACH, describes how many complete room volumes are supplied as clean air in one hour. CADR is a product rating; ACH is the result of putting that product into a particular space.

CADR planning becomes more important when the purifier is being chosen for a known particle load. Wildfire smoke, cooking particles, pet dander, dust, pollen, and crowded rooms can require more turnover than a quiet bedroom. Product labels may show separate smoke, dust, and pollen CADR values because different particle sizes are tested separately. For fine-particle planning, smoke CADR or the lowest listed CADR is usually the cautious reference.

Square-foot rules are useful only as rough checks. A common shortcut puts smoke CADR near two-thirds of the room's floor area for ordinary particle cleaning, but that assumes an 8 ft ceiling and a standard turnover target. Once ceiling height, open doors, or quieter fan speed enter the decision, volume-based ACH sizing gives a clearer answer.

Noise, filter age, and placement can matter as much as arithmetic. CADR is often measured at a strong fan speed, while many people run purifiers lower for sleep, calls, or television. A purifier pressed against curtains, tucked into a corner, or trying to serve a connected hallway will not behave like the same unit in a test chamber. In a large or L-shaped space, two smaller purifiers may mix the room better than one high-output unit.

How to Use This Tool:

Start with the room dimensions and the purifier rating you can verify. Then use the fit checks to decide whether the model, fan speed, or unit count needs to change.

1. Choose the unit beside each room dimension and the Purifier CADR rating so the entered values match the product label or room measurement. CADR results follow the rating unit you choose.
2. Select a Room preset for a starting point, then edit Room length, Room width, and Ceiling height for the whole cleaned space.
3. Pick a Cleaning goal or move Target air changes. Presets set common ACH targets, and the slider can model a custom target from 1 to 10 ACH.
4. Enter the Purifier CADR rating for one unit and set Purifier count. Use the smoke CADR or the lowest listed CADR when fine particles are the main concern.
5. Set Quiet-speed cushion if the purifier should clear the target without relying on its loudest setting. Open Advanced for Usable CADR factor or Open-plan loss when fan speed, filter condition, or connected rooms need allowance.
   A lower Usable CADR factor reduces the candidate value, while Open-plan loss raises the target.
6. Compare Sizing target with cushion with Effective candidate CADR in CADR Sizing Table, then open Fit Checks and ACH Coverage Map to see the status and alternate ACH targets.
   If Check inputs appears, fix any zero room dimension, zero ACH target, negative CADR, missing purifier count, or unusable CADR factor before using the sizing rows.

Interpreting Results:

The main decision is whether Effective candidate CADR meets Sizing target with cushion. The target is the CADR needed for the room volume and ACH after the open-plan and quiet-speed allowances. The candidate value is the product CADR multiplied by purifier count and the usable CADR factor.

• Target only means no product CADR has been entered yet, so the page can show the needed clean-air flow but cannot judge a model.
• More CADR needed means the effective candidate is more than 15% below the adjusted target.
• Borderline fit means the candidate is below target but within 15%, so noise, filter age, placement, and doors can decide the real result.
• Candidate clears target means the effective candidate meets the adjusted target with up to 35% headroom.
• Large CADR headroom means the candidate is more than 35% above target; compare cost, noise, and whether a smaller unit would still meet the goal.

Delivered air changes converts the effective candidate CADR back into ACH for the measured room. Minimum units at this CADR shows how many units of the same rating would be needed after derating. Candidate floor coverage at target ACH is a useful sanity check, but it assumes the same ceiling height and mixing quality.

A good fit status does not prove that every seat in the room receives the same clean air. Before buying or relying on the result, check the product's smoke or lowest CADR, the fan speed used for that rating, the room volume, filter type, and whether the purifier can sit with clear intake and outlet airflow.

Technical Details:

CADR sizing is a volume-flow calculation. In US units, CADR is expressed as cubic feet per minute, so the room volume in cubic feet is multiplied by the desired ACH and divided by 60 minutes. Metric CADR in m3/h is converted to the same CFM basis for calculation, then displayed back with the selected report-unit order.

Three adjustments change the comparison. Open-plan loss raises the target because the purifier is assumed to serve more air than the measured room volume. Quiet-speed cushion also raises the target, leaving buying headroom for lower fan speed, filter loading, and normal placement losses. Usable CADR factor lowers the candidate because the label value may not be delivered at the planned setting.

Formula Core

Base CADR is room volume times the target air changes per hour.

The adjusted target adds the selected open-plan allowance and quiet-speed cushion.

Candidate CADR and delivered ACH reverse the same relationship.

For a 2,400 ft3 room at 4.8 ACH, the base requirement is 192 CFM. Adding 15% quiet-speed cushion and 10% open-plan loss raises the target to about 243 CFM. A 300 CFM purifier counted at 80% usable CADR delivers 240 CFM, so the fit is borderline even though the label CADR looks larger than the room's base formula requirement.

Fit Status Bands

Headroom is measured as the effective candidate CADR minus the adjusted target, divided by the adjusted target. Positive headroom means the candidate clears the selected target.

Limitations:

CADR and ACH estimate particle removal from air that reaches the purifier. They do not certify a product, diagnose indoor air quality, or prove that pollutant sources have been controlled.

• CADR is a particle rating. Gas, volatile organic compound, and odor control require filter media suited to those pollutants.
• Room mixing is simplified. Furniture, corners, open doors, supply vents, and purifier placement can leave uneven clean-air distribution.
• Published CADR may be measured at a high fan speed that is louder than the setting used day to day.
• Dirty or incompatible filters can reduce airflow and particle removal.
• Filtration supplements source control and ventilation; it does not replace removing pollutant sources when that is possible.

Worked Examples:

Advanced Tips:

• Use the smoke CADR or lowest listed CADR when the goal is fine-particle removal, even if the dust or pollen number looks better.
• Keep Quiet-speed cushion above 0% when noise would make the purifier impractical at its highest rated speed.
• Use Open-plan loss when the measured room is connected to a hallway, kitchen, stair, or loft that will mix with the cleaned air.
• Lower Usable CADR factor for worn filters, lower fan settings, or placement that partially blocks intake or outlet airflow.
• Read ACH Coverage Map as a comparison aid, not a promise that every corner receives equal clean air.

FAQ:

Glossary:

CADR

Clean air delivery rate, a particle-filtered airflow rating commonly reported in CFM or m3/h.

ACH

Air changes per hour, the number of room volumes supplied as clean air in one hour.

Effective candidate CADR

The purifier's label CADR after unit count and usable CADR factor are applied.

Quiet-speed cushion

Extra target CADR used to leave margin for lower fan speed, filter loading, and normal placement losses.

Open-plan loss

An allowance for connected spaces and weak mixing that make the purifier serve more air than the measured room.

Smoke CADR

The CADR value commonly used as a conservative reference for small particle removal.

References:

• Air Cleaners and Air Filters in the Home, U.S. Environmental Protection Agency, last updated February 19, 2026.
• Guide to Air Cleaners in the Home, U.S. Environmental Protection Agency, 2nd edition, August 2018.
• Air Changes per Hour FAQ, AHAM Verifide.
• Air Filtration Standards, AHAM Verifide.