Air Purifier CADR Calculator

Room-size claims on air purifier boxes are useful only when the hidden assumptions match the room in front of you. Many labels are built around a standard ceiling height, a specific air changes per hour target, and a high fan speed. A bedroom with a closed door, a vaulted living room, and an open kitchen-lounge zone can all show the same floor area while needing very different clean-air flow.

Clean air delivery rate, or CADR, is the particle-filtered airflow a portable air cleaner can deliver. Air changes per hour, or ACH, describes how many room volumes of clean air are supplied in one hour. CADR belongs to the purifier, while ACH belongs to the purifier-and-room combination. The same purifier can be oversized in a small bedroom and short of the target in a larger room with a high ceiling.

CADR

A tested clean-air flow rating, commonly shown in CFM or m3/h for particles such as smoke, dust, and pollen.

ACH

The number of room volumes supplied as clean air each hour. Higher ACH means faster whole-room turnover.

Room volume

Length times width times ceiling height. Volume is stronger than floor area when ceilings, lofts, or connected spaces vary.

CADR planning matters most when the purifier is being chosen for a specific particle problem. Wildfire smoke, cooking particles, pet dander, dust, pollen, and dense occupancy all change the amount of clean air people may want. Product labels may list separate smoke, dust, and pollen CADR values because those particle sizes test differently. For fine-particle planning, the smoke CADR or the lowest listed value is usually the more cautious starting point.

Square-foot rules work only as rough checks. A common shortcut says the smoke CADR should be about two-thirds of a room's floor area for ordinary particle cleaning, but that shortcut 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 is more reliable.

Noise and placement often decide whether a calculated match works in daily use. CADR is typically measured at a strong fan setting, while many people run purifiers lower for sleep, calls, or television. A purifier pressed against curtains, tucked into a corner, or serving a connected hallway will not behave like the same unit in a test chamber. Multiple smaller purifiers can sometimes mix a large or L-shaped space better than one high-output unit.

How to Use This Tool:

Start with the room and product values you can verify, then use the fit outputs to decide whether the purifier, fan speed, or unit count needs adjustment.

1. Choose Metric / m3/h or Feet / CFM. Match the room dimensions and product label units you have; the result still shows both CADR unit families.
2. Select a Room preset for a starting point, then edit Room length, Room width, and Ceiling height for the actual cleaned space.
3. Pick a Cleaning goal or move Target air changes. The preset changes the ACH target, while the slider lets you 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 concern.
5. Set Quiet-speed cushion if the purifier should meet the target without relying on its loudest setting. Open Advanced when you need a Usable CADR factor or Open-plan loss.
6. Compare Sizing target with cushion with Effective candidate CADR in CADR Sizing Table, then check Fit Checks and ACH Coverage Map for the status and alternate ACH targets.

If Check CADR 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 in both unit systems.

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:

A 4.0 m by 3.2 m bedroom with a 2.4 m ceiling contains about 30.7 m3, or 1,085 ft3, of air. At Quiet bedroom, 3 ACH, the Formula CADR is about 54 CFM. A 120 CFM purifier at a 75% Usable CADR factor gives about 90 CFM effective CADR and nearly 5.0 Delivered air changes, so the fit reads as Large CADR headroom.

A living room measuring 5.5 m by 4.5 m with a 2.6 m ceiling contains about 64.4 m3, or 2,272 ft3. At 4.8 ACH with a 15% Quiet-speed cushion, the Sizing target with cushion is about 209 CFM. A 200 CFM candidate at 100% usable CADR lands a little below target, so CADR fit should read Borderline fit and the next check is noise, placement, or adding another unit.

An open-plan 8.0 m by 5.5 m room with a 2.7 m ceiling contains about 118.8 m3, or 4,195 ft3. At 6.5 ACH with 20% cushion and 15% open-plan loss, the target is about 627 CFM. Two 250 CFM purifiers derated to 80% usable CADR deliver 400 CFM, so More CADR needed is expected and Minimum units at this CADR should point toward four units.

If a measurement is entered as zero, the result area switches to Check CADR inputs. For example, a missing Ceiling height leaves the room volume at zero; increasing the purifier CADR cannot fix the calculation until the room dimension is corrected.

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.