Air Purifier Size & CADR Calculator

Why this calculator matters

Room size and an air-change target (ACH) determine the airflow capacity a purifier must deliver to clean the air at the rate you want. The calculator converts those inputs into a required CADR expressed as CFM (cubic feet per minute) so you can choose a unit that will perform under real conditions rather than relying on vague “room size” labels.

Tool UI (above the fold)

• Inputs: width (ft), length (ft), ceiling height (ft — default 8 ft), desired ACH (preset options: 2, 4, 6, 8), and pollutant type (particles/pollen, smoke, dust).
• Optional adjustments: checkboxes for open-plan, continuous source (smoking/cooking), and occupancy. These change the suggested ACH or add a safety margin but are not required.
• Safety margin toggle: choose 0%, 15% or 25% margin applied to the calculated CADR to account for furniture, leakage and filter aging.
• Visible formula: the calculator displays the exact arithmetic it used: Required CADR = (W × L × H × ACH) ÷ 60 (server-rendered).
• Actions: a “Show matching purifiers” control returns model suggestions that meet or exceed the calculated CADR and links to curated product pages on /best/.
• Microcopy: short tooltips explain each input (e.g., “ACH 4 is a common target for allergy relief; choose 6–8 for smoke or persistent sources”).

Server-rendered formula and worked example

The calculator uses a single, transparent formula. All units are feet and hours:

Required CADR (CFM) = (width (ft) × length (ft) × height (ft) × desired ACH (per hour)) ÷ 60

Worked example (server-rendered): a room 12 ft × 15 ft with an 8 ft ceiling.

• Room volume = 12 × 15 × 8 = 1,440 cubic feet.
• Target ACH = 4 (four air changes per hour).
• Required CADR = (1,440 × 4) ÷ 60 = 96 CFM.
• Apply a 20% safety margin (example): 96 × 1.20 = 115.2 CFM; round up to a practical purchase target — we recommend rounding to the nearest 5–10 CFM, so pick a purifier rated ≥120 CFM.

Rounding rule: always round up, not down. Manufacturers list CADR values by pollutant type (smoke/particle/pollen); match the CADR figure to the pollutant you care about.

How CADR, room volume and ACH relate

CADR is a flow-rate number—how many cubic feet per minute of air the purifier processes for a particular particle size during testing. ACH is a convenience metric: how many times the entire room volume would be processed in one hour at that flow rate. Converting ACH to CADR simply divides the hourly volume processed by 60 to get a per-minute flow.

Important limitations:

• CADR tests assume ideal placement, unimpeded airflow and new filters. Real-world performance is lower because of furniture, curtains, open doors, and filter loading.
• Manufacturers report separate CADR values for smoke (very small particles), pollen and dust (larger particles). Use the CADR appropriate to the pollutant you need to reduce.
• Noise and fan speed affect CADR—published CADR is often measured at a specific speed; running quieter reduces CADR proportionally.

Practical adjustments and edge cases

• High ceilings: use the actual height. For vaulted or partially open ceilings, either divide the space into zones or use an average height and increase the safety margin to 25–40%.
• Open-plan or connected rooms: treat them as one larger volume for calculation. When possible, place multiple units in different zones instead of oversizing a single unit, which improves distribution.
• Continuous sources (cooking, smoking, pets): choose a higher ACH target (6–8) and plan for continuous operation at a higher speed. Expect filters to need earlier replacement.
• Multiple small rooms: calculate each room separately. If you move one unit between rooms, size for the largest room and accept reduced ACH in the smaller spaces.
• Commercial/high-risk settings: consumer calculators are limited. Use the ACH calculator and consult facility guidance or an HVAC professional for clinical or industrial environments.

From result to purchase — shopping checklist

• Confirm the unit’s CADR (CFM) for the pollutant you care about (smoke vs. pollen/particles) is ≥ your recommended CADR after margin.
• Prefer true HEPA filters (not “HEPA-like”) for particle removal; look for an activated carbon stage if odors or smoke gases are a concern.
• Consider noise: parts of the day may require quieter operation, so choose a model whose higher-speed CADR meets your number even if you mainly run it lower.
• Factor running costs: use the running cost calculator and set up replacement reminders with the filter replacement reminder.
• Want help picking models? Click “Show matching purifiers” in the tool or browse curated options on Best Air Purifiers. Use the air purifier finder or the comparison tool to refine choices.

Validation & quick sizing checklist

• Unit CADR (CFM) ≥ recommended CADR after safety margin?
• CADR value corresponds to the pollutant you need to reduce (smoke vs. pollen)?
• Filter type: true HEPA for particles; activated carbon for odors/smoke gases?
• Reasonable running cost and available replacement filters?
• Do you need multiple units for even coverage across zones?

Run the calculator at the top of this page, then use the “Show matching purifiers” action to get result-based picks. For more context on targets and tools, see our collection at Free Air Purifier Calculators and Tools and the curated buying guide on Best Air Purifiers by Room, Need and Budget.