Air Purifier Room Size Calculator
How this air purifier room size calculator works
This calculator estimates the minimum Clean Air Delivery Rate (CADR) you should look for when choosing an air purifier for a specific room. It uses your room dimensions to find the room volume, then applies your chosen air changes per hour (ACH) to calculate the airflow needed to clean that volume of air.
You can enter room dimensions in either feet or meters using the unit selector. The calculator then converts everything internally and reports the recommended CADR in cubic feet per minute (CFM), which is the unit most commonly listed on air purifier packaging and AHAM (Association of Home Appliance Manufacturers) labels.
CADR, room volume, and ACH: the core formula
Three main ideas drive the calculation:
- Room volume – how much air (space) is in the room.
- Air changes per hour (ACH) – how many times per hour that entire volume of air is cleaned.
- CADR – the airflow from the purifier that is effectively cleaned, usually measured in CFM.
The basic relationships are:
- Room volume (imperial): Volume (ft³) = length (ft) × width (ft) × height (ft)
- Room volume (metric): Volume (m³) = length (m) × width (m) × height (m)
- CADR (CFM): CADR = (Room volume × ACH) ÷ 60
Because ACH is “per hour” and CADR is “per minute,” the formula divides by 60 to convert hours to minutes.
CADR formula in MathML
The CADR calculation can be written as:
where V is the room volume (in cubic feet) and ACH is your desired air changes per hour.
Step-by-step: how to use the calculator
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Choose units (feet or meters).
If your tape measure or room listing is in feet, select “Feet.” If you are more comfortable with metric measurements, select “Meters.” The tool will still output CADR in CFM, because that is how most purifiers are labeled. If you prefer metric airflow, a simple conversion is 1 CFM ≈ 1.7 m³/h.
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Measure room length and width.
Measure wall-to-wall at the widest points. For open floor plans, consider just the specific area you want the purifier to serve, not the entire open space (unless you plan to treat it all with one unit).
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Measure ceiling height.
Use the average height if the ceiling is sloped. For very high ceilings or lofts, the required CADR can be significantly higher because there is more air volume to clean.
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Choose desired air changes per hour (ACH).
ACH reflects how aggressively you want to clean the air:
- 2–4 ACH: Light air cleaning for general comfort in low-use rooms.
- 4–5 ACH: Typical target for general indoor air quality in bedrooms and living rooms.
- 5–8 ACH: Common for allergy and asthma relief, or homes with pets.
- 8+ ACH: More intensive cleaning, closer to what you would see in high-risk or specialized environments.
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Enter filter cost and filter life (optional).
These fields estimate ongoing filter replacement costs. They do not change the CADR result. Instead, they help you compare how expensive different purifiers may be to run over time.
- Replacement filter cost: Typical cost for a single replacement filter set for the purifier you are considering.
- Filter life (months): How long the manufacturer says each filter should last under normal use.
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Run the calculation.
Click “Calculate CADR” to see the suggested minimum CADR in CFM for your room and ACH choice. You can copy the result to compare models from different brands.
Interpreting your CADR result
The number you see is a recommended minimum CADR for your room size and chosen ACH. When you compare air purifiers:
- Look for a purifier whose CADR is at least as high as the recommendation.
- If you are sensitive to allergens or smoke, choosing a higher CADR can provide a safety margin.
- Remember that some products list different CADR values for dust, pollen, and smoke; use the one that best matches your concern (smoke CADR is generally the most conservative).
Most consumer air purifiers show either CADR values certified by AHAM or an airflow rating. Certified CADR values are usually more realistic than raw fan airflow because they account for losses in the filter and housing.
Worked example: small bedroom
Suppose you want to size a purifier for a small bedroom and you are mostly worried about dust and pollen. The room is 12 feet long, 10 feet wide, with an 8-foot ceiling, and you want about 5 ACH.
- Room volume: 12 ft × 10 ft × 8 ft = 960 ft³.
- ACH target: 5 air changes per hour.
- CADR: (960 × 5) ÷ 60 = 4,800 ÷ 60 = 80 CFM.
In this case, a purifier with a CADR of around 80 CFM or higher for dust and pollen should be suitable. If you choose a model with 110–120 CFM, you allow for filter clogging over time, higher pollution days, or running the fan at a lower (quieter) speed while still meeting your target ACH.
Metric example: living room in meters
Now consider a living room that measures 5 m by 4 m with a 2.5 m ceiling. You want better cleanup because you have pets and occasional guests, so you choose 6 ACH.
- Volume in cubic meters: 5 m × 4 m × 2.5 m = 50 m³.
- Convert to cubic feet: 1 m³ ≈ 35.3 ft³, so 50 m³ ≈ 1,765 ft³.
- CADR in CFM: (1,765 × 6) ÷ 60 ≈ 176.5 CFM.
When you shop, you would look for a purifier with at least 180 CFM CADR, preferably more if you want faster cleanup after vacuuming, hosting guests, or opening windows.
Quick CADR guidance for common room sizes
The table below shows approximate CADR targets for typical rooms at a mid-range ACH of about 5. These are rounded for simplicity. Actual results will vary with ceiling height and your exact ACH choice.
| Room type | Approx. size | Example dimensions | Ceiling height | ACH assumption | Suggested minimum CADR (CFM) |
|---|---|---|---|---|---|
| Small bedroom | 120–150 ft² | 10 ft × 12 ft | 8 ft | 5 ACH | 75–95 CFM |
| Medium bedroom | 150–200 ft² | 12 ft × 14 ft | 8 ft | 5 ACH | 95–130 CFM |
| Living room | 220–300 ft² | 14 ft × 18 ft | 8–9 ft | 5 ACH | 150–225 CFM |
| Large open area | 350–450 ft² | 18 ft × 22 ft | 9 ft | 5 ACH | 260–340 CFM |
| High-ceiling space | 300 ft² with 10–12 ft ceiling | 15 ft × 20 ft | 10–12 ft | 5 ACH | 310–375 CFM |
Use these values only as a starting point. The calculator will give you more tailored results based on your exact measurements and ACH choice.
Understanding filter costs and long-term operation
The replacement filter cost and filter life inputs help you estimate what it might cost to run an air purifier in your room over time. For a simple monthly cost estimate, you can use:
Monthly filter cost ≈ filter price ÷ filter life in months.
For example, if a filter set costs $40 and lasts 6 months, the filter cost is about $6.67 per month. If another purifier uses $25 filters every 3 months, that is about $8.33 per month. Even if the second purifier is cheaper upfront, the total cost of ownership may be higher over a few years.
These cost estimates are for budgeting and comparison only. Actual filter life will vary based on how polluted your indoor and outdoor air is, how often you run the purifier, and at what fan speed.
To estimate the CADR you need
Here is a short reference you can skim or use as a featured-style summary:
- Measure your room length, width, and ceiling height.
- Compute room volume (ft³) = length × width × height.
- Choose an ACH target (4–5 for typical rooms, 6–8 for allergies or pets).
- Apply CADR (CFM) = (room volume × ACH) ÷ 60.
- Select a purifier with CADR at or above that value for your primary pollutant (dust, pollen, or smoke).
Assumptions and limitations
This calculator is designed to give a practical, easy-to-understand estimate, not an engineering or medical prescription. It makes several assumptions:
- Single, enclosed room: The calculation assumes you are treating one reasonably enclosed space, not a whole house or a space with constant open doors and windows.
- Reasonably mixed air: It assumes the air in the room mixes fairly evenly over time, so the purifier eventually “sees” most of the air volume. Real rooms may have dead zones or drafts.
- Residential or light commercial use: It targets typical homes, apartments, and small offices, not hospitals, laboratories, or industrial facilities with strict ventilation standards.
- Stable pollution levels: It does not account for sudden spikes in pollutants (for example, a wildfire smoke event, heavy indoor smoking, or a major chemical spill).
- Standard filters and maintenance: It assumes the purifier is used with correctly installed filters that are replaced according to manufacturer guidance.
There are also important limitations:
- Not medical advice: This tool does not diagnose, treat, or prevent any disease. If you have asthma, COPD, or other respiratory or cardiovascular conditions, speak with a qualified health professional about your specific needs.
- Different pollutants behave differently: CADR ratings are typically based on standardized tests using dust, pollen, and smoke. Gases and volatile organic compounds (VOCs), such as formaldehyde or cooking fumes, may require specialized filters like activated carbon or other technologies.
- Building leakage and outdoor air: The calculator does not account for air leaks, mechanical ventilation systems, or open windows and doors, all of which can significantly change effective ACH.
- Noise and fan speed: A purifier might only reach its rated CADR at its highest fan speed, which may be too loud for sleeping. In that case, you may want a higher-rated unit so that a medium speed still meets your target.
- Regulatory and standard differences: CADR certifications and test methods may differ slightly across regions. Always review manufacturer documentation and, when available, independent certifications such as AHAM for North American products.
When to consider a higher CADR than the calculator suggests
You may want to size up your air purifier beyond the minimum result in several situations:
- You live in an area with frequent wildfire smoke or heavy outdoor pollution.
- There are smokers in the home and you are trying to reduce secondhand smoke exposure.
- Multiple pets, carpets, and textiles trap dust and dander that you want to control more aggressively.
- You need the purifier to clean the air quickly before or after specific activities (such as hosting guests or sanding a DIY project).
- You prefer to run the purifier on lower, quieter fan speeds while still hitting your ACH target.
In these scenarios, using 7–8 ACH or more in the calculator can give you a more protective target, or you can use the standard 4–5 ACH and then choose a model with extra CADR capacity.
Summary
This air purifier room size calculator turns your room dimensions and desired air changes per hour into a clear CADR target you can use while shopping. By understanding room volume, ACH, and CADR—and by paying attention to ongoing filter costs—you can select a purifier that fits both your air quality goals and your budget. Use the results as a guide, consider your specific pollutants and sensitivity, and consult professionals when air quality is critical to your health or work.