Air Purifier Sizing Calculator: CADR, Room Size, ACH Targets

Last updated: March 2026

Health and Safety Notice: This article provides educational information about air purifier sizing and performance metrics. It is not a substitute for professional indoor air quality assessment. Consult an HVAC or IAQ specialist for building-specific recommendations.

An oversized air purifier wastes money. An undersized one barely moves the needle on pollutant levels. The difference between the two comes down to three numbers: CADR, room volume, and ACH. This guide explains each metric, shows you how to calculate the right unit for any room, and provides quick-reference tables so you can match a purifier to your space in minutes.

Key Metrics Explained

CADR (Clean Air Delivery Rate)

CADR measures the volume of filtered air a purifier delivers per unit of time, expressed in cubic feet per minute (CFM). It is the single most important performance metric for portable air purifiers.

AHAM (Association of Home Appliance Manufacturers) tests CADR separately for three particle types:

Particle Type	Size Range	Examples
Smoke	0.09 - 1.0 microns	Tobacco smoke, wildfire particulates, cooking fumes
Dust	0.5 - 3.0 microns	Household dust, mold spores, pet dander
Pollen	5.0 - 11.0 microns	Tree and grass pollen, large allergens

A purifier’s smoke CADR is the most conservative (hardest to filter) and the best benchmark for general health protection.

ACH (Air Changes Per Hour)

ACH represents how many times the purifier can cycle the entire volume of air in a room in one hour. More air changes mean faster pollutant removal and lower steady-state concentration.

ACH Target	Use Case
2 ACH	Light maintenance in low-pollution rooms
4-5 ACH	AHAM recommendation for general residential use
6+ ACH	Allergy and asthma management, wildfire smoke, post-renovation
8-12 ACH	Hospital-grade / cleanroom environments

The AHAM Verifide certification program uses approximately 4.8 ACH as its benchmark. This is the minimum standard for the “recommended room size” printed on most purifier boxes.

Room Volume

Room volume (in cubic feet) = length (ft) x width (ft) x ceiling height (ft). Standard residential ceiling height is 8 feet. If your ceilings are 9 or 10 feet, you need proportionally more CADR.

The Sizing Formula

The core equation ties all three metrics together:

Required CADR (CFM) = Room Volume (ft3) x Target ACH / 60

Simplified (for 8-foot ceilings):

Required CADR (CFM) = Room Area (ft2) x 8 x Target ACH / 60

Example Calculation

A 200 sq ft bedroom with 8-foot ceilings and a target of 5 ACH:

• Room volume = 200 x 8 = 1,600 ft3
• Required CADR = 1,600 x 5 / 60 = 133 CFM

You would need a purifier with a smoke CADR of at least 133 CFM for that room.

Quick-Reference Sizing Tables

Standard Ceiling Height (8 ft), 5 ACH Target

Room Area (ft2)	Room Type	Minimum CADR (CFM)
100	Small office, nursery	67
150	Bedroom	100
200	Master bedroom	133
250	Living room	167
300	Large living room	200
400	Open-plan area	267
500	Large open-plan	333
600	Studio apartment	400

High Ceiling Adjustment (10 ft), 5 ACH Target

Room Area (ft2)	Minimum CADR (CFM)
150	125
200	167
300	250
400	333
500	417

Elevated ACH Targets (8 ft ceiling)

Room Area (ft2)	4 ACH	5 ACH	6 ACH	8 ACH
150	80	100	120	160
200	107	133	160	213
300	160	200	240	320
400	213	267	320	427

Common Sizing Mistakes

Mistake 1: Trusting the “Recommended Room Size” on the Box

Manufacturers often advertise room coverage based on 1-2 ACH, which is far below the AHAM-recommended 4.8 ACH. A unit rated for “500 sq ft” at 1 ACH actually cleans only 100-170 sq ft at the 5 ACH level that produces meaningful health benefits. Always check the CADR number and run the formula yourself.

Mistake 2: Ignoring Open Floor Plans

If your living room opens to a kitchen and hallway with no doors, you need to size the purifier for the combined area, not just the room where the unit sits. Partially open doorways allow about 50% airflow exchange — close doors when possible to maximize a purifier’s effectiveness.

Mistake 3: Forgetting Ceiling Height

A loft with 12-foot ceilings has 50% more air volume than the same floor area with 8-foot ceilings. Always multiply by actual ceiling height, not the standard 8 feet.

Mistake 4: Using the Wrong CADR Rating

Dust CADR and pollen CADR are always higher than smoke CADR because larger particles are easier to filter. For health-protective sizing, use the smoke CADR — it represents worst-case performance.

Matching Purifiers to Pollutant Types

Different filter technologies excel at different pollutants. Pair the right technology with the right CADR:

Pollutant	Best Filter Type	CADR Relevance
Wildfire smoke / PM2.5	True HEPA (H13+)	Smoke CADR
Pollen / pet dander	HEPA or MERV 13+	Pollen CADR
Dust / mold spores	HEPA or MERV 11+	Dust CADR
VOCs / formaldehyde	Activated carbon	Not captured by CADR
Odors / cooking fumes	Activated carbon	Not captured by CADR

CADR measures particle removal only. For gaseous pollutants like VOCs, activated carbon capacity (measured in weight of carbon media) is the relevant metric. For a detailed comparison of filter technologies, see our HEPA vs MERV vs carbon filter guide.

For more on how AI-powered monitors can track your air quality in real time and help you validate purifier performance, see our indoor air quality monitoring guide.

When One Purifier Is Not Enough

Consider multiple units when:

• The space exceeds 500 sq ft and no single portable unit has sufficient CADR.
• The room has an L-shape or significant obstructions that block airflow.
• You need different filter types in different zones (e.g., HEPA in the bedroom, carbon-heavy in the kitchen).
• You want redundancy during wildfire season so you always have clean air if one unit’s filter is being replaced.

For whole-house solutions that integrate with your HVAC system, explore our HVAC air filtration guide and our air purifier comparison.

Frequently Asked Questions

What CADR do I need for wildfire smoke?

Target at least 6 ACH during active smoke events. For a 200 sq ft room, that means a smoke CADR of at least 160 CFM. During severe smoke events (AQI above 200), consider running two units or moving to a smaller sealed room.

Does fan speed affect CADR?

Yes. Published CADR is measured at the highest fan speed. On lower speed settings, the effective CADR drops proportionally — sometimes by 50% or more. If noise is a concern, buy a unit rated for a larger room so you can run it on medium speed and still achieve your ACH target.

Can I use CADR to compare purifiers from different brands?

Yes, as long as both units are AHAM Verifide certified. AHAM testing uses a standardized protocol (ANSI/AHAM AC-1), so CADR numbers are directly comparable across certified products. Non-certified units may report inflated CADR values.

What about MERV-rated HVAC filters? Do they have CADR?

No. MERV ratings apply to HVAC filters and measure particle capture efficiency, not clean air delivery volume. You cannot directly compare a MERV rating to a CADR number. However, our indoor air quality standards guide covers how MERV and HEPA relate to EPA/WHO benchmarks.

Should I run my air purifier 24/7?

For consistent IAQ, yes. Modern purifiers on low or auto mode consume 5-50 watts — comparable to a light bulb. The health benefit of continuous filtration outweighs the modest electricity cost, especially in homes with asthma or allergy sufferers. Our complete home air quality guide covers strategies for whole-home, around-the-clock air management.

---

Sources:

1. AHAM Verifide, “Air Filtration Standards.” https://ahamverifide.org/ahams-air-filtration-standards/
2. Harvard T.H. Chan School of Public Health, “Portable Air Cleaner Sizing Tool.” https://healthybuildings.hsph.harvard.edu/tools/air-cleaner-calculator/
3. HouseFresh, “CADR Calculator: What Size Air Purifier Do I Need?” https://housefresh.com/cadr-calculator/