Air purifier sizing

Air Purifier Sizing Calculator: Find Your CADR Rating

Instant result calculator Based on AHAM AC-1 CADR standards

Based on AHAM AC-1, EPA, ENERGY STAR

Room details

Adjust any input — the CADR recommendation updates instantly.

Room size

300 sq ft

1001,500

Primary concern

High-pollution environment (pets, smoking, or heavy dust)

Minimum Required CADR

200 CFM

Default for 300 sq ft, general air quality. Adjust your inputs on the left for your exact recommendation.

Calculation breakdown

Room size	300 sq ft
Base CADR (AHAM 2/3 rule)	200 CFM
Concern factor (general ×1.0)	200 CFM
Recommended CADR	200 CFM

Formula: CADR = Sq Ft × 0.67 × Concern Factor × Pollution Factor per AHAM AC-1 standard. Smoke CADR is the most important rating — always check it.

Compare: CADR vs ACH All air purifier sizing guides

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How This Calculator Works: AHAM AC-1 CADR Sizing Methodology

Our air purifier sizing calculator applies the AHAM AC-1 standard, the ANSI-recognized method for testing and rating portable room air cleaners, combined with EPA indoor air quality guidance and ENERGY STAR specifications. Here is the methodology, the meaning of CADR, and how we translate your room into a specific recommendation.

What CADR Actually Means

CADR (Clean Air Delivery Rate) is the AHAM AC-1 measure of how many cubic feet of clean air a purifier delivers per minute (CFM). It is tested and certified independently for three pollutant types: smoke (0.09-1.0 micron particles — the smallest), dust (0.5-3.0 micron), and pollen (5.0-11.0 micron). The smoke CADR is the most demanding test and the most important number for allergy and asthma sufferers, as sub-micron particles penetrate deepest into the lungs.

The AHAM 2/3 Rule (AHAM AC-1, Section 4.2)

The AHAM standard states that your air purifier's CADR should be at least two-thirds of your room's square footage for effective whole-room cleaning at 4.0 air changes per hour (ACH). This is the industry-standard "2/3 rule" used by ENERGY STAR and the EPA:
Minimum CADR = Room Square Footage × 0.67
For a 300 sq ft bedroom: 300 × 0.67 = 201 CADR minimum. The calculation assumes an 8 ft ceiling — the formula can also be expressed in ACH terms.

Concern Type Multipliers

Different air quality concerns require different air exchange rates:
General air quality: 1.00 multiplier — standard 4.0 ACH, adequate for dust control and everyday air cleaning
Allergies & pollen: 1.25 multiplier — targets 4.8 ACH, providing faster allergen removal per EPA IAQ guidance
Pet dander: 1.25 multiplier — pet allergens are sticky and lighter than air, requiring higher air turnover
Smoke & VOCs: 1.35 multiplier — smoke particles are sub-micron (0.1-0.3 μm) and require the highest CADR and often an activated carbon stage for gaseous pollutants

High-Pollution Environment Factor

When multiple concern types coexist (e.g., pets + smoking + heavy dust), the calculator applies an additional ×1.20 pollution multiplier on top of the concern factor. This ensures the purifier can handle the combined particle load without being undersized. In practice, a 500 sq ft smoking household with pets needs CADR 500+ rather than the 335 CADR for a general 500 sq ft space.

CADR and ACH: the Full Picture

ACH (Air Changes per Hour) is the derived metric that ties CADR to room volume:
ACH = (CADR × 60) ÷ Room Volume (cubic ft)
For a 300 sq ft × 8 ft room = 2,400 cu ft with 200 CADR: (200 × 60) ÷ 2,400 = 5.0 ACH. The AHAM 2/3 rule produces ~4.0 ACH for standard rooms; 4.8 ACH is recommended for allergies per EPA guidance. Above 5.0 ACH, diminishing returns set in — you are spending more on electricity and filter replacements without meaningful air quality improvement.

Important Design Limitation

This calculator sizes for a single room. Air purifiers do not clean air around corners or through closed doors. An open-plan living room + kitchen counts as one room. Bedrooms with closed doors need their own purifier. For whole-home air cleaning, a central HVAC-integrated system (MERV 13+ filter or electronic air cleaner) is the correct approach per EPA residential IAQ guidelines.

Why Air Purifier Sizing Matters

Undersized

Air never gets fully cleaned — the purifier can't keep up with the room's volume
Allergens persist, circulating through the room indefinitely
CADR is too low for the room's square footage; pollutants enter faster than they're removed
Money is wasted on a unit that physically cannot do the job, leaving you unprotected

Oversized

Costs more upfront — you're paying for capacity you don't need
Louder on equivalent fan speeds — a larger unit running at medium is often noisier than a properly sized one on high
Uses more electricity, raising your monthly energy bill for no added benefit
Reaches diminishing returns above ~5 ACH — the difference between 5 and 8 ACH is barely perceptible

The sweet spot: Match CADR to your room size so you hit 4— ACH. That's where you get meaningful air cleaning without overspending on hardware or electricity. The AHAM 2/3 rule (smoke CADR — room sq ft × 0.67) is the most reliable shortcut.

5 Common Air Purifier Sizing Mistakes

Buying by the box's room-size recommendation instead of the CADR number. Manufacturers often inflate room-size claims using optimistic assumptions. The CADR is an independently tested, AHAM-certified number that doesn't lie. Always cross-check the smoke CADR against the AHAM 2/3 rule.
Ignoring CADR type — smoke vs. dust vs. pollen. AHAM rates three CADR numbers: smoke (smallest particles), dust (medium), and pollen (largest). Smoke CADR is almost always the lowest and the most important — fine particles are the hardest to filter and the most harmful. Always use smoke CADR for sizing.
Expecting one purifier to clean multiple rooms with closed doors. A purifier only cleans the air in the room it's in. If the bedroom door is closed, the living room purifier isn't helping. You need one purifier per occupied room, or move a single unit between rooms.
Confusing CADR with ACH and not checking air changes. CADR tells you how fast clean air is delivered; ACH tells you how many times the room's air volume is fully filtered per hour. Always calculate ACH: (CADR × 60) ÷ (sq ft × ceiling height). Target 4+ ACH for general use, 5+ for allergies.
Not accounting for specific concerns. The baseline AHAM 2/3 rule is a starting point. Allergy sufferers should add 25% more CADR. Homes with pets should add 15–0%. Homes in wildfire zones may need 20–0% more. Stack these multipliers if multiple factors apply.

Common scenarios for air purifier sizing

If your situation matches one of these, jump straight to a detailed guide:

Bedroom — quiet sleep + allergen relief Large open space —500+ sq ft living area Pet owners — dander + odor control Allergies or asthma — HEPA-grade CADR

Frequently asked questions

What CADR do I need for my room size?

Follow the AHAM AC-1 2/3 rule: your air purifier's CADR should be at least two-thirds of the room's square footage. A 150 sq ft bedroom needs CADR 100+, a 300 sq ft room needs CADR 200+, a 500 sq ft living room needs CADR 330+, and a 1,000 sq ft open plan needs CADR 660+. For smoke or allergies, add 25-35%.

How do I calculate CADR from room size?

Per AHAM AC-1: CADR = room area (sq ft) × 0.67. For example, 250 sq ft × 0.67 = 168 CADR minimum. For allergies or smoke, multiply by 1.25. For heavy pollution, multiply by an additional 1.2. The calculator above applies all of these factors for you automatically.

Is higher CADR always better?

Not necessarily. Oversized CADR costs more upfront and in electricity, runs louder on higher fan speeds, and reaches diminishing returns above ~5.0 ACH. Match CADR to room size per the AHAM 2/3 rule. Exception: allergy and asthma households benefit from higher CADR targeting 4.8+ ACH with a HEPA filter.

What's the difference between CADR and ACH?

CADR (cfm) is the AHAM AC-1 measurement of how fast a purifier delivers clean air. ACH (Air Changes per Hour) is how many times the room's entire air volume is cleaned in one hour. ACH = (CADR × 60) ÷ room volume. A 300 sq ft × 8 ft room with CADR 200 gets (200 × 60) ÷ 2,400 = 5.0 ACH. Target 4-5 ACH for most homes.

Should I look at smoke, dust, or pollen CADR?

Look at the smoke CADR — it represents the smallest particles (0.09-1.0 micron) and is the hardest test to pass. If a purifier has a high smoke CADR, its dust and pollen CADR will be even higher. AHAM-certified purifiers list all three CADR numbers on the box and on ahampurifiers.org.

What does AHAM Verifide certification mean?

AHAM Verifide means the purifier's CADR, room size recommendation, and energy consumption have been independently tested and verified by an ISO 17025 accredited lab per AHAM AC-1. Without this certification, manufacturer CADR claims are not independently verified.

HEPA or True HEPA — what's the difference?

True HEPA (High Efficiency Particulate Air) captures —9.97% of 0.3 micron particles per DOE standard. 'HEPA-type' or 'HEPA-like' are marketing terms with no regulatory definition and may capture only 85-95% of 0.3 micron particles. For allergies or asthma, always choose True HEPA or AHAM Verifide purifiers.

Do I need an activated carbon filter?

Activated carbon removes gaseous pollutants (VOCs, odors, smoke smell) that HEPA filters cannot capture. If your concern is cooking odors, wildfire smoke, chemical sensitivities, or VOCs from new furniture/paint, choose a purifier with a substantial carbon filter (5+ lbs of carbon, not just a thin carbon sheet).

One large air purifier or multiple small ones?

Air purifiers clean only the room they are in. One large unit in an open-plan area is more efficient and quieter than multiple small ones. But bedrooms with closed doors each need their own unit. A 1,000 sq ft open plan benefits from a single CADR 660+ purifier; two 500 sq ft bedrooms each need their own CADR 335+ unit.

How often should I replace the filter?

HEPA filters typically last 6-12 months depending on usage and air quality. Carbon pre-filters need replacement every 3 months. Most purifiers have a filter replacement indicator. Running a clogged filter reduces CADR by 30-50% and strains the fan motor. Check the manufacturer's recommended replacement schedule.

Top Rated Air Purifier Products

These are the most popular air purifiers across key room sizes. All ratings and prices from Amazon as of June 30, 2026.

Product	Spec	Rating	Price
GermGuardian GG1000 Small Air Purifier, Pluggable UVC Air Sanitizer, Room Deodorizer, Kills Germs, Freshens Air, Reduces Odors from Pets, Mold, Smoking, Cooking, Laundry, Germ Guardian Air Purifier	PLUGGABLE AIR SANITIZER - Provides cleaner air and helps reduce airborne germs and reduce household odors caused by bacteria, pets, and cooking fumes. COMPACT DESIGN - This 7-inch wall-pluggable sanitizer is perfect f…	★★★★ 4.1 (12K+)	$15	Amazon
Medify MA-40 Air Purifier with True HEPA H13 Filter \| 546.5 m² Coverage in 1hr for Smoke, Wildfires, Odors, Pollen, Pets \| Quiet 99.9% Removal to 0.1 Microns \| White, 2-Pack	Color: White \| Brand: Medify Air \| Product Dimensions: 18"D x 2"W x 12"H \| Power Source: Plug in \| Item Weight: 1.5 Pounds	★★★★ 4.6 (33)	$450	Amazon
Austin Air HealthMate Plus Air Purifier - Advanced 4- Stage Filtration Air Purifiers For Home with Activated Carbon & Zeolite - Ideal as Air Purifiers for Bedroom or Large room, Model B450 Sandstone	4-STAGE FILTRATION FOR CLEANER AIR - Featuring a filter with integrated Activated Carbon and Zeolite, this air purifier captures airborne particles, including dust, pollen, pet dander, mold, smoke, and VOCs, offering …	★★★★ 4.6 (209+)	$895	Amazon
SHARP Air Purifier and Humidifier. Plasmacluster Ion Technology and Triple Filtration, including True HEPA Filter. Covers up to 1,650 sq. ft. for Large Rooms, for Dust, Smoke, and Pollen. KC860U	Color: White/Grey \| Brand: Sharp \| Product Dimensions: 11.38"D x 15.75"W x 24.75"H \| Power Source: Ac/dc \| Item Weight: 24.9 Pounds	★★★★ 4.7 (400+)	$567	Amazon