# Air purifier room size calculator: the smoke CADR a room needs on AHAM's two-thirds rule, and the air changes per hour a given purifier delivers

> AHAM's rule: smoke CADR at least two-thirds of the room's square feet at an 8 ft ceiling (room size = 1.55 x CADR); ACH = CADR x 60 / volume. 180 sq ft: 120 cfm.

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| Source | https://ownerspec.com/air/calculators/air-purifier-room-size-cadr-calculator/ |
| Publisher | OwnerSpec (https://ownerspec.com/) |
| Published | 2026-10-02 |
| Updated | 2026-10-02 |
| Facts verified | 2026-10-02 |
| Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. |

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## Short answer

A portable air purifier is sized from its smoke CADR, the clean air delivery rate that AHAM
Verifide measures in cubic feet per minute. AHAM's consumer rule for an 8 ft ceiling is a
smoke CADR of at least two-thirds of the room's area in square feet; AHAM's own formula in
its 2026 white paper is room size = 1.55 x smoke CADR, the same rule before rounding, since
1 / 1.55 = 0.645. Air changes per hour follow from CADR x 60 / room volume in cubic feet. A
12 ft x 15 ft bedroom is 180 sq ft and, under an 8 ft ceiling, 1,440 cu ft: the two-thirds
rule asks for a smoke CADR of 120 cfm (204 m3/h), and a 143 cfm unit there delivers 143 x
60 / 1,440 = 6.0 air changes per hour, above the 4.8 that AHAM sizes on. For wildfire smoke
AHAM raises the rule to a smoke CADR equal to the room's square feet, 180 cfm here.

## Fact sheet

- **Answer:** AHAM Verifide's rule at an 8 ft ceiling is a smoke CADR of at least two-thirds of the room's square feet; AHAM's formula is room size = 1.55 x smoke CADR. A 12 x 15 ft room (180 sq ft) needs 120 cfm by the two-thirds rule, 116 by the 1.55 factor; a 143 cfm unit there gives 143 x 60 / 1,440 = 6.0 air changes per hour.
- **Formula:** `Minimum smoke CADR (cfm) = room area (sq ft) x 2/3 at an 8 ft ceiling, equivalent to room area / 1.55; air changes per hour = CADR (cfm) x 60 / room volume (cu ft); at ceilings other than 8 ft the calculator uses CADR for 4.8 ACH = 4.8 x volume / 60; wildfire smoke: CADR (cfm) = room area (sq ft)`
- **Inputs:** Room length: 12 ft; Room width: 15 ft; Ceiling height: 8 ft; Smoke CADR of the purifier being considered: 143 cfm
- **Output:** Area 180 sq ft, volume 1,440 cu ft. Minimum smoke CADR 120 cfm (204 m3/h) by the two-thirds rule, 116 cfm by AHAM's 1.55 factor, 180 cfm for wildfire smoke. The 143 cfm unit gives 6.0 air changes per hour.
- **Assumptions:** AHAM and EPA both state the room-size rule for an 8 ft ceiling; for other heights the calculator holds AHAM's 4.8 air changes per hour and scales the CADR with the room volume, which is arithmetic on AHAM's printed equations, not a figure AHAM prints; CADR values are the maker's published smoke CADR figures; none was measured for this page; 1 cfm = 1.699 m3/h and 1 ft = 0.3048 m are unit conversions, not citations
- **Method:** The calculator multiplies length by width for area and by ceiling height for volume, converts metres to feet at 0.3048, returns area x 2/3 and area / 1.55 as minimum smoke CADR in cfm and m3/h, area x 1 for wildfire smoke, and CADR x 60 / volume as air changes per hour for the CADR the reader types. The three listed units are a comparison of maker-published CADR figures against the same rule.
- **Primary source:** [AHAM, Consumer Room Air Cleaners and Equivalent Air Changes per Hour, January 2026 (white paper PDF)](https://ahamverifide.org/wp-content/uploads/2026/02/AHAM-White-Paper_Air-Cleaner-ACH_2025.pdf) (manufacturer or vendor document)
- **Facts verified:** 2026-10-02
- **Last updated:** 2026-10-02
- **Canonical URL (cite this):** https://ownerspec.com/air/calculators/air-purifier-room-size-cadr-calculator/

If you are sizing from the "covers up to 1,748 sq ft" on the box, stop: Coway prints that figure at 1 air change an hour and 361 sq ft at the 4.8 AHAM sizes on.

> Interactive calculator, "Smoke CADR a room needs, and the air changes per hour a purifier delivers": use it on the HTML page at the URL above; the formula, constants and sources are written out below.

## What is the formula?

**CADR** is the clean air delivery rate: AHAM Verifide describes it as the volume of filtered air an air cleaner delivers, scored separately for tobacco smoke, dust and pollen, in cubic feet per minute or cubic metres per hour. **Air changes per hour (ACH)** is how many room volumes pass through the filter in an hour.

| Rule | Formula | Who prints it |
| --- | --- | --- |
| Two-thirds rule | smoke CADR (cfm) >= 2/3 x room area (sq ft), at an 8 ft ceiling | AHAM Verifide, Air Filtration Standards page |
| Suggested room size | room size (sq ft) = 1.55 x smoke CADR (cfm) | AHAM white paper, January 2026; 10 CFR 429.68(a)(4) |
| Air changes per hour | eACH = smoke CADR x 60 / room volume (cu ft) | AHAM white paper, January 2026 |
| Wildfire smoke | smoke CADR (cfm) = room area (sq ft) | AHAM Verifide; AHAM white paper (7.5 eACH) |

The two-thirds rule is the 1.55 factor rounded for a consumer: 1 / 1.55 = 0.645, two-thirds is 0.667, slightly stricter. AHAM derives the factor from its air-change target, room size = 60 / (8 x 4.816) x smoke CADR = 1.557 x smoke CADR, with 4.816 equivalent air changes per hour rounded to 4.8 and 8 the ceiling in feet, on an 80 percent steady-state particle reduction with 1 ACH of infiltration.

EPA's Guide to Air Cleaners in the Home prints the same rule as a chart at an 8 ft ceiling: 65 cfm for 100 sq ft, 130 for 200, 195 for 300, 260 for 400, 325 for 500, 390 for 600. EPA says a higher ceiling needs a cleaner rated for a larger area but prints no factor, so the calculator holds AHAM's 4.8 ACH and scales CADR with volume: arithmetic on AHAM's equation, not a figure either body prints.

## What does a worked example look like?

A 12 ft x 15 ft bedroom, 8 ft ceiling, with a Levoit Core 300-P (143 CFM on Levoit's page) on the shortlist.

1. Area: 12 x 15 = 180 sq ft. Volume: 180 x 8 = 1,440 cu ft.
2. Two-thirds rule: 180 x 2/3 = 120 cfm, or 120 x 1.699 = 204 m3/h.
3. AHAM's factor: 180 / 1.55 = 116 cfm. EPA's chart, at 65 cfm per 100 sq ft, interpolates to 117.
4. Wildfire smoke: 180 cfm.
5. Air changes: 143 x 60 / 1,440 = 5.96, so 6.0 ACH, above AHAM's 4.8.

The factor reproduces three makers' published room sizes.

| Unit (maker's published figures) | Smoke CADR (cfm) | Room size at 4.8 ACH | 1.55 x smoke CADR | Room size at 1 ACH |
| --- | --- | --- | --- | --- |
| Levoit Core 300-P | 143 (printed as 143 CFM) | 222 sq ft | 221.7 | 1,066 sq ft |
| Coway Airmega Mighty AP-1512HH | 233 | 361 sq ft | 361.2 | 1,748 sq ft |
| Winix 5510 | 253 | 392 sq ft (AHAM verified) | 392.2 | up to 1,882 sq ft in 1 hour |

The 1 ACH column is the number on the box: Coway's 1,748 is 233 x 60 / 8, the same CADR at one air change an hour. The Core 300-P covers the worked example; the Coway and the Winix cover a living room of 300 to 390 sq ft, and the [bedroom and living room purifier comparison](/air/reviews/best-air-purifier-for-bedroom-and-living-room/) argues the choice.

## What do the three CADR numbers mean?

Each is a particle size; the test method is the one ENERGY STAR cites as ANSI/AHAM AC-1-2020.

| CADR | Particle (AHAM) | Size class (EPA) | Used for room size? |
| --- | --- | --- | --- |
| Smoke | Engineered tobacco smoke | Small | Yes: the two-thirds rule, the 1.55 factor and 10 CFR 429.68 all use smoke CADR |
| Dust | Fine dust | Medium | No |
| Pollen | Paper mulberry pollen | Large | No |

## What do ENERGY STAR and CARB certify, and what do they not?

Neither rates cleaning. EPA's guide says the ENERGY STAR label "does not refer to its air-cleaning effectiveness in any way". Version 2.0, effective October 17, 2020, set a minimum smoke CADR per watt of 1.9 for 30 to under 100 cfm, 2.4 for 100 to under 150 and 2.9 for 150 and above, tested to ANSI/AHAM AC-1-2020, ozone not above 50 ppb under UL 867. The Version 3.0 cover memo makes it effective October 9, 2025 and moves the metric to PM2.5 CADR per watt, so a Version 2.0 CADR-per-watt figure is not comparable with a Version 3.0 one. The final Version 3.0 thresholds were not read for this page.

CARB's list of certified air cleaning devices is an ozone test: every portable indoor air cleaning device sold to people or businesses in California must be certified, mandatory since 2010 (electronic in-duct cleaners added October 2020), against an ozone emission concentration limit of 0.050 parts per million (50 ppb). CARB says certification does not reflect effectiveness.

**Measure the room before the cart**

Measure the room, run the calculator, then match the smoke CADR: the Levoit Core 300-P at 143 cfm to about 220 sq ft at 4.8 ACH, the Coway Airmega Mighty AP-1512HH at 233 cfm to about 360, the Winix 5510 at 253 cfm to about 390. Skip all three for an open-plan space over 600 sq ft: EPA's chart ends at 390 cfm.

## The picks

- **Levoit Core 300-P**: A bedroom up to about 220 sq ft at an 8 ft ceiling (Levoit's page prints 143 CFM, 56 W, 222 sq ft at 4.8 ACH and 1,066 sq ft at 1 ACH, a 2-year limited warranty and CARB compliance; 143 x 1.55 = 222) Amazon: https://www.amazon.com/dp/B07VVK39F7/?tag=ownerspec-20
- **Coway Airmega Mighty AP-1512HH (white)**: A living room up to about 360 sq ft at an 8 ft ceiling (Coway's page prints smoke CADR 233, dust 246, pollen 240, 77 W, 361 sq ft at 4.8 air changes an hour and 1,748 sq ft at 1; 233 x 1.55 = 361) Amazon: https://www.amazon.com/dp/B01728NLRG/?tag=ownerspec-20
- **Winix 5510**: A large room up to about 390 sq ft, or a smaller room during wildfire smoke (Winix's page prints smoke CADR 253, dust 252, pollen 247, an AHAM verified room size of 392 sq ft at 4.8 ACH and 'AHAM / UL / CARB / Prop65' under certifications; 253 x 1.55 = 392) Amazon: https://www.amazon.com/dp/B0DJG1731C/?tag=ownerspec-20

## Frequently asked questions

### What size air purifier do I need for a 200 sq ft room?

AHAM's blog applies the two-thirds rule: a 200 sq ft room needs a smoke CADR of at least 132 cfm at an 8 ft ceiling. EPA's chart in its Guide to Air Cleaners in the Home prints 130 cfm for 200 sq ft, the same rule rounded the other way. For wildfire smoke AHAM raises the figure to a smoke CADR equal to the square footage, 200 cfm.

### Why does the box say 1,748 sq ft when the calculator says 361?

Both numbers come from the same CADR at different air-change rates. Coway prints 1,748 sq ft for the Airmega Mighty AP-1512HH at 1 air change an hour and 361 sq ft at 4.8. AHAM's formula, room size = 1.55 x smoke CADR, is built on 4.8 equivalent air changes per hour, so the smaller figure is the one AHAM Verifide sizes on.

### What is the difference between smoke, dust and pollen CADR?

AHAM Verifide rates each air cleaner on three particle sizes: tobacco smoke, dust and pollen, which EPA's guide describes as small, medium and large particles. AHAM's white paper names the test particles as engineered tobacco smoke, fine dust and paper mulberry pollen, cited in ANSI/AHAM AC-1. Room-size rules use the smoke CADR, the hardest of the three.

### Does ENERGY STAR on an air purifier say anything about how well it cleans?

No. EPA's Guide to Air Cleaners in the Home says the ENERGY STAR label does not refer to air-cleaning effectiveness in any way. ENERGY STAR Version 3.0, effective October 9, 2025, rates efficiency as PM2.5 CADR per watt, replacing the smoke CADR per watt of Version 2.0, so a CADR-per-watt figure on an older listing is not comparable with a new one.

### What does CARB certification of an air cleaner mean?

CARB's list page says all portable indoor air cleaning devices sold to people or businesses in California must be certified, mandatory since 2010, with electronic in-duct cleaners added in October 2020. The ozone emission concentration limit is 0.050 parts per million (50 ppb). CARB states the certification does not reflect an air cleaner's effectiveness, so it is an ozone test, not a performance rating.


## Sources

- [AHAM, Consumer Room Air Cleaners and Equivalent Air Changes per Hour, January 2026 (white paper PDF)](https://ahamverifide.org/wp-content/uploads/2026/02/AHAM-White-Paper_Air-Cleaner-ACH_2025.pdf) (manufacturer or vendor document)
- [AHAM Verifide, Air Filtration Standards (CADR, the two-thirds rule, 8 ft ceilings, wildfire)](https://ahamverifide.org/ahams-air-filtration-standards/) (manufacturer or vendor document)
- [AHAM, Portable Air Cleaners and Air Changes per Hour, October 2021 (white paper PDF)](https://ahamverifide.org/wp-content/uploads/2021/11/White-Paper-Portable-Air-Cleaners-and-AIr-Changes-per-Hour-FINAL-00106301.pdf) (manufacturer or vendor document)
- [AHAM Consumer Blog, Tips for Choosing the Ideal Portable Room Air Cleaner (200 sq ft needs 132)](https://blog.aham.org/tips-for-choosing-the-ideal-portable-room-air-cleaner/) (manufacturer or vendor document)
- [EPA, Guide to Air Cleaners in the Home (sizing chart, CADR particle sizes, ENERGY STAR and ozone notes)](https://www.epa.gov/indoor-air-quality-iaq/guide-air-cleaners-home) (government or regulatory source)
- [Cornell Law, 10 CFR 429.68 (effective room size = 1.55 x represented smoke CADR)](https://www.law.cornell.edu/cfr/text/10/429.68) (university or extension service)
- [ENERGY STAR, Room Air Cleaners Version 2.0 Specification Rev. May 2022 (PDF)](https://www.energystar.gov/sites/default/files/2024-08/ENERGY%20STAR%20Version%202.0%20Room%20Air%20Cleaners%20Specification%20Rev.%20May%202022.pdf) (government or regulatory source)
- [ENERGY STAR, Room Air Cleaners Version 3.0 Final Specification cover memo (PDF)](https://www.energystar.gov/sites/default/files/2025-01/ENERGY%20STAR%20Version%203.0%20Room%20Air%20Cleaners%20Final%20Specification%20Cover%20Memo%20(002).pdf) (government or regulatory source)
- [CARB, List of CARB-Certified Air Cleaning Devices (ozone limit, who must certify)](https://ww2.arb.ca.gov/list-carb-certified-air-cleaning-devices) (government or regulatory source)
- [Levoit, Core 300-P air purifier product page (143 CFM, 56 W, 222 sq ft at 4.8 ACH)](https://levoit.com/products/core300-p-air-purifier) (manufacturer or vendor document)
- [Coway, Airmega Mighty AP-1512HH product page (CADR smoke 233, dust 246, pollen 240; 361 sq ft at 4.8 air changes)](https://cowaymega.com/products/mighty-ap-1512hh) (manufacturer or vendor document)
- [Winix America, 5510 product page (CADR smoke 253, dust 252, pollen 247; 392 sq ft AHAM verified)](https://www.winixamerica.com/product/5510/) (manufacturer or vendor document)

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