1 · Room → minimum CADR

Enter room dimensions and pick a use case. The recommendation is always rounded up.

Unit system

Length (ft)

Width (ft)

Ceiling height (ft) CADR need scales with volume: × (ceiling / 8).

Use case

Recommended smoke CADR

— CFM

smoke CADR (recommended, rounded up)

— — — —

Three rules, side by side

Rule	Min CADR	ACH @ your ceiling	Honest note

The ⅔ rule (5.0 ACH @ 8 ft) is the most conservative of the three; the AHAM seal (4.8 ACH) and the EPA table (4.875 ACH) are within 3% of each other. Always size by smoke CADR — the smallest-particle rating, and the strictest.

2 · ACH target solver

Pick a target air-change rate and get the CADR it demands: CADR = volume × ACH / 60.

Length (ft)

Width (ft)

Ceiling height (ft)

Target ACH

CADR for your target ACH

— CFM

smoke CADR required

— — —

3 · Inverse — CADR → max room size

You own a purifier. How big a room does its smoke CADR honestly cover?

Smoke CADR (CFM) Use the smoke CADR from the AHAM label or spec sheet, not the pollen/dust numbers.

Maximum room size

— sq ft

AHAM seal formula: CADR × 1.55 (4.8 ACH @ 8 ft)

— —

The AHAM-seal number is the official one for comparing certified models. The ⅔-rule inverse (CADR × 1.5) is shown for reference — it assumes a slightly stricter 5.0 ACH.

4 · ACH checker

Check a purifier you already own against your actual room.

Your purifier's smoke CADR (CFM)

Length (ft)

Width (ft)

Ceiling height (ft)

Your actual ACH

— ACH

filtered air changes per hour

— —

5 · Ceiling-height adjuster

Vaulted or loft ceiling? Drag the slider — CADR need scales with volume (× ceiling / 8).

Room area (sq ft)

Ceiling height

10 ft

Ceiling adjustment

—

volume multiplier (ceiling / 8)

— —

6 · Multi-unit planner

Two smaller purifiers beat one big one when placed at opposite ends. CADR adds up — enter up to 4 units.

Room length (ft)

Room width (ft)

Ceiling height (ft)

Unit 1 smoke CADR

Unit 2 smoke CADR

Unit 3 smoke CADR

Unit 4 smoke CADR

Combined coverage

— CFM

total smoke CADR across units

— —

Assumes decent circulation: place units at opposite ends of the room, keep interior doors closed, and don't block airflow with curtains or furniture.

7 · Converter & manufacturer-claim translator

CFM ↔ m³/h and sq ft ↔ m², live. Plus: translate a brand's "covers X sq ft" claim into its true 5-ACH coverage.

CFM

m³/h

Square feet

Square meters

Manufacturer-claim translator

Many brands advertise coverage at just 1–2 ACH. True 5-ACH coverage = claimed coverage × 0.4.

Brand's claimed coverage (sq ft)

True coverage

— sq ft

real coverage at 5 ACH (not the marketing number)

—

A claim of "covers 1,000 sq ft" at ~2 ACH implies a smoke CADR of only ~333 CFM (2 × 1,000 × 8 / 60), which covers 400 sq ft at a proper 5 ACH. Always compare by smoke CADR, not by headline coverage.

How the math works

Transparent, primary-source model — every number above comes from these formulas:

ACH = CADR(CFM) × 60 / (room area sq ft × ceiling ft) CADR_min = area × ⅔ (2/3 rule) room = smoke CADR × 1.55 (AHAM Verifide seal) ceiling adjustment: CADR × (ceiling_ft / 8) wildfire: CADR = room area (sq ft)

The honest 3-way comparison (8-ft ceiling)

Rule	Formula	ACH @ 8 ft	Source
⅔ rule	area × ⅔	5.0 (exact)	Popular rule, attributed to AHAM
AHAM seal	CADR × 1.55	4.8 (4.84)	AHAM Verifide program documents
EPA table	area × 0.65	4.875	EPA Guide to Air Cleaners in the Home

All three agree within 3%. The ⅔ rule is the most conservative; this calculator rounds its result up and labels each rule's ACH honestly instead of pretending they are identical.

EPA sizing table (verbatim, 8-ft ceiling)

Room area (sq ft)	100	200	300	400	500	600
Minimum CADR (CFM)	65	130	195	260	325	390

EPA: "The CADRs are calculated based on an 8-foot ceiling. If you have higher ceilings, you may want to select a portable air cleaner with a higher CADR." For rooms off the table we use the same 0.65 CFM/sq ft ratio.

Wildfire, ceiling, and multi-unit notes

Case	Rule	Example
Wildfire smoke	smoke CADR = room sq ft	300 sq ft → 300 CFM = 7.5 ACH (EPA DIY-air-cleaner research)
High ceiling	CADR × (h / 8)	400 sq ft, 10 ft: 260 × 1.25 = 325 CFM (conservative pick)
Multiple units	CADR adds up	2 × 150 = 300 CFM covers 450 sq ft at 5.0 ACH
Conversions	1 CFM = 1.699 m³/h	200 CFM = 339.8 m³/h

Worked examples

Room	⅔-rule min	EPA equiv.	Pick
Bedroom 12×15×8 ft (180 sq ft)	120 CFM	117 CFM	120 CFM
Living room 20×20×8 ft (400 sq ft)	267 CFM	260 CFM (table)	267 CFM
Open plan 20×30×8 ft (600 sq ft)	400 CFM	390 CFM (table)	400 CFM
Wildfire: 300 sq ft bedroom	200 CFM	195 CFM	300 CFM (7.5 ACH)
400 sq ft, 10-ft ceiling	267 CFM	260×1.25 = 325 CFM	325 CFM
Two 150-CFM units, 450 sq ft	300 CFM	292.5 CFM	5.0 ACH ✓

Examples computed with the same engine as the calculators above — change any input and re-run to verify.

Boundary: what this tool is not

Canonical boundary: An air purifier changes particle concentration, not temperature, humidity, or air pressure.

Related tool	Its job	Why this tool is different
Bathroom fan (CFM)	Exhausts air outdoors	A purifier recirculates and filters indoor air — it is not an exhaust
Range hood (CFM)	Kitchen exhaust + makeup air	No makeup-air or venting math here; purifiers move no air outside
Relative humidity	Moisture in air	HEPA filtration neither adds nor removes humidity
AC room size	Thermal cooling load (BTU)	ACH here = filtered air changes, not cooling capacity

Frequently asked questions

What size air purifier do I need for a 300 sq ft room?

A smoke CADR of at least 200 CFM. That is the 2/3 rule: 300 x 2/3 = 200 CFM, which delivers exactly 5.0 air changes per hour (ACH) at a standard 8-ft ceiling. The EPA sizing table lists 195 CFM for 300 sq ft at the same ceiling height; we recommend 200 as the conservative round-up. Honest difference: the 2/3 rule targets 5.0 ACH while the EPA table works out to 4.875 ACH, so the rule is slightly stricter - the safer direction to err.

My purifier has a smoke CADR of 200 - what room size does it cover?

Two honest answers from two different rules. The AHAM Verifide seal formula (room = smoke CADR x 1.55) gives 310 sq ft, calculated at 4.8 ACH with an 8-ft ceiling. The 2/3 rule inverted (room = CADR x 1.5) gives 300 sq ft at 5.0 ACH. The AHAM seal is the official number: use 310 sq ft when comparing AHAM-Verifide models, and treat it as an upper bound, not a guarantee.

What does the AHAM Verifide 'suggested room size' actually mean?

It is smoke CADR x 1.55, sized so the purifier delivers 4.8 air changes per hour in a room with an 8-ft ceiling. Example: a smoke CADR of 234 -> 234 x 1.55 = 362.7, about 363 sq ft. Real-world check: the Levoit Core 300-P lists smoke CADR 143 and a 222-sq-ft suggested room size (143 x 1.55 = 221.65). Coverage numbers on brand packaging that assume only 1-2 ACH are not comparable; our claim translator converts them - a 'covers 1,000 sq ft' claim at about 2 ACH is really about 400 sq ft at a proper 5.0 ACH.

How do high ceilings change the CADR I need?

CADR need scales with air volume, not floor area: multiply by ceiling height / 8. For a 400 sq ft room with a 10-ft ceiling, the volume method gives 400 x 10 x 4.8 / 60 = 320 CFM, while the EPA 8-ft value of 260 x 1.25 = 325 CFM. We recommend at least 325 CFM - take the conservative of the two.

What CADR do I need for wildfire smoke?

Size so that smoke CADR equals your room's square footage: a 300 sq ft room needs 300 CFM, which is exactly 7.5 ACH at an 8-ft ceiling (EPA DIY-air-cleaner research cites this AHAM guidance for wildfire smoke). During smoke events outdoor air is a continuous particle source pushing smoke back indoors through the building envelope, so the higher rate is what holds indoor levels below outdoor levels rather than merely tracking them.

Can I use two smaller purifiers instead of one big one?

Yes - CADR adds up, assuming decent circulation. Two units at 150 CFM each give 300 CFM total, which covers a 450 sq ft room needing 450 x 2/3 = 300 CFM at exactly 5.0 ACH. Place the units at opposite ends of the room and keep doors closed so each one serves its own air volume.

Smoke vs dust vs pollen CADR - which number do I size by?

Size by smoke CADR. It is measured with the smallest test particles, so it is the most conservative of the three AHAM numbers; a unit that handles smoke handles dust and pollen at least as well. Ozone safety note: since 2010 every portable air cleaner sold in California must be CARB-certified to emit no more than 0.050 ppm of ozone (verified against carb.ca.gov, regulation 17 CCR 94802). EPA also notes that CARB and AHAM maintain lists of air cleaners tested to emit little or no ozone - check them before buying ionizer or UV units.

Is higher ACH always better?

Up to a point. CADR is measured at maximum fan speed, and sleep or low modes typically deliver only 1/3 to 1/4 of the rated figure - a unit rated 400 CFM may give just 100-133 CFM overnight. So a 300 sq ft room needing 200 CFM is underserved on sleep mode by that 400-CFM unit. The fix is sizing up 10-20% for allergies or pets (target 221-240 CFM here) so quiet speeds still meet the requirement - not chasing extreme ACH numbers that just mean noise and power draw. Also note: the ENERGY STAR label on a purifier means energy efficiency only; per EPA it does not refer to air-cleaning effectiveness in any way, so it is not a sizing signal.

Air Purifier Room Size Calculator (CADR)

1 · Room → minimum CADR

Recommended smoke CADR

Three rules, side by side

2 · ACH target solver

CADR for your target ACH

3 · Inverse — CADR → max room size

Maximum room size

4 · ACH checker

Your actual ACH

5 · Ceiling-height adjuster

Ceiling adjustment

6 · Multi-unit planner

Combined coverage

7 · Converter & manufacturer-claim translator

Manufacturer-claim translator

True coverage

How the math works

The honest 3-way comparison (8-ft ceiling)

EPA sizing table (verbatim, 8-ft ceiling)

Wildfire, ceiling, and multi-unit notes

Worked examples

Boundary: what this tool is not

Frequently asked questions

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