The short answer
For a 215 ft² (20 m²) bedroom with an 8 ft ceiling, look for a smoke CADR of at least 145 CFM, and check that the purifier still delivers useful cleaning at a speed quiet enough to sleep beside. CADR stands for clean air delivery rate, a standardized test result for how quickly a machine removes particles, and it is measured in CFM, cubic feet of air per minute. Size up if you want it to run quietly on a lower setting.
How we get to 145 CFM
- AHAM guideline: the Association of Home Appliance Manufacturers, whose certification program tests purifier ratings, advises a smoke CADR of at least two-thirds of the floor area. 215 ft² × 2/3 = 143 CFM.
- Air changes: 215 ft² × 8 ft = 1,720 ft³. Five air changes an hour = 8,600 ft³/h, or 143 CFM.
- These are two expressions of the same sizing logic at an 8 ft ceiling, not independent checks. Both yield about 143 CFM, which we round up to 145 CFM.
- Taller ceilings raise the figure: at 9 ft, five air changes need about 161 CFM.
Why five air changes, not two
Marketing coverage often quotes one or two air changes an hour. An air change means the room's full volume of air passes through the filter once; the abbreviation for air changes per hour is ACH, which is how the table below is labeled. One or two ACH is enough for light dust, but for allergy control or smoke we model closer to five. The CADR on the box is measured at the highest speed, which few people can sleep beside, so the real question is how much air the unit moves on the setting you will actually use.
What we look for
- Published smoke CADR matched to the room volume
- Low-speed sound data in decibels, not only a vague “sleep mode”
- Filter cost and availability over three years
- No mandatory app or bright status light beside the bed
Choose for the speed you will use at night
A purifier's listed CADR is normally measured at its highest setting. That number is useful for comparing maximum output, but it may not describe the setting you can sleep beside. Look for a complete sound table and output by speed. If a maker publishes only a minimum sound level without saying how much clean air the machine delivers at that level, the quiet claim is incomplete. For a light sleeper, sizing up can let a larger unit meet the room's need at a lower, less intrusive speed.
How quiet is quiet enough?
There is no universal sleep number because tone, cycling and distance matter as much as a single decibel reading. As a practical screen, look for roughly 20 to 30 dBA (decibels on the A-weighted scale used for everyday sound; for context, a whisper is near 30) on the setting intended for overnight use, then confirm that the machine still moves meaningful air at that setting. A smooth fan sound is usually easier to tolerate than motor whine, rattling or frequent speed changes. Reviews can help identify those qualities, but they do not replace published output data.
Count the filter cost over three years
The purchase price is only the first line of the cost. Suppose a purifier costs $220 and its combined replacement filter costs $55 every nine months. Three years requires four replacements, adding $220, so the three-year equipment and filter cost is about $440 before electricity. If another machine costs $300 but uses one $45 filter each year, its comparable three-year total is $435. Check the actual replacement schedule, whether filters remain in stock, and whether separate carbon prefilters add another recurring cost.
Where to place it
- Leave several inches of clearance around every intake and outlet; use the maker's larger clearance if specified.
- Keep the clean-air outlet unobstructed rather than aiming it into curtains, bedding or a wall.
- Place it within the occupied bedroom, not in the hall, and keep the door position consistent with how you sleep.
- Avoid a spot where the airflow crosses the pillow if the draft or sound will make you switch the unit off.
- Do not place the purifier directly beside a humidifier; concentrated moisture can wet the filter and encourage growth.
What CADR cannot tell you
CADR describes how quickly a machine removes a defined particle type under test conditions. It does not describe carbon dioxide removal, gas removal, long-term reliability, filter cost or sound quality. A carbon layer may reduce some odors, but thin carbon sheets saturate quickly and should not be treated as broad protection from gases or VOCs (volatile organic compounds, gases released by many furnishings and household products). For a bedroom that feels stuffy because two people sleep behind a closed door, ventilation is the relevant intervention; a particle filter cannot add fresh air.
What you will notice, and what you will not
A correctly sized purifier reduces airborne particles, pet dander and smoke within an hour or so. It will not remove carbon dioxide, so a closed bedroom can still feel heavy by 3 a.m.; that is a ventilation question. It will not fix humidity, and it will not remove allergens settled in bedding.
How we judge
We currently analyze published specifications, independent laboratory data where available, and recurring patterns in verified owner reports. We do not yet claim hands-on testing.
Bedroom CADR sizing table
AHAM's two-thirds smoke-CADR rule is equivalent to five air changes per hour with an 8 ft ceiling. A 9 ft ceiling needs more airflow for the same five-air-change target.
| Room area | Metric | 5 ACH at 8 ft | 5 ACH at 9 ft |
|---|---|---|---|
| 100 ft² | 9 m² | 67 CFM | 75 CFM |
| 150 ft² | 14 m² | 100 CFM | 113 CFM |
| 215 ft² | 20 m² | 143 CFM | 161 CFM |
| 250 ft² | 23 m² | 167 CFM | 188 CFM |
| 300 ft² | 28 m² | 200 CFM | 225 CFM |
| 400 ft² | 37 m² | 267 CFM | 300 CFM |
Round up to an available model. Increase the target for ceilings above 8 ft or if you need useful cleaning at a quieter speed.
Interactive tool
Bedroom CADR calculator
Starting target
143 CFM smoke CADR
For 215 ft² at 8 ft, 5 air changes per hour requires 143 CFM. AHAM's two-thirds rule gives 143 CFM and represents about five air changes at an 8 ft ceiling. Round up for quiet overnight use.
Questions, answered
What CADR do I need for a bedroom?
Multiply bedroom floor area in square feet by two-thirds for the minimum smoke CADR. A 215 ft² room starts at about 143 CFM, rounded up to 145 CFM. Size up for a taller ceiling or quieter low-speed operation.
Should an air purifier run all night?
Particle levels usually stay lower when a correctly sized purifier runs continuously. Use a speed you can tolerate, disable unnecessary lights, and follow the maker's safety and filter guidance.
Where should I put an air purifier in a bedroom?
Keep it in the bedroom with its intake and outlet clear. Avoid curtains, bedding, corners that block airflow, and direct proximity to a humidifier. The best position is one that lets it run consistently without blowing across the sleeper.
Will an air purifier fix a stuffy bedroom?
Not when stuffiness comes from accumulated carbon dioxide. A HEPA purifier removes particles but does not add outdoor air. Measure CO₂ if the room becomes heavy overnight and improve ventilation when conditions allow.
Is a bigger air purifier louder?
Not necessarily. A larger purifier can often meet the same clean-air target at a lower, quieter speed. Compare sound and airflow at the speed you will actually use, not only maximum CADR and minimum decibels.
Do I need an app or a status light?
No. Neither improves filtration. For a bedroom, physical controls, a dimmable display and settings that remain after a power interruption are often more useful than an app.
How often should I replace the filter?
Follow the maker's schedule, commonly every 6 to 12 months, and replace sooner after wildfire smoke, renovation dust or heavy use. Include replacement filters when comparing three-year cost.
Are two small purifiers better than one large one?
Two units can distribute clean air more evenly in an awkward or divided room, but they usually cost more to buy and maintain. In a simple bedroom, one correctly sized unit with clear airflow is normally easier.
Sources
- AHAM Verifide: Air Filtration Standards and the two-thirds smoke CADR rule
- US EPA: Guide to Air Cleaners in the Home
- US EPA: Residential Air Cleaners technical summary
General information, not medical advice. Sources and specifications were reviewed on October 1, 2026. We do not yet conduct hands-on testing.