Blueair 511i Max vs Coway Airmega Mighty: which to buy

The short answer

Two machines, two different jobs. The deciding question is the size of the room, not the quality of the brand.

• Buy the Blueair Blue Pure 511i Max for an average bedroom where fan noise, running cost and sticker price are what you care about. $139.99, 19 dB floor, 17 watts, smoke CADR 153 cfm.
• Buy the Coway Airmega Mighty for a large bedroom, an open-plan sleeping space or a wildfire-smoke season. $239.99, 24 dB(A) floor, 77 watts, smoke CADR 233 cfm.
• Our pick for most readers is the Blueair, on the basis that most US bedrooms are inside its published 237 square foot coverage and the extra Coway capacity is paid for three times over.
• Neither is a medical device. The EPA says improvements in health from portable HEPA cleaners are typically small and not always noticeable to the individual.

Spec sheet, side by side

Every figure is from each vendor's own US pages, read on October 13, 2026. Where a vendor publishes two different numbers for the same thing, both appear here rather than the flattering one.

• List price: Blueair $139.99. Coway $239.99.
• CADR: Blueair pollen 174, dust 159, smoke 153 cfm. Coway smoke 233, dust 246, pollen 240.
• Strict coverage claim: Blueair 237 sq ft at 4.8 air changes per hour. Coway rooms up to 361 sq ft.
• Loose coverage claim: Blueair does not publish one. Coway states it purifies 1,748 sq ft in 60 minutes.
• Filtration stages: Blueair one filter, HEPASilent electrostatic plus mechanical. Coway four stages: pre-filter, deodorization, Green True HEPA, bipolar device.
• Capture claim: Blueair 99.97% of airborne particles down to 0.1 microns, with a footnote marker. Coway 99.97% of airborne allergens as small as 0.3 microns.
• Filter life: Blueair up to 9 months, tracked by its RealTrack algorithm. Coway deodorization 6 months, HEPA 1 year.
• Filter price: Blueair $29.99 standard, $38.99 SmokeBlock, $44.99 AllergenBlock, $8.99 fabric pre-filter. Coway $59.79 for the set of one HEPA and two deodorization filters.
• Noise: Blueair 19 to 48 db in the spec table, with the body copy on the same page claiming no louder than 42 dB. Coway 24 to 53 dB(A).
• Power: Blueair 17 W. Coway 77 W.
• Size: Blueair 14.5 x 9.3 x 9.3 in, 4.2 lb. Coway 16.8 x 18.3 x 9.6 in, 12.3 lb.
• Controls: Blueair speeds 1 to 4 plus Auto and Night. Coway five airflow stages including Eco, plus a 1, 4 or 8 hour timer and a filter change indicator.
• Warranty: Blueair 1 year, extended to 3 with registration. Coway 1 year on internal and functional parts, 3 years on motor and electronic parts for this model.
• Returns: Blueair 30-day home trial. Coway 30 days from shipped date, buyer pays return shipping unless Coway erred, up to 15% restocking fee on incomplete returns.
• On-page rating: Blueair 4.6 from 2,228 reviews with a published star breakdown. Coway 4.7 from 544 reviews with no published breakdown.

Coway moves half again as much air, and its coverage claim survives the arithmetic

On the only metric that governs sizing, Coway wins outright. Its published smoke CADR is 233 cfm against Blueair's 153 cfm.

AHAM, which defines the measurement, says CADR should be at least two thirds of the room's area, and gives a worked example: a 120 square foot room wants a smoke CADR of at least 80.

Run that backwards, which is our arithmetic rather than either vendor's. Blueair's 153 cfm satisfies the two-thirds rule up to about 229 square feet. Coway's 233 cfm satisfies it up to about 349 square feet.

Now compare those numbers to what each vendor claims. Blueair says 237 square feet. Coway says 361 square feet. Both claims sit within a few percent of the AHAM arithmetic, which is the mark of a coverage figure you can trust.

The EPA's own sizing table lands in the same place: 200 square feet wants a minimum CADR of 130 cfm, 300 square feet wants 195, and 400 square feet wants 260. Blueair clears 200. Coway clears 300 and misses 400.

Ignore Coway's 1,748 square feet in 60 minutes claim when you are choosing. It is a different measurement and it will mislead you into undersizing.

Blueair is quieter at both ends, but its own page cannot agree on how quiet

Blueair's spec row says 19 to 48 db. Coway's says 24 to 53 dB(A). On those figures Blueair wins at both the floor and the ceiling by about five decibels.

The complication is that Blueair's own marketing paragraph, on the same page, says the unit starts at 19 db and is no louder than 42 dB on high fan speed. That is a six decibel gap inside one page.

We cannot resolve it. This is a documentation comparison and we do not take sound measurements.

What we can say is that on either Blueair figure, the quiet end is lower than Coway's, and on the pessimistic Blueair figure the ceilings are within five decibels of each other.

For a light sleeper the floor is the number that matters, because that is the setting you will actually run overnight.

Blueair's Night mode and Coway's Eco mode are different solutions to the same problem: Blueair runs very slowly, Coway stops the fan entirely until it detects pollution again.

If a stopped fan is what you want, Coway's Eco mode is the only setting of its kind here. If a permanently low hum is what you want, Blueair's 19 dB is the lower number.

Neither of these replaces earplugs if the noise in the room is a person rather than a motor.

Pricing: Blueair is $100 cheaper up front and cheaper to feed every year

Blueair is $139.99. Coway is $239.99. Both were checked on October 13, 2026.

The consumables widen the gap. Blueair's standard Particle + Carbon filter is $29.99 and rated up to nine months, which our arithmetic puts at about $40 a year.

Coway's set is $59.79 and maps almost exactly to one year of use, so about $59.79 a year.

Electricity widens it again. Our arithmetic, from each vendor's own wattage, run flat out for a full year: Blueair at 17 watts gives roughly 149 kilowatt hours, Coway at 77 watts gives roughly 675.

Multiply by your own utility rate rather than ours, because we are not going to invent an electricity price.

Add the first three years, device plus filters only and ignoring power. Blueair comes to roughly $260. Coway comes to roughly $419. Those totals are editorial arithmetic from published prices, not figures either vendor publishes.

The third option worth pricing is the Levoit Core 300S-P at $149.99 with $29.99 filters every 6 to 8 months, which lands between the two on cost and below both on published CADR detail.

Coway has four filtration stages, and the fourth one needs an answer before you buy

Blueair runs a single combined filter using what it calls HEPASilent dual filtration, which pairs electrostatic charging with mechanical filtration. Coway runs four stages: a pre-filter, a deodorization filter, a Green True HEPA filter, and a bipolar device.

On paper the four-stage path is the more complete one, and the two deodorization filters in Coway's standard replacement set tell you the brand expects odour to be a real part of the job.

The bipolar device is the open question. Coway's product page names it and does not say what it emits.

The EPA says to avoid portable air cleaners that intentionally produce ozone, because ozone is a lung irritant.

It also says that air cleaners containing electrostatic precipitators, ionizers, UV lights without adequate lamp coatings, and plasma air cleaners may have the potential to emit ozone, and it points readers to the California Air Resources Board.

That warning technically touches both machines, since HEPASilent uses electrostatic charging as part of its design. Neither page we read carried an ozone statement or a CARB certification mark on October 13, 2026.

We are not saying either unit emits ozone. We are saying the documentation does not answer the question, and if anyone in the bedroom has an airway condition, that question belongs with the vendor and a clinician before it belongs with a checkout button.

Warranty and returns: Coway covers the motor longer; Blueair asks you to register

Blueair's page lists a one-year warranty as standard and says registering the product extends it to three years. Coway's warranty page lists one year on internal and functional parts across all Airmega models, with three years on motor and electronic parts for this model.

In effect both can reach three years, but Blueair requires an action from you and Coway does not for the motor.

The returns small print is where they really separate. Blueair advertises a 30-day home trial on the product page, with free extended warranty mentioned alongside it.

Coway's refund policy gives 30 days from the shipped date on cowaymega.com, states that shipping, handling and processing costs on a refund are the buyer's responsibility except where Coway was at fault, and allows a restocking fee equal to the higher of 15% of the original price or the retail value of missing parts.

On a $239.99 device, a 15% restocking fee is about $36. Keep the box.

Coway also excludes damage caused by non-genuine or unauthorised filters and requires proof of original retail purchase from an authorised dealer.

That is standard, and it is a reason to buy filters from the vendor rather than a marketplace listing, the same caution that applies to wellness gear bought on a supermarket run.

The review averages favour Coway; the published breakdown favours Blueair

Coway's page showed 4.7 out of 5 from 544 reviews on October 13, 2026. Blueair's showed 4.6 from 2,228.

Taken alone, Coway wins. Taken with what each vendor publishes underneath, the picture changes.

Blueair publishes the full star breakdown: 1,806 five-star, 162 four-star, 79 three-star, 46 two-star and 135 one-star reviews.

Our arithmetic makes the one-star share about 6% of the total, which is a useful thing to know and a slightly uncomfortable thing for a brand to publish.

Coway publishes no breakdown on the page we read. A 4.7 average with no distribution behind it is a weaker signal than a 4.6 with one, because you cannot see whether the complaints cluster.

In the Blueair reviews published on its own page, buyers frequently report units that stopped powering on and replacements handled under warranty, which is also why we treat the registration step as important rather than optional.

In the Coway reviews visible on its page, buyers frequently report running it overnight in a bedroom and in apartments with little ventilation, and describe the fan as audible but tolerable.

We did not contact any buyer and we are not quoting individuals. This is what each vendor's public feed showed on the date we read it.

What neither of these will do

Neither machine is a treatment, and the EPA's own words are the ceiling on what to expect.

It says several studies using portable HEPA air cleaners have demonstrated small improvements in cardiovascular and respiratory health, and that the improvements are typically small and not always noticeable to the individual, although they may be measurable by health professionals.

On allergies and asthma it says multiple studies found improvements in one or more symptoms, that the studies do not always show major improvements, and that they do not show improvements in all symptoms.

It also says filtration does not replace the need to control pollutants and ventilate, and that portable air cleaners do not address the cause of mould.

Neither will cool a hot room, which is the Eight Sleep Pod question. Neither will deal with light, which is the sunrise alarm question. Neither will quiet a partner.

If you are not yet sure which variable is wrecking your sleep, the drug-free sleep toolkit is the cheaper place to start than either of these.

Nothing on this site is medical advice. We are not clinicians, and a worsening cough, wheeze or sleep problem belongs with one.

Choose the Blueair Blue Pure 511i Max if

This is the default recommendation for a bedroom, and these are the conditions under which it is clearly right.

• Your bedroom is under about 230 square feet, which covers the large majority of US bedrooms.
• Fan noise is what wakes you. A 19 dB floor plus Night mode is the lowest published figure of the two.
• You want the lowest total cost. $139.99 plus about $40 a year in filters plus 17 watts is the cheapest line in this comparison by some distance.
• You want a vendor that publishes its star distribution rather than only an average.
• You will register the product, because without that step you are buying a one-year warranty.

Choose the Coway Airmega Mighty if

Capacity is the only reason to pay the premium, and it is a good enough reason when the room demands it.

• The room is larger than about 230 square feet, or the bedroom opens onto another space.
• Wildfire smoke is the reason you are shopping. AHAM asks for a smoke CADR equal to the room's square footage in that case, which only 233 cfm gets anywhere near.
• Odours are part of the problem. Four stages, two of them deodorizing in the replacement set, is the stronger hand.
• You want the fan to stop entirely when the air is clean. Eco mode does that; nothing on the Blueair does.
• You want three years on the motor without filling in a registration form.

Bottom line

For most readers of this site, the Blueair Blue Pure 511i Max is the better purchase.

It is $100 cheaper, roughly a third cheaper to feed, draws a quarter of the power, is quieter at both ends of its range on its own spec table, and publishes all three AHAM CADR figures so you can check the claim yourself.

The Coway Airmega Mighty is the right machine for a room the Blueair cannot cover. Its smoke CADR of 233 cfm is the real differentiator, its coverage claim stands up to AHAM arithmetic, and its Eco mode is genuinely useful.

You pay for all of it in price, noise, weight and watts.

Both leave one question unanswered, and it is the same question: what, exactly, the electrostatic or bipolar part of the airflow emits. Ask before the return window closes.

All figures in this comparison were read from vendor pages and from EPA and AHAM guidance on October 13, 2026. Nothing here is medical advice.