Choose by the room you need to clean. The 211i Max has more clean-air capacity; the 311i Max takes less space and weighs less. The smaller model is not automatically the better purchase for a bedroom, and a large one-hour coverage claim is not equivalent to frequent air cleaning.

The useful specification comparison

These are Blueair’s U.S. technical specifications checked on September 10, 2026, not our laboratory measurements.

Specification Blue Pure 211i Max Blue Pure 311i Max
Smoke CADR 435 cubic feet per minute 283 cubic feet per minute
Recommended room at 4.8 air changes/hour 674 sq ft 439 sq ft
Height × width × depth 22.7 × 14.4 × 14.4 inches 19 × 12.5 × 12.5 inches
Weight 16.2 lb 7.8 lb
Published sound range 23–53 dB 23–50 dB

Sources: 211i Max technical specifications and 311i Max technical specifications . Blueair says its CADR and room-size values have changed with DOE standards, so older packaging may show different figures. The 311i page also has conflicting headline copy; this comparison uses its technical-specifications section consistently. The 311i Max and 311i+ Max are different listings.

Translate that into your room

Measure the space that stays connected while the purifier runs. A 12 × 15 ft room with an 8 ft ceiling contains 1,440 cubic feet. At a chosen target of 4.8 air changes per hour, the calculation is:

1,440 × 4.8 ÷ 60 = 115.2 cubic feet per minute.

Both models’ published maximum smoke CADR exceeds that example. A 20 × 25 ft room with the same ceiling needs 320 cfm at that target: above the 311i Max’s published 283 cfm, but below the 211i Max’s 435 cfm. That is a concrete reason to consider the larger model. Tall ceilings and open doorways change the calculation.

Rated CADR does not describe every fan speed. A quiet night setting may provide less clean air than the published maximum. If you want strong cleaning while sleeping, evaluate the speed you can tolerate during the seller’s return period rather than assuming the lowest advertised noise and maximum CADR happen together.

Compare the exact offers

These are named searches, not verified stock offers. Check the exact model, included filter, seller and return terms. A search can also show replacement filters or the 311i+ Max, neither of which is the same purchase.

Compare a replacement filter for each exact model before checkout. Add its current cost to the appliance price and check the recommended replacement conditions; subscription discounts are not the same as a one-time price. Leave the intake and outlet clear according to the manual rather than squeezing either purifier behind furniture.

Which should you buy?

  • 311i Max: a room within its capacity where its smaller footprint and lower weight solve a real placement or moving problem.
  • 211i Max: a larger space, higher ceiling or need for more capacity headroom, provided the dock-sized footprint fits with clear airflow.
  • Neither yet: you have not measured the connected room, cannot tolerate the needed fan speed, or expect a particle purifier to remove every gas or make smoking indoors safe.

The EPA’s air-cleaner guidance explains that filtration supplements source control and cannot remove every pollutant. Pick the capacity for your actual space, then compare the cost and placement of the two exact models.