Air Purifier Size Calculator: CADR Needed for Your Room

CADR in one paragraph

Clean air delivery rate (CADR) is the volume of particle-free air a portable purifier delivers, in cubic feet per minute, measured under the AHAM AC-1 test. It combines fan airflow and filter capture into one number. Each purifier gets three CADRs: smoke (the smallest particles), dust and pollen (the largest). Smoke CADR is usually the lowest and the most demanding, so it is the one to size with. More on the label in the CADR rating guide.

The sizing formula

Room volume (ft³) = Floor area × Ceiling height
CADR needed (CFM) = Volume × Target ACH ÷ 60
AHAM rule: Smoke CADR ≥ ⅔ × Floor area (for 8 ft ceilings)

The AHAM two-thirds rule is a shortcut built on an 8-foot ceiling and the equivalent of 5 air changes per hour (⅔ CFM per sq ft × 60 ÷ 8 ft), so it matches the full formula at 5 ACH. Where they differ, trust the volume formula: it accounts for tall ceilings, which the two-thirds rule ignores. The formula is the same one used in the air changes per hour calculator, with CADR in place of fan CFM.

Worked example

A 300 sq ft living room with an 8-foot ceiling, sized for allergies at 5 ACH, with a purifier rated at 250 CFM smoke CADR:

1. Volume: 300 × 8 = 2,400 ft³.
2. CADR needed: 2,400 × 5 ÷ 60 = 200 CFM.
3. Two-thirds rule check: 300 × ⅔ = 200 CFM. The two methods agree.
4. The candidate purifier gives 250 × 60 ÷ 2,400 = 6.3 ACH, and could cover up to 250 × 60 ÷ (5 × 8) = 375 sq ft at this goal.

These numbers match the calculator's default result.

Quick CADR chart

Smoke CADR needed at 5 ACH:

Values are rounded up to the nearest 5 CFM. The AHAM smoke CADR scale tops out around 450, so very large or open spaces are better served by two units or by better filtration in the central system.

Why real-world results fall short

• CADR is measured on the highest speed. Most people run purifiers on a quieter setting. Check the dB or sone level at the speed you will actually use (the sones to decibels converter helps), and size up so a quiet speed still hits your target.
• Filters load up. CADR drops as the filter fills; replace on schedule.
• Placement matters. A unit wedged behind furniture or in a corner recirculates its own air. See where to place an air purifier.
• Open doors and open plans spread the clean air into a larger volume than you calculated.

Purifier, HVAC filter or both?

A central system with a MERV 11–13 filter cleans the whole house whenever the blower runs, but only while it runs. A portable HEPA purifier treats one room continuously. For allergy sufferers the usual plan is both: the best filter your blower can handle without restricting airflow (see the MERV rating chart), plus a correctly sized purifier in the bedroom. True HEPA media captures at least 99.97% of 0.3-micron particles; the HEPA filter guide explains the grades.

Ozone generators and "ionic" units that emit ozone are not a substitute for CADR. Ozone is a lung irritant; choose purifiers that are certified for low or no ozone emission.

Running a purifier for best results

• Run it continuously on auto or a low-to-medium speed rather than in short bursts at high speed; particles are constantly re-entering the air.
• Close the door in the room you are treating, especially a bedroom at night, so the CADR is spent on the volume you sized for.
• Start it before you need it, such as an hour before bed or as soon as wildfire smoke is forecast.
• Keep the pre-filter clean if the unit has one; it protects the main filter and keeps airflow up.
• Watch the energy use. Most portable units draw roughly 20–100 W depending on size and speed, so a year of continuous running costs far less than an air conditioner; the appliance electricity usage page shows how to estimate it.

Frequently asked questions