Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.
Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.
Summary
Every air cleaner has a CADR (Clean Air Delivery Rate) number in cubic feet per minute (CFM) on its packaging. That number must match your room size, and the sizing calculation depends entirely on ceiling height — a factor that nearly every retailer and product page omits. Most rooms have 8-foot ceilings, and the calculation assumes this as a baseline. Taller ceilings require a proportionally higher CADR. This page teaches the method so you can calculate your own requirement and then match a cleaner to it. If your room has an open floor plan or unusual shape, or if your cleaner needs to serve multiple adjoining spaces, this page stops and sends you to the manufacturer for a custom recommendation.
Necessary equipment
| Item | Note |
|---|---|
| Tape measure | Measure the length, width, and height of the room in feet. |
| Pencil and paper | Record your measurements and calculations. |
| Calculator | Calculate the room area and required CADR value. |
| Air cleaner specifications | The CADR rating is on the product package and the manufacturer website. |
Specifications
Source: Guide to Air Cleaners in the Home · tier 1 · accessed 2026-07-28
Source: Guide to Air Cleaners in the Home · tier 1 · accessed 2026-07-28
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
Measure your room
Estimated 1 min
- Measure the length of the room in feet from wall to wall.<1mReport a problem with step 1, Measure your room
- Measure the width of the room in feet from wall to wall.<1mReport a problem with step 2, Measure your room
Measure the ceiling height
Estimated under a minute
- Measure the distance from the floor to the ceiling in feet.<1mReport a problem with step 1, Measure the ceiling height
- Record this measurement precisely.
- Most ceilings are 8 feet.
- Commercial spaces often are 9 or 10 feet.
- Vaulted ceilings vary.
Calculate the room area
Estimated 1 min
- Multiply the length and width together to calculate the area in square feet.1mReport a problem with step 1, Calculate the room area
- A room 12 feet long and 15 feet wide has an area of 12 × 15 = 180 square feet.
Understand CADR ratings
Estimated 2 min
- Locate the CADR rating on the air cleaner package or datasheet.2mReport a problem with step 1, Understand CADR ratings
- AHAM-certified air cleaners publish three separate CADR numbers.
- The numbers cover tobacco smoke, dust, and pollen.
- Each number represents a different particle size.
- Tobacco smoke CADR is most useful for general air cleaning.
CADR DOES NOT REMOVE CARBON MONOXIDE.
Carbon monoxide is colourless and odourless. Exposure can cause death.
A HIGH CADR DOES NOT REMOVE ODOURS, GASES, OR VAPOURS.
CADR rates only particles.
- Understand that CADR measures only particle removal, not gases.<1mReport a problem with step 2, Understand CADR ratings
- CADR does not remove odours or smoke smell.
- CADR does not remove volatile organic compounds.
- If you need to remove gases, consult the air cleaner specifications for activated carbon filters.
Calculate required CADR
Estimated 3 min
- Multiply your room area by two-thirds to find the minimum required CADR.1mReport a problem with step 1, Calculate required CADR
- For a room of 180 square feet, the minimum required CADR is 120 CFM.
- The EPA rule: every 100 square feet at 8-foot ceiling needs about 65 CFM CADR.
- This is equivalent to the two-thirds rule.
ACCOUNT FOR CEILING HEIGHT WHEN YOU CALCULATE THE CADR YOU NEED.
An air cleaner sized for 8 feet will not clean a room with 10-foot ceilings.
- For ceilings higher than 8 feet, adjust your CADR requirement upward.2mReport a problem with step 2, Calculate required CADR
- Divide your ceiling height by 8.
- Multiply the CADR from the previous step by this result.
- Example: a 10-foot ceiling in a room of 180 square feet needs 150 CFM.
Select an air cleaner
Estimated 4 min
DO NOT CHOOSE AN AIR CLEANER ON ROOM SIZE ALONE.
The manufacturer label 'for rooms up to 400 square feet' assumes an 8-foot ceiling.
- Choose an air cleaner with a CADR equal to or greater than your calculated requirement.3mReport a problem with step 1, Select an air cleaner
- If your requirement is 120 CFM, choose a cleaner with a tobacco smoke CADR of at least 120.
- A higher CADR is acceptable.
- A lower CADR will not clean adequately.
- Review the pollen and dust CADR values as well if those are a concern.
- Look for the AHAM Verifide seal on the air cleaner.1mReport a problem with step 2, Select an air cleaner
- AHAM is the Association of Home Appliance Manufacturers.
- AHAM Verifide certifies air cleaners through standard testing.
Terminology
This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.
- Clean Air Delivery Rate (CADR)
- Also called: no common alternatives
- room square footage
- Also called: no common alternatives
- ceiling height
- Also called: no common alternatives
When to use a professional
For open floor plans or rooms that connect to adjacent spaces, consult the manufacturer. For rooms with vaulted ceilings or irregular shapes, obtain a custom recommendation.
Common questions
What does the 2/3 rule mean?
The EPA recommends that an air cleaner's CADR should be at least 2/3 (or 67%) of your room's square footage. A 300 square foot room needs a minimum CADR of 300 × 0.67 = 201 CFM. This rule assumes a standard 8-foot ceiling.
Why is ceiling height important?
Ceiling height determines the volume of air in the room. A room with a 10-foot ceiling contains 25% more air than an identical room with an 8-foot ceiling. An air cleaner sized for 8 feet will not change the air fast enough in a 10-foot space. The higher the ceiling, the larger the CADR you need.
What if my room has a non-standard shape?
If your room is L-shaped, triangular, or has an unusual layout, measure it in separate rectangular sections and calculate the area of each. Add the sections together to get the total area. Then apply the sizing rule. If the room has an open floor plan or connects to adjacent spaces, the sizing becomes much more complex and you should contact the manufacturer for a custom recommendation.
Do I need to consider the CADR for dust and pollen or just tobacco smoke?
All three CADR ratings (tobacco smoke, dust, pollen) should be at least as high as your calculated requirement. They are often close to each other, but not always. Tobacco smoke CADR is the most stringent rating and usually the lowest of the three, so if it meets your requirement, the others usually will too. Check all three to be certain.
What if no air cleaner available has a CADR high enough?
If no single air cleaner meets your requirement, you have two options: use two smaller cleaners (place one on each side of the room), or choose a cleaner with the highest available CADR and accept that it will clean the air more slowly. Consult the manufacturer for guidance on multiple units in the same space.
Will an air cleaner with a higher CADR than I need be a problem?
No. An air cleaner with a CADR higher than your calculated requirement will clean the room faster and more thoroughly. The only downside may be noise — higher-CADR units often operate at higher fan speeds. Check the manufacturer's noise rating if noise is a concern.
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Sources
Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.
Revision history
Current version 1 · updated 2026-08-03 · next review due 2027-08-03
- v1 · 2026-08-03First publication.
Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.
Comments and corrections
Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.
Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.