CADR and ACH: two different numbers that work together
CADR and ACH get used interchangeably, but they measure different things. See how the sizing formula connects a product's CADR to your room's ACH target.
Key takeaways
- CADR is a fixed specification of a product, measured in cubic metres per hour of clean air delivered.
- ACH is a target you set for a room based on how it's used, not a number printed on any product.
- The sizing formula links the two: floor area times ceiling height times target ACH gives you the CADR you need.
- Checking a product's CADR rating against your room's actual required figure avoids both under-sizing and unnecessary overspending.
Why CADR and ACH get used interchangeably and shouldn't be
It's common to see both terms dropped into product descriptions and buying guides as if they mean roughly the same thing. They don't. Confusing them leads to two common mistakes: choosing a unit based on CADR alone without checking whether it actually meets your room's ACH target, or setting an ACH target without knowing how to translate it into anything a product spec sheet actually states.
Keeping the two separate in your head, one describes the product, one describes the room, makes the rest of the sizing process much easier to follow.
This confusion isn't unique to air purifiers, similar mix-ups happen with aircon sizing, but it's worth untangling here since getting it wrong means either an undersized unit or an unnecessarily large one.
CADR as a product spec, ACH as a room-sizing target
CADR, or clean-air delivery rate, is a fixed number stated on a product, measured in cubic metres per hour, describing how much clean air that specific unit can produce. It doesn't change based on where you put the unit, it's a property of the machine itself.
ACH, or air changes per hour, is the opposite: it's not a product property at all, it's a target you set based on how a room is used, recommended at 3 to 4 for homes, 4 to 5 for offices, 5 or more for clinics, and 6 or more for dental and procedure rooms. The same unit could meet the ACH target in a small bedroom and fall well short of it in a large open office.
This is precisely why quoting a unit's CADR on its own, without knowing the room it's going into, tells you very little about whether it's actually the right choice.
How the sizing formula connects the two numbers
The formula that links them is: required CADR equals floor area in square metres, multiplied by ceiling height in metres, multiplied by target ACH. This converts a room-based target into a product-based specification you can actually shop against, which is the whole point of doing the calculation before buying anything.
Without this step, a buyer is left comparing CADR numbers between products with no reference point for whether either number actually suits their room, which is how oversized or undersized purchases happen.
It also means two different rooms with the same recommended ACH target can end up needing very different CADR figures, purely because their floor areas and ceiling heights differ.
A worked example from floor area to required CADR
Take a 40 square metre Singapore office room with a 2.5 metre ceiling, aiming for the recommended 4 ACH for offices. The calculation is 40 multiplied by 2.5 multiplied by 4, giving a required CADR of 400 cubic metres per hour. A unit rated below that figure won't meet the target ACH in that room, regardless of how the product is marketed.
Run the same room through a clinic's recommended ACH of 5 instead, and the required CADR rises to 500 cubic metres per hour, showing how the same physical room needs a different product depending on its actual use.
The same worked example also shows why a single model rarely suits every room in a building: a waiting area, a consult room and a procedure room can each demand a different CADR even under one roof.
Where to check a product's CADR rating
A genuine CADR rating should appear on the product's specification page, often alongside the recommended floor area range it's suited to. AIRE states CADR for each model on its products page, which is the figure to compare against whatever your room's calculation returns.
If a product page only gives a vague coverage claim without a stated CADR number, that's worth treating with caution, since coverage claims without the underlying figure are hard to verify against your own room.
Comparing CADR figures side by side across a shortlist is only meaningful once each figure has been checked against the same room calculation, rather than compared as raw numbers alone.
Next step
Run your own room's numbers through the free Air Purifier Size Calculator to get a CADR figure you can shop against directly.
Frequently asked
What is the difference between CADR and ACH?
CADR, clean-air delivery rate, is a fixed product specification measured in cubic metres per hour, describing how much clean air a unit produces. ACH, air changes per hour, is a target you set for a room based on how it's used, and it isn't printed on any product.
How does the sizing formula connect CADR and ACH?
Multiply the room's floor area in square metres by its ceiling height in metres, then by your target ACH, and the result is the required CADR in cubic metres per hour. This turns a room-based target into a product specification you can actually compare units against.
Where can a product's CADR rating be checked?
A genuine CADR rating should be stated clearly on the product's specification page, usually alongside its recommended floor area range. A coverage claim without a stated CADR figure is difficult to verify against your own room's requirement.
Related reading
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