ACH vs CADR: what's the difference?
In one line: CADR is a property of the purifier (how much clean air it delivers per hour); ACH is a property of the purifier in your room (how many times it replaces that room's air per hour). ACH = CADR ÷ room volume. Use CADR to compare products; use ACH to check a unit is big enough for your space.
They measure different things
CADR (clean-air delivery rate) is a fixed spec of the machine — say 300 m³/h. It tells you how much particle-free air the unit pushes out, and it's the fairest way to compare two purifiers head-to-head. ACH (air changes per hour) only exists once you put that unit in a specific room: the same 300 m³/h purifier gives lots of air changes in a small bedroom and few in a large lounge.
How they connect
ACH = CADR (m³/h) ÷ room volume (m³) — where room volume = floor area × ceiling height.
Example: a 300 m³/h purifier in a 20 m² room with a 2.4 m ceiling (48 m³) delivers 300 ÷ 48 ≈ 6.3 ACH. Put the same unit in a 40 m² open-plan space (96 m³) and it drops to ~3.1 ACH — same machine, half the effect, because the room is twice the size.
| CADR | ACH | |
|---|---|---|
| What it describes | The purifier | The purifier in a room |
| Depends on room size? | No | Yes |
| Best used for | Comparing products | Checking it fits your space |
| Units | m³/h (or CFM) | times per hour |
How to use both when buying
Compare shortlisted units on CADR (higher = cleans faster and can run quieter). Then divide your chosen unit's CADR by your room's volume to check you'll get at least 4–5 ACH — more for allergies or infection control. If the ACH comes out low, size up or add a second unit.
Next step
Compare real units on measured CADR at Best Air Purifier UK, or work out running cost with the running-cost calculator.
FAQ
Which matters more, ACH or CADR? Neither alone — CADR compares machines, ACH confirms one is big enough for your room. You need both.
What ACH should I aim for? Roughly 4–5 for general clean air, 5–6+ for allergies, smoke or infection control.