If you've ever tried to shop for an air purifier by the numbers, you've almost certainly run into two acronyms: CADR and ACH. Manufacturers plaster CADR on the front of the box. ACH rarely shows up at all — even though it's arguably the more useful figure for deciding whether a unit will actually clean your room. Most reviews gloss over the difference, or worse, treat the two as interchangeable. They're not.
This guide explains what each number means in practice, why marketers love one and ignore the other, and how to use both together to size an air purifier for your actual space — without overpaying for coverage you won't get.
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Short on time? Skip to the sizing framework at the bottom — it's a four-step calculation that works for any room without a spreadsheet.
What CADR actually measures
CADR stands for Clean Air Delivery Rate. It's a single number — expressed in cubic feet per minute (CFM) — that represents how quickly a purifier can deliver clean air for a specific particle size. The Association of Home Appliance Manufacturers (AHAM) administers the most widely cited version of the test, and manufacturers report three separate CADR figures: one each for tobacco smoke, dust, and pollen.
A higher CADR means the unit moves more clean air per minute. Simple enough. The problem is how marketers use it.
| CADR Range | Typical Room Size (AHAM guideline) | What it means day-to-day |
|---|---|---|
| Under 100 CFM | Up to ~150 sq ft | Small bedroom or office nook |
| 100–200 CFM | ~150–300 sq ft | Average bedroom or home office |
| 200–300 CFM | ~300–450 sq ft | Large bedroom or open kitchen |
| 300+ CFM | 450+ sq ft | Open-plan living area or large great room |
Why CADR is useful — and where it falls short
CADR gives you an apples-to-apples comparison between machines. Because AHAM's test is standardized, a unit rated at 200 CFM for dust has genuinely been measured against another unit rated at 200 CFM for dust in the same conditions. That consistency is valuable when you're comparing models side by side.
The shortfall: CADR doesn't tell you how many times the purifier cycles all the air in your room per hour. That's where ACH comes in.
What ACH actually measures
ACH stands for Air Changes per Hour. It's the number of times a purifier can theoretically push all the air in a room through its filter in sixty minutes. The formula is straightforward:
ACH = (CADR × 60) ÷ Room Volume (cubic feet)
Room volume is just your floor area multiplied by ceiling height. A 200 sq ft bedroom with 8-foot ceilings has 1,600 cubic feet of air. A purifier rated at 200 CFM would achieve an ACH of (200 × 60) ÷ 1,600 = 7.5 — meaning it cycles the room's air about seven and a half times per hour.
Why does this matter? Because particle concentration in a room doesn't just depend on how clean the filtered air is — it depends on how often that clean air replaces the contaminated air sitting around your furniture, near the floor, and in corners. For general maintenance, 4–5 ACH is considered adequate. For allergy or asthma management, 5–6 ACH is a commonly cited target. For wildfire smoke or heavy cooking odors, you want closer to 6+ ACH.
The ceiling-height trap
Most manufacturer room-size ratings assume 8-foot ceilings. If your open-plan apartment has 10-foot ceilings, that same space holds 25% more air than the spec sheet assumes. A unit rated for 400 sq ft drops to an effective coverage closer to 320 sq ft once you account for the extra volume. This is one of the most common reasons people feel their air purifier "isn't working" — it's working fine, just undersized for the actual cubic footage.
The marketing number vs. the engineering number
Manufacturers love CADR because it's a single, impressive-sounding figure with a standardized body behind it. "CADR 300!" reads well on packaging. ACH requires you to know your room dimensions, do arithmetic, and think about ceiling height — it doesn't fit on the front of a box.
The result is that brands optimized for shelf appeal quote CADR (usually the highest of the three particle-size figures, which is typically pollen) and suggest a room size that assumes the most favorable conditions: short ceilings, no furniture obstructing airflow, the unit centered in the room. In real homes, that room-size claim often overshoots by 20–30%.
Priya's practical note
"When I'm evaluating a unit for my own home, I calculate ACH before I read anything else on the spec sheet. I measure my room, account for my ceiling height (I have 9-foot ceilings in my Oakland place), and run the math. If the unit can't hit 4 ACH in the room I actually want to use it in, the rest of the spec sheet doesn't matter much. CADR is just the input — ACH is the answer."
Which spec should you actually lead with?
Use CADR to compare machines against each other. Use ACH to decide whether a machine is right for your room. You need both, but in that order.
A few situations where this distinction changes the purchase decision:
- Open-plan spaces: A unit with a CADR of 250 that a brand claims covers "up to 500 sq ft" may only achieve 3 ACH in your 500 sq ft room with 10-foot ceilings. You'd need a CADR closer to 350 to hit 4 ACH in that space.
- Bedrooms: You almost certainly don't need a 400 CFM unit for a 150 sq ft bedroom. At that scale you'd be running it on its lowest fan speed to avoid noise — which often means the CADR drops significantly (low-speed CADR is rarely what's printed on the box).
- Wildfire smoke events: During active smoke days, hitting 6+ ACH in the rooms where you spend the most time is worth prioritizing over coverage area. A smaller, high-ACH unit in the bedroom where you sleep beats a large unit running quietly in the living room where you aren't.
A four-step sizing framework
If you're ready to match a unit to your room, work through these steps before you read a single product review:
- Measure your room's square footage. Length × width. For open-plan spaces, measure the area you realistically want the purifier to serve — don't include adjacent rooms unless there are no walls between them.
- Multiply by your ceiling height. This gives you cubic footage. A 300 sq ft room with 9-foot ceilings = 2,700 cubic feet.
- Decide your target ACH. 4 ACH for general use, 5–6 ACH for allergies or pets, 6+ ACH for smoke or mold concerns.
- Back-calculate the CADR you need. Required CADR = (Target ACH × Room Volume) ÷ 60. For the example above at 5 ACH: (5 × 2,700) ÷ 60 = 225 CFM minimum.
That number — your minimum required CADR — is your filter when shopping. Any unit that doesn't meet it at a real-world fan speed is undersized for your room, regardless of what the box says.
What to watch out for when comparing specs
- CADR quoted at max fan speed only. Many purifiers are run on their lowest setting 90% of the time to reduce noise. Ask whether the brand publishes CADR at each speed — a few do. If not, assume real-world CADR is meaningfully lower than the headline figure.
- Room size claims without ACH context. "Covers 800 sq ft!" means nothing without knowing ceiling height and how many air changes that actually delivers.
- CADR for pollen only. Pollen CADR is always the highest of the three figures because pollen particles are larger and easier to capture. A purifier with CADR 300 for pollen might only score 220 for dust and 200 for smoke — the numbers that matter most if those are your concerns.
- Ignoring filter replacement cost. A unit that delivers excellent CADR at a $200 sticker price but requires $80 filters every three months costs more over two years than a slightly pricier unit with annual $50 filters.
The bottom line
CADR is a useful, standardized input. ACH is the output that actually tells you whether a purifier will keep up with your room. Neither number alone is enough — but together, and with your actual room dimensions in hand, they give you everything you need to buy without guessing.
If you're ready to shop by category, our Best True HEPA Air Purifiers guide applies exactly this framework to the units we've spent the most time with — including how each one performs at real-world fan speeds, not just max.
Ready to find the right unit for your room?
See our editor-reviewed ranking of the best True HEPA air purifiers, sized by real ACH calculations.
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