🌿 Air Purifier CADR Calculator

What CADR actually means

CADR — Clean Air Delivery Rate — is the single number that tells you how powerful an air purifier is. It measures the volume of filtered air a purifier delivers per minute, in cubic feet per minute (CFM), tested independently for smoke, dust and pollen. A purifier's CADR for smoke is usually the headline figure and the one to shop by. Marketing terms like "covers up to 1,000 sq ft" are unregulated; CADR is not. If two purifiers both claim a large room but one has a CADR of 120 and the other 300, the second cleans more than twice as fast.

The AHAM rule: CADR ≥ room area × 0.67

The Association of Home Appliance Manufacturers recommends a CADR at least two-thirds of your room's square footage. A 300 sq ft living room needs a CADR of at least 201; a 150 sq ft bedroom needs 101. This rule assumes standard 8 ft ceilings and roughly 4–5 air changes per hour (ACH) — the rate at which the purifier processes the room's entire air volume — which is the level studies link to meaningful reductions in particles, allergens and odours.

Air changes per hour (ACH), explained

ACH tells you how many times per hour a purifier filters the room's full air volume. For allergy and asthma relief, aim for 4–5 ACH; for general freshness, 2–3 ACH is fine. You can estimate ACH as (CADR × 60) ÷ room volume in cubic feet. The AHAM two-thirds rule bakes in roughly 4.8 ACH for 8 ft ceilings. High ceilings dilute the effect — a 12 ft ceiling needs about 50% more CADR than the basic rule suggests, so size up.

One big purifier or two smaller ones?

For large or L-shaped rooms, two smaller purifiers often beat one big one. Air purifiers clean best within about 10–15 feet; a single unit in a long room leaves dead zones at the far end. Two units also give redundancy — one can run on quiet low in the bedroom while the other works the living area — and replacement filters for small models are cheaper. The trade-off is two sets of filters to maintain and slightly higher total energy use. Compare on total CADR: two 150-CADR units (300 combined) outperform one 250-CADR unit in a 400 sq ft space.

HEPA, carbon and what to ignore

True HEPA (H13) captures 99.97% of 0.3-micron particles — the standard worth paying for. "HEPA-type" or "HEPA-like" are marketing terms with no guaranteed performance. An activated carbon filter handles odours, smoke and VOCs that HEPA cannot catch; it is essential if cooking smells or wildfire smoke are your concern. Ignore ionisers and UV lights as primary features — independent testing shows their contribution is minor next to a properly sized HEPA + carbon combination, and some ionisers produce ozone.

Placement and running tips

• Give it breathing room: 1–2 feet of clearance on all sides, especially the intake.
• Put it where the pollution is: near the kitchen for cooking particles, in the bedroom for allergies.
• Run it continuously on low rather than blasting on high for an hour — steady filtration wins.
• Close doors and windows in the room you are cleaning; you cannot filter the whole neighbourhood.
• Replace filters on schedule: a clogged filter can halve real-world CADR. Most last 6–12 months.

How much should you spend?

Price roughly tracks CADR. Expect to pay for about $1 per CADR point in mid-range models — a 200-CADR purifier around $200. Budget models with honest CADR ratings exist, but beware no-name units that list room coverage without a CADR figure at all. Factor in filter costs: a cheap purifier with $80 replacement filters every 6 months quickly becomes the expensive option.

Size by CADR, buy True HEPA + carbon, run it low and steady — clean air is mostly arithmetic.

CADR vs "covers up to" claims

Manufacturers love room-coverage claims — "purifies up to 1,500 sq ft!" — but these are unregulated and usually assume one air change per hour, far below the 4–5 ACH that actually cleans air. A unit claiming 1,500 sq ft might have a CADR of 150, which by the AHAM rule properly serves about 225 sq ft. The rule of thumb: ignore coverage claims entirely and shop by CADR. If a listing hides its CADR, treat that as information — reputable brands publish it prominently because independent AHAM testing is a selling point.

Wildfire smoke: sizing up

During wildfire season, normal sizing rules go out the window. Smoke particles are fine (PM2.5) and infiltrate constantly through every gap, so target 5–6 ACH instead of 4–5 — roughly 25% more CADR than the AHAM minimum. Seal the room as best you can (tape, towels under doors), run the purifier on high before the smoke arrives to build a clean buffer, and prioritise the bedroom: eight hours of clean sleep air matters more than a pristine living room. Activated carbon becomes essential here — HEPA catches the particles, but only carbon adsorbs the gases and smell.

Noise, energy and running costs

A purifier running 24/7 on low is the effective strategy, so check the low-speed noise rating — under 30 dB is whisper-quiet; over 50 dB on high will annoy in a bedroom. Energy use is modest: most units draw 30–70 W, costing a few dollars a month continuously. The real running cost is filters: budget $50–100 per year per unit. When comparing models, divide the purchase price plus five years of filters by CADR — the "cost per clean air" reveals that some cheap units are the expensive choice.

Pre-filters and the maintenance schedule

The washable pre-filter is the unsung hero: it catches hair, dust and pet fluff before they reach the expensive HEPA filter, and vacuuming it monthly can double HEPA life. Mark your calendar: monthly — vacuum pre-filter, wipe the unit exterior; every 6 months — check carbon filter for odour breakthrough (if smells return, it is saturated); every 6–12 months — replace HEPA per the indicator or when airflow audibly drops. Running a purifier with clogged filters is like driving with the handbrake on — the motor works harder for a fraction of the CADR you paid for.

Purifiers and pets

Pet dander is exactly what HEPA filters excel at — 99.97% of 0.3-micron particles covers dander, dust-mite debris and most pollen. Size up one notch for pet homes since dander generates continuously, and place the unit where the pet spends time, not where you do. A carbon filter handles the litter-box and wet-dog odours HEPA cannot touch. One honest caveat: no purifier fixes an unwashed pet — regular grooming and washing bedding does as much as any machine. For severe allergies, the bedroom purifier plus keeping pets out of the bedroom beats any single device.

A worked example: sizing a two-bedroom flat

Take a 140 sq ft bedroom with 8 ft ceilings. AHAM minimum: 140 × 0.67 ≈ 94 CADR; buy 120+ for margin. Check ACH: (120 × 60) ÷ (140 × 8) ≈ 6.4 air changes per hour — excellent for allergies. For a 350 sq ft open-plan living area: minimum 235 CADR, so shop for 250+. Either one 250-CADR unit centrally placed, or two 130-CADR units at opposite ends for more even coverage. Total filter budget: roughly $100–150 per year across both rooms.

Five buying mistakes to avoid

• Shopping by room-coverage claims instead of CADR — the number that actually predicts cleaning speed.
• Buying "HEPA-type" instead of True HEPA (H13); only the latter guarantees 99.97% capture.
• Undersizing the bedroom, where you spend eight hours breathing the same air.
• Ignoring filter prices — a $99 purifier with $70 filters costs far more over five years.
• Running it only when the air looks bad — particles you cannot see still need continuous filtration.