Air Changes per Hour Calculator: Is Your Room Getting Enough Clean Air?

Air changes per hour (ACH) is how many times an hour a room’s whole volume of air passes through a filter, or is swapped for outdoor air. It’s the number public health agencies use to judge whether a room gets enough clean air, and it’s easy to work out once you know your purifier’s CADR. Use this calculator when you already have a purifier (or a shortlist) and want to know how well it covers a specific room.

If you’re still choosing a purifier and want the CADR to look for, start with the air purifier size calculator instead: it runs the same formula the other way round.

How the calculator works

Air changes per hour = clean air (cfm) × 60 ÷ room volume (cu ft)

• Clean air is the purifier’s smoke CADR times the number of identical units, plus any other clean air you enter. CDC’s advice is to work out the ACH of every strategy (purifiers, filtered HVAC air, outdoor air) and add them.
• Room volume is length × width × ceiling height. Metric sizes are converted to feet first, because CADR is published in cubic feet per minute.

For the clean-up times, the calculator uses the formula behind CDC’s Table B.1:

Minutes to remove a share of airborne particles = −ln(1 − share) ÷ ACH × 60

That table assumes an empty room, perfect mixing and nothing new being released, so treat the times as best cases.

What the targets mean

Air changes per hour	Who says so	What it’s for
4 / 6 / more than 6	Lancet COVID-19 Commission (2022)	“Good”, “better” and “best” levels of clean air (the Commission says they are not a standard for homes, but they are useful context)
4.8	AHAM	The room size printed on the box: enough to keep smoke-sized particles at least 80% below their starting level
5 or more	CDC/NIOSH	A rough guide for occupied rooms to reduce airborne viruses; CDC says it doesn’t guarantee safe air
About 7.5	AHAM	Wildfire smoke: a smoke CADR at least equal to the floor area (8 ft ceiling)

The Lancet Commission also gives per-person rates: 21 cfm per person is “good” and 30 cfm is “better”. As summarized by AHAM, ASHRAE Standard 241 calls for 30 to 50 cfm of clean air per person in homes when infection risk is a concern. Enter the number of people to see where you land.

Note: There is no official ACH target for allergies. AHAM’s 4.8 applies to all airborne particles, not allergens specifically. For allergy-specific advice, see the best air purifier for allergies.

Worked examples

• Bedroom, 15 x 12 ft, 8 ft ceiling (1,440 cu ft), one purifier with a smoke CADR of 233 cfm: 233 × 60 ÷ 1,440 = 9.7 ACH. 99% of airborne particles clear in about 28 minutes in theory. With two people that’s about 117 cfm each.
• Living room, 20 x 15 ft, 8 ft ceiling (2,400 cu ft), the same purifier: 233 × 60 ÷ 2,400 = 5.8 ACH, which meets CDC’s 5 but not the Lancet “best” level. 99% clears in about 47 minutes.
• Home office, 12 x 10 ft, 8 ft ceiling (960 cu ft), a small purifier with a smoke CADR of 100 cfm: 100 × 60 ÷ 960 = 6.25 ACH, about 44 minutes to 99%.
• Open-plan room, 25 x 20 ft, 9 ft ceiling (4,500 cu ft), two purifiers at 300 cfm each: 600 × 60 ÷ 4,500 = 8.0 ACH, about 35 minutes to 99%.

You can check the time formula against CDC’s table: at 6 ACH it gives 46 minutes for 99% and 69 minutes for 99.9%; at 2 ACH, 138 and 207 minutes. The calculator returns the same numbers.

Why your real ACH is lower than the result

• CADR is measured on the highest fan speed. The EPA notes that CADR at quieter settings is usually not published and can be much lower. If you run a purifier on low overnight, your real ACH is a fraction of the result. Our guide on how long to run an air purifier covers when to turn it up.
• Mixing is never perfect. CDC says removal times are longer in rooms with stagnant corners. Furniture, closed doors and a purifier pushed against a wall all slow things down; see where to put an air purifier.
• Sources keep releasing particles. Cooking, pets, people walking on carpet and outdoor air leaking in all add new particles, so a real room settles at a lower level rather than reaching zero.

Outdoor air and your HVAC system

Most homes swap far less air with the outdoors than people expect. The EPA’s Exposure Factors Handbook recommends 0.45 air changes per hour as the central estimate for US homes and 0.18 for a tight home, while ASHRAE 62.2 calls for at least 0.35. That outdoor air also brings outdoor pollution with it, so on smoky days don’t count it as clean air at all.

A central HVAC system only cleans air while the fan runs, and only as well as its filter. EPA recommends MERV 13 or the highest rating your system can handle; our MERV rating chart explains what each number captures. If you know your system’s filtered airflow, convert it to ACH with the same formula and enter it in the optional field.

What air changes per hour can’t tell you

ACH describes particles: smoke, dust, pollen, pet dander and mold spores. It says nothing about gases and odors, which depend on how much activated carbon a filter holds and have no CADR-style rating. The CADR explainer covers what the rating measures and what it leaves out. To see what running a purifier around the clock costs, use the air purifier electricity cost calculator.