Air Changes per Hour Calculator
Work out air changes per hour from room size and airflow, or the cfm, m³/h or L/s needed for a target ACH, plus the ASHRAE 62.2 whole-house ventilation rate.
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About this toolWhat is the Air Changes per Hour Calculator?
Air changes per hour (ACH) says how many times the volume of air in a room is replaced, or cleaned, every hour. It is the airflow divided by the room volume: 120 cfm moving through a 12 by 15 ft room with an 8 ft ceiling is 120 x 60 / 1,440 = 5 ACH, one complete change every 12 minutes.
This calculator works in both directions. Give it the room size and an airflow from a fan, a supply diffuser or a portable air cleaner's CADR and it returns ACH, or give it a target ACH and it returns the airflow you need in cfm, m³/h and L/s. A second panel calculates the continuous whole-house ventilation rate from ASHRAE Standard 62.2, and a guidance table lists ACH figures from the CDC, ASHRAE 62.2, ASHRAE 170 for healthcare and the IECC blower-door limit, each labelled with its source.
- ACH from airflow or airflow from a target ACH
- Room volume from dimensions or entered directly, in feet or metres
- Airflow in cfm, m³/h and L/s together
- ASHRAE 62.2 whole-house ventilation rate from floor area and bedrooms
- Guidance table from CDC, ASHRAE 62.2, ASHRAE 170 and IECC with sources
How to use the Air Changes per Hour Calculator
- Choose US or metric units, and enter the room length, width and height or its volume directly.
- Pick whether you want ACH from an airflow or the airflow needed for a target ACH.
- Enter the airflow (supply, exhaust or a portable air cleaner's CADR) or the target ACH.
- Read ACH, airflow in cfm, m³/h and L/s, the room volume and the time per air change.
- Optionally enter the floor area and bedrooms to get the ASHRAE 62.2 whole-house ventilation rate.
Worked examples
Bedroom with a portable air cleaner (page default)
Input
15 x 12 x 8 ft, 120 cfm
Output
5 ACH, one change every 12 minutes, 99% removal in about 55 minutes
A cleaner with a CADR of 120 cfm meets the CDC target of 5 ACH for this room on its own.
Airflow for a target in metric
Input
6 x 5 x 2.7 m office, target 6 ACH
Output
486 m³/h, 135 L/s, 286 cfm
At 10 L/s per person for outdoor air, this airflow would also cover about 13 people, but outdoor and recirculated air are usually supplied together.
ASHRAE 62.2 whole house
Input
1,500 ft², 3 bedrooms
Output
75 cfm continuous (35.4 L/s), about 0.35 ACH
The per-person part (7.5 x 4 = 30 cfm) and the floor area part (0.03 x 1,500 = 45 cfm) add up to 75 cfm.
Background and insight
ACH is a convenient way to compare rooms of different sizes, but it is not how every standard sets ventilation. ASHRAE 62.1 for commercial buildings gives outdoor air per person plus per square foot of floor, and ASHRAE 62.2 for homes uses 0.03 cfm per square foot of floor area plus 7.5 cfm for each occupant, counting bedrooms plus one. A 1,500 ft² three-bedroom house needs 75 cfm of continuous ventilation, which is only about 0.35 ACH.
Higher figures appear where airborne contaminants matter. In 2023 the CDC and NIOSH recommended aiming for 5 or more air changes per hour of clean air in occupied rooms, counting outdoor air, filtered return air and portable air cleaners together. Healthcare design under ASHRAE 170 sets 12 ACH for new airborne infection isolation rooms and 20 total ACH for operating rooms. Kitchens and bathrooms in homes get local exhaust under 62.2: a 100 cfm range hood or 5 ACH continuous for kitchens, and 50 cfm intermittent or 20 cfm continuous for bathrooms.
ACH50, measured with a blower door at 50 Pa, is a different number entirely. It describes airtightness under test pressure, not ventilation, and the 2021 IECC limits new homes to 3 ACH50 in most climate zones. Natural infiltration under normal conditions is a small fraction of the ACH50 value. With perfect mixing, each air change removes 63% of a contaminant, so 99% removal takes about 4.6 air changes.
Common use cases
Sizing portable air cleaners
Check whether a HEPA unit's CADR delivers 5 ACH in a classroom, office or bedroom, or how many units a large room needs.
Bathroom and kitchen fans
Turn a room volume into the fan cfm needed for a target number of air changes, then compare with the 62.2 minimums.
Whole-house ventilation
Get the ASHRAE 62.2 continuous rate for an ERV, HRV or exhaust-only system in a new or retrofitted home.
Workshops and spray booths
Estimate the extract airflow for a target ACH, then confirm against the solvent or dust guidance that applies.
Tips and limitations
- Air cleaners are rated by CADR, which already accounts for filter efficiency. Use the CADR, not the raw fan airflow.
- Mixing matters. Supply and return grilles close together short-circuit the air, so the room sees fewer effective air changes than the number suggests.
- Ventilation rates set by people (ASHRAE 62.1) are more meaningful than ACH in rooms with high or low occupancy.
- Do not compare ACH50 from a blower door with ventilation ACH; divide ACH50 by roughly 15 to 20 for a rough natural infiltration estimate only.
- Outdoor air brings heat or cooling load with it. Pair high ventilation rates with heat recovery in cold or hot climates.
Frequently asked questions
How do you calculate air changes per hour?
ACH = airflow (cfm) x 60 / room volume (ft³), or airflow (m³/h) / room volume (m³). A 1,440 ft³ room with 120 cfm gets 5 ACH.
How many air changes per hour do I need?
It depends on the space. CDC guidance for reducing airborne virus is 5 or more ACH of clean air; homes need the ASHRAE 62.2 rate, usually around 0.3 to 0.5 ACH of outdoor air; isolation rooms need 12 ACH and operating rooms 20 ACH.
How do I convert ACH to CFM?
CFM = ACH x room volume (ft³) / 60. For 6 ACH in a 2,000 ft³ room, 6 x 2,000 / 60 = 200 cfm.
What is the ASHRAE 62.2 ventilation formula?
Required continuous ventilation (cfm) = 0.03 x conditioned floor area (ft²) + 7.5 x (number of bedrooms + 1). In SI, 0.15 L/s per m² plus 3.5 L/s per person.
Does a HEPA air purifier count toward air changes?
Yes, as equivalent air changes of clean air. Divide its CADR in cfm by the room volume in ft³ and multiply by 60. The CDC counts it toward the 5 ACH target alongside ventilation.
What is the difference between ACH and ACH50?
ACH is a ventilation or cleaning rate under normal conditions. ACH50 is the air leakage measured with a blower door at 50 Pa of pressure, used to rate airtightness; it is much larger than the natural air change rate.
Is there an in-depth guide to the Air Changes per Hour Calculator?
Yes. “Air Changes per Hour: What ACH Means and How Much Ventilation You Need” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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