CFM and Air Changes per Hour Calculator Widget

How it works

The widget multiplies length, width and ceiling height to get the room volume, then relates airflow to volume per hour. In feet, one CFM moves 60 cubic feet an hour, so ACH = CFM x 60 / volume; in metric it is simply m³/h divided by m³. Switch Find to Airflow and it runs the other way, CFM = ACH x volume / 60. The cubic foot is exactly 0.028316846592 m³, so 1 CFM is 1.699 m³/h or 0.4719 L/s, and all three airflow units appear together. The last card applies the dilution relation used in CDC's infection-control guideline table: in a perfectly mixed room with no ongoing source, removing 99% takes ln(100) / ACH hours (276 minutes divided by ACH) and 99.9% takes ln(1000) / ACH hours. A 20 x 15 x 8 ft room with 200 CFM gets 5 ACH and needs about 55 minutes for 99%.

Calculation method

• Volume V = length x width x ceiling height
• ACH = CFM x 60 / V(ft³), or ACH = Q(m³/h) / V(m³)
• CFM = ACH x V(ft³) / 60; m³/h = ACH x V(m³)
• 1 ft³ = 0.028316846592 m³, so 1 CFM = 1.699011 m³/h = 0.471947 L/s; L/s = m³/h / 3.6
• Removal time t(min) = ln(1 / (1 - E)) / ACH x 60, for E = 99% and 99.9% (perfect mixing, no source)

Worked examples

Limitations

• Assumes a rectangular room; for vaulted ceilings or L-shaped rooms enter an equivalent volume.
• ACH counts total airflow; it does not tell outdoor air from recirculated air or say whether a code is met.
• Removal times assume perfect mixing and no continuing source, so they are a best case.

Where publishers use it

• An air-purifier or HEPA filter review comparing a unit's CADR against the room size
• Bathroom and kitchen extractor-fan product pages showing the air changes a fan achieves
• A clinic, dental practice or classroom ventilation explainer
• Grow-room and workshop guides sizing an inline exhaust fan
• Facilities pages documenting supply airflow for a meeting room

Questions

Sources

1. Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B. Air, Table B.1 - US Centers for Disease Control and Prevention . Air changes per hour and minutes for 99% and 99.9% airborne-contaminant removal (e.g. 6 ACH: 46 and 69 min), assuming an empty room with no aerosol source and perfect mixing. Checked 2026-10-01.
2. Whole-House Ventilation Strategies for New Homes - US DOE Building America Solution Center (PNNL) . States that ASHRAE 62.2 sets the whole-dwelling ventilation rate from floor area and number of bedrooms (0.03A from the 2013 edition), i.e. in CFM, not as air changes per hour.

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A2Z Tools CFM and Air Changes per Hour Calculator
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