Ventilation Air Change Rate (ACH) Calculator

ACH = Air Changes per Hour. Calculates room air change rate from airflow and volume, and the time required to clear airborne contaminants to a target efficiency, per CDC environmental infection control guidance.

ACH & Airflow

Contaminant Clearance Time

Room Dimensions

Length (ft)

Width (ft)

Ceiling Height (ft)

Airflow

Supply/Exhaust Airflow Rate (CFM) Total clean airflow delivered to (or removed from) the room. If using a portable HEPA unit alongside HVAC, add both flow rates together.

Or: Target ACH (to calculate required airflow instead) Leave blank if you entered an airflow rate above.

0 ft³

Room Volume

Air Changes per Hour

Air Changes per Hour (ACH)

Target Removal Efficiency (%)

Mixing Factor (k) 1.0 = perfect air mixing (theoretical). Real rooms with imperfect air distribution can need a factor up to 10 — use a conservative (higher) value if actual mixing performance is unknown.

MINUTES TO TARGET CLEARANCE

Formula Reference

Room Volume = Length × Width × Height
ACH = (Airflow Rate × 60) ÷ Room Volume
Required Airflow = (Target ACH × Room Volume) ÷ 60
Clearance Time (minutes) = − ln(1 − Efficiency÷100) ÷ ACH × 60 × Mixing Factor (k)

CDC Reference: Time for Airborne Contaminant Removal (k = 1)

4 ACH	69 min (99%) / 104 min (99.9%)
6 ACH	46 min (99%) / 69 min (99.9%)
10 ACH	28 min (99%) / 41 min (99.9%)
12 ACH	23 min (99%) / 35 min (99.9%)
15 ACH	18 min (99%) / 28 min (99.9%)
20 ACH	14 min (99%) / 21 min (99.9%)

Commonly Cited ACH Recommendations

Operating rooms	≥ 15–20 ACH
Airborne infection isolation room (new construction)	≥ 12 ACH
Airborne infection isolation room (existing)	≥ 6 ACH
Patient/treatment rooms, ER waiting areas	6–12 ACH

Based on CDC "Guidelines for Environmental Infection Control in Health-Care Facilities" (Table B.1 methodology) and ASHRAE ventilation guidance. Clearance times assume perfect air mixing unless a mixing factor is applied — real-world air distribution is frequently imperfect, and CDC notes the mixing factor can be as high as 10 for poorly-mixed spaces. This tool supports ventilation planning; final design and verification should involve a qualified HVAC/industrial hygiene professional, particularly for healthcare isolation and other critical applications. For customized calculators tailored to your organization, please contact Impact Tech Solutions.