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

Methodology, interpretation & sources

Purpose. 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.

How to use the result. Start with source data you can verify, enter values on a consistent unit and time basis, and record the assumptions used. The main inputs on this page include Length (ft), Width (ft), Ceiling Height (ft), 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., Air Changes per Hour (ACH). Re-run the calculation when those assumptions change rather than carrying an old result forward.

Decision use. Use this result as a planning and screening aid within a wider risk-management process. The calculator should support, not replace, competent judgement and site-specific verification.

Limitations. Document the assumptions and input source so another person can reproduce the calculation and confirm that the result remains valid when conditions change.

Primary references. Use the current edition/version that applies to your jurisdiction and organization:

Editorial note. Prepared and maintained by Impact Tech Solutions. Editorially updated 28 September 2026. No individual professional credential is claimed on this page; high-consequence, statutory, tax, engineering or health decisions should be independently verified against the current primary source and, where required, a qualified professional.