🔄 Air Changes Per Hour

ACH Calculator

Q: How do you calculate air changes per hour (ACH)?

Air changes per hour equals airflow in CFM multiplied by 60, divided by room volume in cubic feet: ACH = (CFM x 60) / Volume. For a room 20 feet by 15 feet with 9 foot ceilings (2,700 cubic feet) receiving 450 CFM of airflow, ACH = (450 x 60) / 2700 = 10 air changes per hour. In metric units, ACH = (L/s x 3.6) / Volume in cubic metres. ACH measures how many times the entire room's air volume is replaced or recirculated in one hour, but does not by itself indicate how much of that air is fresh outdoor air versus recirculated air, which is a separate consideration governed by ASHRAE 62.1 and 62.2.

Q: What ACH rate is required for isolation rooms in Canadian hospitals?

Canadian healthcare facilities follow CSA Z317.2 for HVAC requirements in health care facilities, which generally aligns with ASHRAE 170 minimum ACH rates. Airborne infection isolation rooms typically require 12 ACH total with negative pressure relative to adjacent spaces. Protective environment rooms for immunocompromised patients require 12 ACH with positive pressure. Operating rooms require 20 ACH minimum. General patient rooms typically require 6 ACH. Always verify current requirements against the specific edition of CSA Z317.2 and any provincial health facility guidelines in effect for your project, as these standards are periodically updated.

Calculate air changes per hour from room volume and airflow, or find the airflow needed for a target ACH. Includes a reference table of recommended ACH rates for residential, commercial, and healthcare spaces. Use with the ventilation rate calculator for outdoor air requirements.

⚙ Calculation Direction

🏠 Room Dimensions

Units

Room Length *

Room Width *

Ceiling Height *

Supply Airflow *

CFM

Target ACH *

ACH

🔄 ACH Results

Parameter	Value	Note

Export:

Recommended ACH Reference Table

Space Type	Recommended ACH	Notes
Residential living space	0.35-1 (whole-house)	Per ASHRAE 62.2 continuous rate
Residential bathroom (exhaust)	8-10	Intermittent / on-demand
Office space	4-6	Typical commercial design
Classroom	6-8	Higher for younger age groups
Retail store	4-6	—
Restaurant dining	8-12	Higher with smoking or open kitchen
Commercial kitchen	15-30+	Driven by hood exhaust requirement
Gym / fitness centre	6-10	—
General hospital patient room	6	Per CSA Z317.2 / ASHRAE 170
Airborne infection isolation room	12	Negative pressure, per ASHRAE 170
Operating room	20+	Per ASHRAE 170
Laboratory (general)	6-12	Higher for fume hood-equipped labs
Parking garage	4-6 (or CO-controlled)	See garage ventilation calculator

Understanding ACH: what it measures and what it doesn't

Air Changes per Hour (ACH) measures how many times the air volume in a space is theoretically replaced in one hour, calculated as airflow divided by room volume, scaled to an hourly basis. It's a simple, intuitive metric widely used to communicate ventilation intensity, especially in healthcare, laboratory, and infection control contexts where dilution of airborne contaminants is the primary goal.

It's important to understand what ACH does not tell you: it says nothing about how much of that airflow is fresh outdoor air versus recirculated air. A room can have a very high ACH using 100% recirculated air with zero outdoor air, providing excellent mixing and filtration but no actual fresh air ventilation. ASHRAE 62.1 and 62.2 govern outdoor air requirements separately from total ACH — use the ventilation rate calculator for that calculation.

ACH for infection control and healthcare design

Higher ACH rates are specified in healthcare and laboratory settings primarily to dilute and remove airborne contaminants quickly, reducing the time any infectious particles or hazardous substances remain in the breathing zone. Canadian healthcare facilities follow CSA Z317.2, generally aligned with ASHRAE 170 minimum rates: 6 ACH for general patient rooms, 12 ACH for isolation rooms (with appropriate pressure relationships), and 20+ ACH for operating rooms. These high rates also typically require dedicated air handling with HEPA filtration and pressure control, not just a larger fan.

Calculating room volume correctly

Room volume should reflect the actual conditioned air volume, not just floor area times a nominal height. For rooms with sloped ceilings, mezzanines, or significant furniture/equipment volume displacement, adjust your volume calculation accordingly for accurate ACH results. This calculator uses simple length × width × height; for complex geometries, calculate volume separately and use the direct airflow comparison.

Frequently Asked Questions

How do you calculate air changes per hour (ACH)?+

ACH = (CFM × 60) ÷ Room Volume (ft³). For a 2,700 cubic foot room receiving 450 CFM: ACH = (450×60)/2700 = 10 air changes per hour. In metric: ACH = (L/s × 3.6) ÷ Volume (m³). ACH measures total air volume turnover, not how much is fresh outdoor air — that's a separate calculation governed by ASHRAE 62.1/62.2. Use the ventilation rate calculator for outdoor air requirements.

What ACH rate is required for isolation rooms in Canadian hospitals?+

Canadian healthcare facilities follow CSA Z317.2, generally aligned with ASHRAE 170. Airborne infection isolation rooms typically require 12 ACH with negative pressure. Protective environment rooms require 12 ACH with positive pressure. Operating rooms require 20 ACH minimum. General patient rooms require 6 ACH. Always verify against the current CSA Z317.2 edition and provincial health facility guidelines for your specific project.