Formula & Calculator
Air Changes per Hour (Ventilation Rate)
Measures how many times the entire air volume in a room is replaced with fresh air each hour, a key indoor air quality metric.
Interpretation
ACH = Airflow Rate (m³/h) / Room Volume (m³). The number of times the air in a room is replaced per hour. Used for ventilation design and indoor air quality assessment.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| ACH | Air changes per hour | 1/hr |
| Airflow Rate | Volumetric ventilation airflow rate | m3/hr |
| Room Volume | Total room air volume | m3 |
What it means
Air Changes per Hour (ACH) is a measure of how often the air in a space is completely replaced with outdoor air. It is calculated by dividing the supplied airflow rate (m³/h) by the room volume (m³). ACH is a key parameter for designing heating, ventilation, and air conditioning (HVAC) systems and for ensuring acceptable indoor air quality. Recommended ACH values vary by application: residential (0.5-1), offices (1-3), operating rooms (15-20). Higher ACH reduces contaminant build‑up but increases energy consumption. Example: A room of 50 m³ receives an airflow of 100 m³/h. ACH = 100/50 = 2 h⁻¹. This means the air in the room is replaced twice every hour, which is adequate for a typical office.
Worked example
Air Changes per Hour – Two Examples
Real‑World| Parameter | Value |
|---|---|
| Airflow rate | 500 m³/hr |
| Room volume | 100 m³ |
| Parameter | Value |
|---|---|
| Airflow | 1,000 m³/hr |
| Volume | 150 m³ |
Common mistakes
- Airflow rate: The volume of outdoor air supplied to the space – in m³/hr.
- Room volume: The total volume of the room – in m³.
- ACH: Number of air changes per hour – a measure of ventilation rate.
- Typical values: Residential: 0.35‑0.5 ACH; offices: 2‑6 ACH; hospitals: 6‑15 ACH.
- Effective ventilation: This is the theoretical ACH; actual mixing efficiency may be lower.
Applications
Air changes per hour (ACH) is the number of times the entire volume of air within a room is replaced by outdoor or conditioned air in one hour, calculated as airflow rate divided by room volume. This metric is crucial for ventilation design in buildings, ensuring adequate indoor air quality, controlling odours, and removing pollutants. Engineers use ACH to size HVAC systems, to comply with building codes (e.g., ASHRAE 62.1), and to design cleanrooms and healthcare facilities. Higher ACH rates are required in kitchens, laboratories, and waiting rooms to dilute contaminants. By calculating ACH, building professionals can balance energy efficiency with occupant health and comfort.
- Design of ventilation systems for residential, commercial, and industrial buildings
- Compliance with indoor air quality standards (ASHRAE, LEED)
- Assessment of infectious disease transmission risk (e.g., COVID‑19)
- Design of cleanrooms, operating theatres, and pharmaceutical facilities
- Energy performance evaluation and retrofitting of ventilation systems
Frequently Asked Questions
ACH is a measure of how many times the entire volume of air in a room is replaced with fresh air each hour. It is calculated as ACH = (Airflow Rate (m³/hr)) / Room Volume (m³). It is used to assess ventilation effectiveness.
Assuming a target ACH value applies universally. Recommended ACH varies significantly by room use (e.g., offices typically need 2‑4 ACH, hospitals may need 6‑12 ACH, labs may need 10‑20 ACH). Always refer to standards for the specific application.
- Residential: 0.3‑1.0 (natural ventilation) or 2‑3 (mechanical).
- Offices: 2‑4 ACH.
- Schools: 3‑5 ACH.
- Hospitals: 6‑12 ACH (depending on room type).
- Laboratories: 10‑20 ACH.
- Industrial: varies based on contaminant generation.
Required airflow = Target ACH × Room Volume. For example, a room of 100 m³ with target ACH of 4 needs airflow = 4 × 100 = 400 m³/h.
ACH is a room‑based metric. Per‑person ventilation (e.g., L/s/person) is used for IAQ to control CO₂ and bioeffluents. Both are used together: the maximum of the two requirements often governs.
Higher ACH dilutes indoor pollutants (VOCs, CO₂, particulates) and reduces the concentration of airborne pathogens. However, excessive ACH increases energy consumption for heating/cooling. Balancing IAQ and energy is key.
- Tracer gas decay (e.g., using CO₂ or SF₆).
- Direct measurement of supply airflow and room volume.
- Calculated from fan speed and duct sizes (if known).
ACH = 500 / 200 = 2.5 air changes per hour.
ACH is a volume‑based rate of air exchange. Air velocity is the speed of air movement (m/s) within the space. Both are important: ACH for dilution, velocity for occupant comfort and contaminant removal.
Higher ACH requires more energy to condition the incoming air (heating, cooling, humidification). Energy recovery ventilators (ERVs) can reduce this energy penalty by transferring heat and moisture between exhaust and supply air.