Air Change Rate Calculator

Calculate air changes per hour (ACH), required ventilation rates, and CO₂ decay curves based on ASHRAE 62.1 and room characteristics.

Or enter volume directly:
m³/h
Office: 10x8x3 m, 500 m³/h
Classroom: 6 ACH target
Auditorium: 20 people, 300 m²
Calculating...

Understanding Air Change Rate (ACH)

Air Changes per Hour (ACH) is a measure of how many times the air within a defined space is replaced by supply air in one hour. It is a critical parameter for indoor air quality (IAQ), ventilation design, and infection control.

Basic Formula

ACH = Q / V where:

  • Q = volumetric airflow rate (m³/h or CFM)
  • V = room volume (m³ or ft³)

For SI units: if Q is in m³/h and V in m³, ACH = Q/V directly. For IP: if Q in CFM, convert to ft³/h (multiply by 60) then divide by volume in ft³.

ASHRAE Standard 62.1

The standard specifies minimum ventilation rates for acceptable IAQ. The breathing zone outdoor airflow is calculated as:

Vbz = Rp × Pz + Ra × Az

  • Rp = outdoor airflow rate per person (default 5 CFM/person ≈ 2.5 L/s·person for office)
  • Pz = number of people
  • Ra = outdoor airflow rate per unit area (default 0.06 CFM/ft² ≈ 0.3 L/s·m²)
  • Az = floor area

You can customize Rp and Ra in the calculator.

CO₂ as a Ventilation Indicator

Indoor CO₂ concentration reflects the balance between occupant generation and ventilation. Under steady‑state, CO₂ can be used to estimate ventilation rate per person. The decay curve shows how quickly CO₂ returns to outdoor levels after occupancy ends, assuming perfect mixing.

Calculator features:

  • Three calculation modes: ACH from flow, flow from ACH, and ASHRAE 62.1 compliance
  • Customizable Rp and Ra for ASHRAE mode
  • Option to set volume = area × height for consistency
  • Copy results with one click
  • Automatic unit conversion (SI / IP) for all inputs and outputs
  • CO₂ decay curve with adjustable initial and outdoor concentrations

Applications

  • HVAC design: Sizing fans, ducts, and air handlers based on required ACH
  • Infection control: CDC recommends 6‑12 ACH for airborne infection isolation rooms
  • Building codes: Meeting minimum ventilation standards (ASHRAE, local codes)
  • IAQ investigations: Estimating ventilation effectiveness using CO₂ decay

Frequently Asked Questions

Recommended ACH varies by space type: homes 0.5‑1.5, offices 4‑8, classrooms 4‑6, healthcare isolation rooms 6‑12, laboratories 6‑10. Check local building codes for mandatory minimums.

1 CFM = 1.699 m³/h. So to convert CFM to m³/h, multiply by 1.699. Conversely, divide m³/h by 1.699 to get CFM. Our calculator handles this automatically when you switch units.

After occupants leave, CO₂ concentration decays exponentially: C(t) = C_out + (C_initial - C_out)·e^(-ACH·t). By measuring the decay over time, you can back‑calculate the actual ACH in a space. The chart above visualizes this decay assuming perfect mixing.

ACH typically refers to the total outdoor air supply rate. If you have recirculation, the effective air changes of outdoor air may be lower. For IAQ purposes, outdoor air ACH is what matters for diluting contaminants. Our calculator assumes you input outdoor air flow.

The simplified rates (Rp and Ra) are default values from ASHRAE Standard 62.1-2019. They are based on decades of research and are widely accepted for design. Actual required ventilation may vary based on specific pollutants, activities, and local codes. You can customize Rp and Ra in this calculator to match your specific application.