Formula used
- Air changes per hour: ACH = (Q × 3600) / V
- Airflow from ACH: Q = (ACH × V) / 3600
- Occupancy method: Q = N × qpp
- Steady-state mass balance: Q = G / (Ci − Co)
Q is airflow (m³/s), V is volume (m³), N is people, qpp is per-person airflow, G is generation (mg/s), and concentrations are in mg/m³. A safety factor multiplies the final requirement.
How to use this calculator
- Select a calculation mode that matches your known inputs.
- Enter values and choose units for each input you provide.
- Set a safety factor if you want extra margin.
- Press Calculate to display results above the form.
- Use Download CSV or Download PDF for your latest run.
Example data table
| Scenario | Inputs | Typical output |
|---|---|---|
| ACH from airflow | Q = 250 m³/h, V = 120 m³, SF = 1.00 | ACH ≈ 2.083 1/h |
| Airflow from ACH | ACH = 6, V = 180 m³, SF = 1.10 | Q ≈ 1188 m³/h |
| Occupancy method | N = 10, qpp = 8 L/s-person, SF = 1.00 | Q ≈ 80 L/s (≈ 288 m³/h) |
| Mass balance (CO₂) | G = 18 g/h, Ci = 1000 ppm, Co = 420 ppm, SF = 1.00 | Q ≈ 0.0073 m³/s (≈ 26.4 m³/h) |
The mass-balance example uses a simplified ppm-to-mg/m³ conversion for CO₂. For precision work, use measured density conditions or convert concentrations directly to mg/m³.
Professional guide to ventilation rate decisions
1. Why ventilation rate matters
Ventilation rate is the controlled delivery of outdoor air that dilutes heat, moisture, and pollutants. In enclosed rooms, inadequate air exchange can raise carbon dioxide, odors, and aerosol concentrations. Sizing ventilation targets improves comfort, productivity, and risk control, especially during high occupancy or intensive activities.
2. Understanding airflow and ACH
Airflow (Q) is the volume of air supplied per time, while air changes per hour (ACH) expresses how many room volumes are replaced each hour. The relation is ACH = (Q × 3600) / V. A small room can have high ACH with modest flow; large halls require much higher flow.
3. Unit conversions used in practice
Engineering drawings and field measurements often mix units. Common conversions include 1 m³/h = 0.2778 L/s and 1 CFM ≈ 0.4719 L/s. This calculator converts between m³/s, m³/h, L/s, and CFM so you can compare fan data, diffusers, and guidelines consistently.
4. Occupancy-based ventilation planning
Many design approaches start from people. If a guideline specifies 8 L/s-person and the room holds 10 people, the minimum outdoor air is 80 L/s. Adding a safety factor accounts for imperfect mixing, short-circuiting, or future occupancy growth. If you also enter room volume, the calculator estimates the implied ACH.
5. Mass-balance method for contaminant control
When you know a generation rate (G) and a target indoor concentration (Ci) above outdoor (Co), steady-state mass balance provides Q = G / (Ci − Co). This is helpful for CO₂ screening, laboratory emissions, or odor control. The calculator supports mg/m³ directly and an approximate ppm conversion for CO₂.
6. Choosing realistic inputs
Use measured dimensions for volume, and select representative occupancy and activity levels. For CO₂, typical outdoor values are near 400–450 ppm, while indoor targets often range from 800–1200 ppm depending on policy. If Ci is not greater than Co, the model cannot produce a meaningful airflow requirement.
7. Interpreting results with engineering judgment
Ventilation targets should be cross-checked against fan curves, duct pressure losses, diffuser noise, and filtration strategy. High ACH improves dilution but may increase energy use in hot or humid climates. Consider heat recovery, demand control ventilation, and zoning to meet indoor air goals efficiently.
8. Reporting and documentation
Clear documentation accelerates reviews. Export the latest run to CSV for spreadsheets, or to PDF for project submittals and audits. Record the selected method, units, safety factor, and assumptions (mixing, steady state, pollutant identity). Consistent reporting makes recalculations easy when layouts or occupancies change.
FAQs
1) What is a good ACH value for a room?
There is no single value. Offices may operate around 2–6 ACH, while laboratories or isolation spaces can be higher. Use local codes, risk goals, and system capability, then confirm with measurements where possible.
2) How does room volume affect ACH?
ACH scales inversely with volume for a fixed airflow. If the airflow stays constant and volume doubles, ACH is halved. Always verify dimensions, ceiling height, and any large voids that change the effective volume.
3) Why add a safety factor?
Real rooms are not perfectly mixed. Supply short-circuiting, door leakage, filter loading, and fan tolerance can reduce effective outdoor air. A modest factor (for example, 1.05–1.20) provides margin for these uncertainties.
4) When should I use the occupancy method?
Use it when you have a per-person guideline or when occupancy drives pollutant load. It is quick for classrooms, meeting rooms, and retail spaces. Enter volume too if you want the implied ACH for comparison.
5) What does the mass-balance mode assume?
It assumes steady state and well-mixed air, with constant generation and constant outdoor concentration. It is best for screening and early design. Dynamic conditions may require time-dependent modeling or measured tracer decay tests.
6) Is the ppm conversion accurate for all pollutants?
No. The ppm option is intended for CO₂ with a fixed approximate conversion under typical indoor conditions. For other gases, or for high accuracy, convert using molecular weight and actual temperature and pressure.
7) Can this replace a commissioning test?
It helps estimate targets and compare scenarios, but it does not measure delivered outdoor air. For compliance and performance, verify with balancing reports, airflow measurements, or tracer-gas methods, and document the final conditions.