Compute air changes per hour from airflow. Use room size, leakage, and correction factors easily. See results instantly and document decisions for site planning.
| Scenario | Room (L×W×H) | Supply Airflow | Outdoor Fraction | Leakage | Estimated ACH (total) |
|---|---|---|---|---|---|
| Site office | 30 ft × 20 ft × 10 ft | 1200 CFM | 30% | 5% | 12.000 |
| Temporary enclosure | 18 m × 12 m × 4 m | 10,000 m³/h | 100% | 10% | 11.574 |
| Mechanical room | 40 ft × 25 ft × 12 ft | 1500 CFM | 20% | 0% | 7.500 |
Air Changes per Hour (ACH) measures how often the air in a space is replaced each hour.
These calculations support planning and comparisons. Always apply site standards, equipment limits, and safety guidance.
Tip: If you measured exhaust too, compare supply and exhaust to understand pressurization trends in temporary enclosures and controlled work zones.
Air change rate (ACH) indicates how many times the air volume in a workspace is replaced in one hour. In construction, ACH helps control dust, fumes, and moisture in temporary enclosures, site offices, welding bays, and curing zones. Higher ACH can reduce contaminant buildup and improve comfort when heat sources, solvents, or equipment emissions are present.
ACH depends on room volume and delivered airflow. Measure length, width, and average height, or enter a verified volume for irregular spaces. Use the airflow unit that matches field data, then apply outdoor-air fraction to reflect mixed-air systems. Add a leakage allowance for imperfect sealing and a correction factor to reflect distribution effectiveness or short-circuiting.
Total ACH uses the full supply airflow and describes overall mixing and turnover. Outdoor ACH uses only effective outdoor airflow after leakage and correction adjustments, representing the ventilation component that actually refreshes air. For odor control, dilution of gases, and oxygen replenishment, outdoor ACH is the more meaningful metric.
To size fans or air handlers, select a target ACH based on task risk and project requirements, then compute required airflow. Compare required airflow against available equipment capacity, duct losses, and power constraints. Track minutes per air change to understand how quickly conditions can recover after a high-emission activity or door opening.
When using recirculated air, consider filtration and capture at the source to prevent spreading dust. In negative-pressure containment, confirm make-up air paths and avoid starving appliances. Seasonal temperature differences can change density and fan performance, so recheck flows after shifts.
Verify airflow with hood readings, fan curves, or anemometer traverse where feasible. Check that exhaust and supply flows maintain intended pressurization to limit infiltration from dirty zones into clean areas. Record inputs, assumptions, and outputs to support safety plans, QA documentation, and commissioning handover.
Targets vary by occupancy and local guidance, but many teams start around 4–8 ACH for comfort. Use outdoor ACH when verifying ventilation effectiveness, especially where odors or contaminants are concerns.
Use outdoor ACH for dilution and contaminant control because it represents fresh-air replacement. Total ACH is useful for mixing and turnover, especially where recirculation is significant.
Temporary barriers and doors leak. Leakage reduces effective outdoor airflow and can change pressure balance. Including an allowance keeps estimates realistic and supports better equipment sizing.
It adjusts for distribution effectiveness. Poor diffuser placement, short-circuiting between supply and exhaust, or dead zones can lower actual ventilation performance compared with nameplate airflow.
Use a flow hood for grilles, anemometer traverse for ducts, or fan curve readings with static pressure. Recheck after layout changes, filter loading, or significant temperature shifts.
Yes. If dimensions are hard to measure, enter a verified volume from a takeoff or model. Then add airflow, outdoor fraction, and adjustments to compute ACH.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.