Estimate ACH from room volume and airflow easily. Switch units, set targets, plan upgrades fast. Get practical results to support healthier indoor spaces today.
Air Changes per Hour (ACH) represents how many times the air within a space is replaced in one hour. The core relationship is:
This tool converts common units (m³/s, L/s, CFM, ft³) into consistent SI units before calculating.
| Room Volume (m³) | Airflow (m³/h) | Computed ACH | Interpretation |
|---|---|---|---|
| 50 | 150 | 3.00 | Moderate air turnover |
| 75 | 300 | 4.00 | Good for many occupied rooms |
| 120 | 720 | 6.00 | Higher turnover for heavier use |
Air Changes per Hour (ACH) describes how many times the air inside a space is replaced in one hour. It is a simple way to compare ventilation performance across rooms of different sizes. Higher ACH generally means faster dilution of indoor pollutants, odors, moisture, and heat gains, but it also increases fan energy and conditioning load.
The calculator uses the standard relationship between airflow and volume. If you know the supply or exhaust flow rate, ACH is found from the flow converted to cubic meters per hour and divided by room volume. If you know a target ACH, the required airflow is the target multiplied by the volume.
Design data arrives in mixed units: cubic meters per second, liters per second, or CFM. Small unit mistakes create large ACH errors because the equation scales linearly. For example, treating 200 CFM as 200 m³/h overstates flow by about a factor of 1.7, pushing a realistic 3.5 ACH calculation to nearly 6 ACH.
ACH is a rate, not a guarantee of air quality. Mixing patterns, short-circuiting between supply and exhaust, and door undercuts can lower effective removal in the occupied zone. Use ACH as a screening KPI, then validate with placement, balancing, and where possible, measurements of airflow and pressure.
To size a fan or outdoor-air requirement, start with the room dimensions or measured volume, choose a target ACH, and compute the required flow. If you later add partitions or increase occupancy, re-run the calculation; the required airflow rises proportionally with volume and target rate.
Increasing ACH can improve freshness and humidity control, but it can also raise heating and cooling loads because more air must be conditioned. If your results are high, check whether demand-controlled ventilation, heat recovery, or improved source control could meet indoor-air goals with lower energy use.
Prefer measured airflow values from commissioning or balancing reports. If you estimate flow from fan curves, include the expected static pressure and filter loading. For volume, use the actual conditioned zone height and subtract large obstructions only when they materially reduce free air volume.
There is no single “good” value. Targets depend on occupancy, pollutant sources, and local requirements. Use ACH as a comparative metric, and confirm the final target against codes, standards, and your building’s operating goals.
Not always. Poor diffuser placement or short-circuiting can reduce effective dilution in the occupied zone. Higher airflow can help, but distribution, filtration, and source control also matter for overall indoor air performance.
Yes, if exhaust flow is representative of the air leaving the space and the room is reasonably balanced. In some systems, make-up air paths and leakage change the effective exchange rate, so verify with balancing data when possible.
ACH is “per hour,” so the airflow must be expressed per hour as well. Converting to m³/h keeps the equation consistent: ACH = (m³/h) ÷ (m³). The same idea applies to any consistent unit set.
Measure length, width, and the actual effective height of the ventilated zone. For sloped ceilings, use an average height. If you cannot measure precisely, run a low and high volume estimate to see the sensitivity of ACH.
Start with the space type, occupancy schedule, and contaminant sources, then reference your local building code and design criteria. Higher-risk activities may warrant higher targets, while energy constraints may favor recovery and better filtration.
No. ACH can describe total supply air, which may include recirculated air. Outdoor-air rate depends on how much fresh air is introduced. For health-focused evaluations, confirm the outdoor-air fraction and filtration level, not ACH alone.
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.