| CFM/Fan | Fans | Time (min) | Duty (%) | Efficiency (%) | Duct (%) | Filter (%) | Effective CFM | Volume (ft³) | Volume (m³) |
|---|---|---|---|---|---|---|---|---|---|
| 200 | 1 | 30 | 100 | 90 | 5 | 5 | 162.45 | 4,873.50 | 137.99 |
The calculator estimates how much air a ventilation system moves over time. It starts with rated airflow, then applies real-world factors that reduce flow.
- Base airflow:
BaseCFM = CFM_per_fan × Fans - Effective airflow:
EffectiveCFM = BaseCFM × (Duty/100) × (Efficiency/100) × (1−DuctLoss/100) × (1−FilterLoss/100) - Volume moved:
Volume_ft³ = EffectiveCFM × Minutes - Air changes per hour (optional):
ACH = (EffectiveCFM × 60) / EnclosureVolume_ft³
- Enter your fan airflow (CFM) and the number of fans.
- Set the run time per cycle in minutes or hours.
- Adjust duty cycle if the fan runs intermittently.
- Use efficiency and loss fields to match real ducting.
- Enable enclosure dimensions to estimate ACH for grow spaces.
For healthier plants, pair airflow with temperature and humidity targets. Re-check filter loss often as carbon filters clog over time.
Ventilation volume in real grow spaces
Grow tents and greenhouses need predictable air movement to control heat, humidity, and odor. This calculator converts fan airflow into total volume moved over a cycle, helping you compare equipment and timer settings. Enter CFM per fan, number of fans, and run time to estimate how much air is exchanged during each ventilation event.
Why effective CFM matters
Rated airflow is measured under ideal conditions, but ducting and filters add resistance. The efficiency field represents fan curve reduction from static pressure, while duct and filter losses model extra restrictions and dirty media. Using effective CFM prevents undersizing that can lead to condensation, slow transpiration, and elevated disease pressure.
Using duty cycle for timers and controllers
Many gardens run exhaust intermittently, especially in cooler periods or when using CO₂ enrichment. Duty cycle converts on/off schedules into an equivalent average airflow over the selected time window. For example, a fan running 15 minutes each hour is a 25% duty cycle, which lowers total volume moved while still allowing periodic air refresh.
Linking volume moved to air changes per hour
If you enable enclosure dimensions, the tool calculates enclosure volume and ACH. ACH describes how many times the full space volume is replaced each hour. Higher ACH can remove heat faster and reduce humidity spikes, but may also increase heater load in winter. Use ACH to balance plant comfort with energy cost.
Interpreting results for healthier crops
Compare the calculated volume to your enclosure volume to judge whether a single cycle provides meaningful exchange. In high-transpiration stages, consider higher effective airflow or longer run times. After installation, revisit losses; adding elbows, silencers, or a carbon filter can reduce flow substantially and may require fan speed adjustments. For quick checks, target one to two complete exchanges per minute in warm, dense canopies, and less during lights-off. Record readings with a cheap anemometer if available. Update the inputs when filters age or seasons change. to keep results realistic. always.
What does CFM mean in garden ventilation?
CFM is cubic feet per minute, a fan airflow rating. Higher CFM can remove heat and humidity faster, but real performance drops with ducting, bends, and filters.
Why does the calculator include duct and filter losses?
Loss fields approximate airflow reduction from resistance. Long duct runs, tight elbows, backdraft dampers, and loaded carbon filters can significantly reduce delivered airflow compared with the label rating.
How do I choose an efficiency percentage?
Start with 85–95% for short, straight ducting, and 70–85% for restrictive setups. If you have measured airflow or manufacturer pressure curves, adjust the efficiency until effective CFM matches your observed performance.
What run time should I enter?
Enter the time the fan operates for one ventilation cycle, not the whole day. If you run 10 minutes every hour, use 10 minutes and set duty cycle to 100%, or use 60 minutes and duty cycle to 16.7%.
What is ACH and why is it useful?
ACH is air changes per hour: how many times the enclosure volume is replaced each hour. It helps compare different fan sizes and schedules, and supports humidity and temperature planning across growth stages.
Can I use this for greenhouses, tents, and drying rooms?
Yes. Use the enclosure option for any defined space. Drying rooms often need gentler exchange to avoid overdrying, so lower duty cycles and conservative losses usually give more realistic results.
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