| Scenario | Space | Method | Target | Base Airflow | Adjusted Airflow |
|---|---|---|---|---|---|
| Grow tent | 4×4×7 ft | ACH | 12 ACH | 224 CFM | ~277 CFM (10% safety, 10% derate) |
| Small greenhouse | 3×4×2.4 m | Heat | 800 W, ΔT 3°C | ~467 m³/h | ~578 m³/h (10% safety, 10% derate) |
| Garden shed | 10×12×8 ft | ACH | 6 ACH | 96 CFM | ~119 CFM (10% safety, 10% derate) |
Volume = Length × Width × HeightCFM = Volume(ft³) × ACH ÷ 60m³/h = Volume(m³) × ACH
BTU/hr = Watts × 3.412CFM = BTU/hr ÷ (1.08 × ΔT°F)- Metric model:
m³/s = W ÷ (ρ × cₚ × ΔT°C), thenm³/h = m³/s × 3600
Area = π × (Diameter²) ÷ 4CFM = Area(ft²) × Velocity(FPM)m³/h = Area(m²) × Velocity(m/s) × 3600
Adjusted = Base × (1 + Safety%) ÷ (1 − Derate%)- Safety adds buffer. Derate accounts for filters, bends, and backpressure.
- Select a calculation method: ACH, Heat, or Duct Velocity.
- Choose units, then enter room dimensions or a volume override.
- Fill in method inputs (ACH, heat load and ΔT, or duct details).
- Optionally add a fan rating, safety factor, and system derate.
- Click “Calculate Airflow” to view results under the header.
- Use the download buttons to export CSV or PDF reports.
1) Why airflow matters in enclosed grow spaces
Air movement supports transpiration, CO₂ replenishment, and uniform canopy temperatures. In small tents, stale pockets can form within minutes, especially behind filters and duct bends. A practical baseline is 5–20 air changes per hour (ACH), depending on odor control, humidity, and plant density.
2) Using ACH to size ventilation quickly
The ACH method links room volume to your target refresh rate. For example, a 4×4×7 ft tent is 112 ft³. At 12 ACH, the base airflow is 112×12÷60 ≈ 22.4 CFM before adjustments. Many growers intentionally oversize because real systems lose airflow through filters, long duct runs, and tight louvers.
3) Heat load and temperature rise planning
Lighting and equipment convert most power into heat. A 600 W setup adds roughly 600 W of sensible heat. If you want to limit the temperature rise to 8°F, the calculator estimates airflow using the sensible-heat relationship. Lower ΔT targets require higher airflow, so small changes in ΔT can noticeably change the fan selection.
4) Duct velocity and common ranges
When you know duct size, velocity offers a direct flow estimate. Typical garden ventilation ducts run near 600–1200 feet per minute (about 3–6 m/s). Higher velocities increase noise and pressure losses. Keeping velocity moderate often improves delivered airflow and reduces vibration on lightweight framing.
5) Safety factor and derate are the difference makers
Fan labels are usually free-air ratings. Real delivered flow can drop 10–40% once you add a carbon filter, bends, insect screens, or long ducting. This calculator applies a safety factor and a derate to produce an adjusted airflow that better matches field performance and seasonal peaks.
1) What ACH should I use for a grow tent?
Many setups start around 8–12 ACH. Dense canopies, high humidity, or strong odor control often benefit from 12–20 ACH, especially with filters that reduce delivered airflow.
2) Why does the adjusted airflow increase so much?
Adjusted airflow adds a safety buffer and compensates for real-world losses. Filters, bends, backpressure, and dirty screens can reduce flow, so adjusted values better match delivered performance.
3) Should I enter fan rating in CFM or m³/h?
Either is fine. Select the correct unit and the calculator converts internally. Use the manufacturer’s rating for the setup closest to your real installation when possible.
4) How do I estimate heat load in Watts?
Add the power of lights, drivers, pumps, and equipment inside the space. Most electrical power becomes heat. If a device is outside the room, do not include it.
5) What is a reasonable duct velocity for garden ventilation?
A common range is 600–1200 FPM (about 3–6 m/s). This range balances noise, pressure loss, and stable airflow for typical flexible ducting.
6) Can I use volume override instead of dimensions?
Yes. If you already know volume, enter it and leave dimensions blank. Ensure the override matches the selected unit system so the airflow calculations remain consistent.
7) Will this replace a full ventilation design?
No. It provides planning estimates but does not model full static-pressure fan curves or complex airflow paths. For critical builds, validate with fan curves and measurements.