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Ventilation Calculator
Work out the exhaust airflow your grow room or tent needs, and check a fan against it
Room & Units
Air Changes
A preset fills in a target ACH. Typing your own switches this to Custom.
Extra capacity on top of the requirement. 20-50% is the usual allowance.
Fan Check (optional)
Leave empty to size a fan from the target instead.
Heat Load
Leave blank for 100%. Lower it only if some light heat leaves another way.
Fans, pumps, dehumidifiers and anything else running in the room.
How far above the intake air the room may run. Leave blank for 5 °C.
Duct & Filter Losses
Engineering estimates from duct-design equations, published test data and fan curves. Leave a field blank to use its default. Your fan's own pressure curve is the better guide when you have it.
Usually the fan's size. Leave blank for 150 mm.
The whole run the fan pushes or pulls through.
Two 45° bends count as about one 90° bend.
From the fan's spec sheet or curve (the pressure at zero airflow). Leave blank to estimate it from the duct size: about 2 Pa per mm, 300 Pa for 150 mm.
How this is calculated · Sources
The model in plain words
- A fan's rated airflow is what it moves with nothing attached. Ducting, bends and a filter push back, and the fan settles where its own pressure curve meets that resistance. That point is the airflow it really delivers.
- Straight duct: friction from the Darcy equation with the Altshul–Tsal friction factor. Roughness 0.09 mm for rigid duct and 1.5 mm for flexible, raised further when flex is not pulled taut (compression ratio 5%, 15% or 30%).
- Bends: each 90° bend costs a loss coefficient times the velocity pressure (0.6 × air speed² in Pa): 0.33 for a rigid elbow, 1.0 for a flexible bend.
- Carbon filter: its rated pressure drop (150 Pa if blank) at the fan's rated airflow, scaled by airflow to the power 1.5. With no filter, the open duct entry costs half a velocity pressure.
- Fan: a generic curve, pressure = max pressure × (1 − (airflow ÷ rated airflow)1.3), using your fan's max static pressure or about 2 Pa per mm of duct diameter.
- Range: the uncertain figures are varied one group at a time (bend coefficient 0.65–2.0 flex or 0.22–0.42 rigid, filter 60–180 Pa, flex slack and, when estimated, fan max pressure 1.5–3.5 Pa per mm) and the lowest and highest results are shown.
Sources
- Handbook ASHRAE Handbook—Fundamentals, Duct Design chapter: Darcy and Altshul–Tsal friction equations; Table 1, duct roughness.
- Handbook ASHRAE Fitting Loss Coefficients (Duct Fitting Database): fitting 1-1 (duct entry) and 3-2 (90° round elbow).
- Peer-reviewed Abushakra, Walker & Sherman (2004). Compression Effects on Pressure Loss in Flexible HVAC Ducts. HVAC&R Research 10(3):275–289. doi:10.1080/10789669.2004.10391104; free copy at escholarship.org/uc/item/0d76400v
- Peer-reviewed Dai et al. (2021). Investigation of pressure drop in flexible ventilation ducts under different compression ratios and bending angles. Building Simulation 14:1251–1261. doi:10.1007/s12273-020-0737-8
- Manufacturer Can-Filters published pressure-drop data: 180 Pa (0.75 in. w.g.) at maximum airflow. canfilters.com
- Manufacturer Fan curves and maximum pressures: Vents TT PRO and Systemair K EC datasheets, and AC Infinity Cloudline specifications.
What is estimated
The filter's 1.5 power law, the 1.0 coefficient for a flexible bend, the generic fan-curve shape and the default fan max pressure (2.0 Pa per mm; makers' figures run from 1.2 to 4.2) are engineering estimates fitted to the sources above, not measurements of your equipment. Treat every result as a planning estimate with the range shown.
Enter a target ACH to size a fan, a fan's rated flow to check it, or both.
Hydro Oasis • Ventilation Calculator