How to Size an Extraction Fan for Your Grow Tent
The right extraction fan is chosen by airflow, not duct diameter alone. Calculate the tent’s volume in cubic metres, decide how often that air should be exchanged, then add capacity for the carbon filter, ducting bends, heat and summer conditions. Match the fan, filter and duct diameter as one system.
Step 1: calculate the tent volume
Use metres throughout. A 1.2m × 1.2m × 2m tent has a volume of 2.88m³. Exchanging that air once per minute gives a theoretical minimum of about 173m³/hr because 2.88 × 60 = 172.8. At one exchange every two minutes, the theoretical figure is about 86m³/hr.
Theoretical minimums assume no filter, no duct bends and no heat load. Real systems always have resistance, so do not buy exactly on the base calculation.
| Tent | Volume | One exchange/minute | Practical starting range |
|---|---|---|---|
| 0.6 × 0.6 × 1.7m | 0.61m³ | 37m³/hr | 100–200m³/hr |
| 1.2 × 1.2 × 2m | 2.88m³ | 173m³/hr | 250–500m³/hr |
| 1.2 × 2.4 × 2m | 5.76m³ | 346m³/hr | 500–850m³/hr |
These are starting ranges, not guarantees. Room temperature, light heat, intake restriction and ambient Johannesburg summer conditions can change the requirement.
Step 2: size the fan and filter together
A carbon filter creates resistance. Its airflow rating should meet or exceed the intended working airflow of the fan. Skyline’s RAM Pro Active Carbon Filters are currently listed at 170, 500, 850 and 1080m³/hr across the 100–250mm range. That aligns naturally with several RAM Inline Duct Fans, rated at 277, 580, 998 and 1088m³/hr.
Do not automatically pair equal duct diameters without checking airflow. For example, the 100mm RAM filter is rated at 170m³/hr while the 100mm duct fan is rated at 277m³/hr. Running the fan at full output could exceed the filter rating, so a suitable controller or different combination may be necessary.
Step 3: choose the correct fan type
- Compact axial exhaust fans: economical for simple air movement and short low-resistance runs. See Garden HighPro Exhaust Fans.
- Mixed-flow fans: a middle ground with useful airflow for moderate duct systems. The RAM Mixed Flow range is listed at 199, 517 and 840m³/hr.
- Inline duct fans: better suited to systems with a carbon filter, longer ducting or higher resistance.
Fan type matters because a free-air rating falls once the system must pull through carbon and push around bends.
Step 4: allow for real-world resistance
Add capacity for every restriction: carbon filtration, insect screens, long flexible ducting, reducers and sharp bends. Keep RAM ALUDUCT as short and straight as the room allows. A fan controller such as the RAM Air-Pro II can provide adjustment and temperature-based control, but confirm motor compatibility before connecting any controller.
Final buying checklist
- Calculate tent volume in m³.
- Choose the desired exchange rate.
- Add 25–50% for filter and duct losses.
- Increase allowance for high heat or hot intake air.
- Match fan, filter and duct diameter.
- Confirm the filter rating is not below the working fan airflow.
- Plan power leads, mounting, noise control and safe exhaust location.
Browse inline and exhaust fans, carbon filters and ducting products. For broader planning, read how to choose a grow-tent size and the humidity-control guide.
Frequently asked questions
How do I calculate grow-tent fan size?
Multiply tent length by width by height to obtain cubic metres, then choose a fan that can exchange that volume roughly every one to three minutes after allowing for filters, duct length, bends and heat load.
Should the fan or carbon filter have the higher airflow rating?
The filter should normally meet or exceed the working airflow of the fan. A filter rated below the fan can restrict performance and reduce effective filtration.
Do duct bends reduce airflow?
Yes. Tight bends, long duct runs, filters and small duct diameters add resistance. Keep ducting short, straight and correctly sized where possible.
Is a small axial fan enough for a carbon filter?
Often not. Axial fans are useful for simple extraction or boosting short duct runs, while mixed-flow or inline duct fans generally handle system resistance better.
