You have a tent, you have plants that need fresh air, and you want to know what size fan will actually do the job without guessing. Here is how to work it out properly.
Start with air changes per minute
The standard rule of thumb for grow tent extraction is to replace the entire volume of air in the tent at least once per minute. Some growers aim for every 30 to 45 seconds under high heat load, but once per minute is a solid baseline to size from.
Fan airflow is measured in cubic metres per hour (m³/h) or cubic feet per minute (CFM). To use the once-per-minute rule, you need the volume of your tent in cubic metres, then multiply by 60 to get the minimum m³/h rating you are looking for.
Volume maths for common tent sizes
Tent sizes are given as width × depth × height. Most standard tents are around 2 metres tall. Here is the working for the sizes you are most likely to be looking at:
- 60 × 60 × 160 cm — Volume: 0.6 × 0.6 × 1.6 = 0.576 m³. Multiply by 60 = 35 m³/h minimum. A 4-inch fan handles this comfortably.
- 1.0 × 1.0 × 200 cm — Volume: 1.0 × 1.0 × 2.0 = 2.0 m³. Multiply by 60 = 120 m³/h minimum. A 4-inch fan is marginal; a 5- or 6-inch is the right choice.
- 1.2 × 1.2 × 200 cm — Volume: 1.2 × 1.2 × 2.0 = 2.88 m³. Multiply by 60 = 173 m³/h minimum. A 6-inch fan is the standard fit here.
- 1.5 × 1.5 × 200 cm — Volume: 1.5 × 1.5 × 2.0 = 4.5 m³. Multiply by 60 = 270 m³/h minimum. You need a strong 6-inch or move to an 8-inch.
These are the numbers before you account for anything that reduces the fan's real-world performance — and there are a couple of things that will.
Add a carbon filter and size up
A carbon filter creates resistance. Air has to be pulled through a dense bed of activated carbon, and that work costs you airflow — typically 20 to 25 per cent of the fan's rated output, sometimes more as the filter ages and the carbon loads up. If you are running a filter on the exhaust (and most people are, for odour control), you should add roughly 25 per cent to your minimum m³/h figure before choosing a fan.
So for a 1.2 m tent, instead of targeting 173 m³/h, you are now looking for a fan rated at around 215 m³/h or above to maintain your air-change rate once the filter is in line. Ducting runs and bends add further resistance, so every elbow in your duct is another small loss worth accounting for. Buying the length you actually need helps here — the 4 & 6 Inch Flexible Ducting Hose comes in 100 mm and 150 mm, from 2 m to 10 m, so you are not coiling a spare four metres behind the tent.
Why a controllable fan beats a perfectly-sized one
It is tempting to calculate your exact requirement and buy a fan that just meets it. In practice, a fan that is slightly too large and fitted with a speed controller is a much better tool. Here is why:
- Plants and lights produce more heat in summer and less in winter. A controllable fan lets you dial output up or down to match conditions rather than running flat out permanently.
- A new carbon filter has less resistance than one that has been running for six months. With a controller you compensate for this automatically as the filter loads.
- Running a larger fan at 60 to 70 per cent speed is quieter than running a smaller fan at 100 per cent, and puts less wear on the motor over time.
- If your setup changes — a bigger light, a second tent connected in series — you have headroom to cope without buying a new fan.
A fan that is too small, running flat out, cannot give you more than its maximum. A fan with room to spare gives you flexibility in both directions.
What to look for in a 6-inch fan
For the 1.2 m tent, which is probably the most common size in UK homes, a 6-inch inline fan is the go-to choice. Look for an EC (electronically commutated) motor rather than an older AC type — EC motors are significantly more energy efficient, run cooler, and the speed can be adjusted smoothly rather than in fixed steps. Airflow ratings above 300 m³/h give you enough headroom to run a filter and a reasonable length of ducting without dropping below your target air-change rate.
The 6 Inch EC Inline Duct Fan from our range covers 350 CFM (roughly 595 m³/h), which gives solid headroom for a 1.2 m or even a 1.5 m tent with a filter in line.
The one thing worth doing next
Measure your tent and do the sum above before you buy anything. Volume in cubic metres, multiplied by 60, then add 25 per cent for a carbon filter. That single number tells you the minimum fan rating you need, and buying one step above it gives you a setup that can handle whatever the seasons and your growing environment throw at it.
Read next
- How Do I Match a Carbon Filter to My Fan?
- Will a Grow Tent Fan Be Too Loud for a Bedroom?
- What size grow tent do I need?
Looking for the kit? Start here, or browse the full grow range.