How to Calculate the Size of an Exhaust Fan for Your Grow Space

Getting the fan size right is the single decision that most affects how your grow space behaves. Too small and it runs flat out, gets loud, gets hot and wears out early. Too large and you have paid for ducting and a filter you did not need. Put your room dimensions into the calculator below and it will work it out from there.

Fan size calculator

Enter your space and setup. Results update as you type.

Common tent sizes
Your space needs about
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How the number is worked out

Nothing here is a secret formula — you can do it on paper in a minute, and it is worth understanding so you can sanity-check the answer.

  1. Volume. Length × width × height, in metres. A 1.2 × 1.2 × 2 m tent is 2.88 m³.
  2. Air changes. The working target is to replace the whole volume once a minute, so multiply by 60 to get cubic metres per hour. That 2.88 m³ tent wants about 173 m³/h before anything else is added.
  3. Carbon filter. A filter is a restriction. Add 25%.
  4. Ducting. Add roughly 3% per metre of flexible duct.
  5. Bends. Add 20% for each sharp 90° bend. A gentle sweep costs far less — count it as half.
Why we show three answers. The minimum is the smallest fan that can physically do the job — it will spend its life near full speed, which is the loudest, hottest and shortest-lived way to run a fan. The recommended pick sits at roughly two-thirds load, which is where these fans are quiet and comfortable. The third option leaves room for a hot summer, a second light, or a filter that has loaded up. If two options cost about the same, take the larger one.

Airflow at each speed setting

Every CLOUDLINE fan is adjustable, and the figure on the box is only what it does flat out. This is AC Infinity's measured data for each speed step. Pick a model to see it.

Checked with AC Infinity, September 2026. We asked the factory whether this data was still current after three years. It is — and they confirmed the figures are recorded with a 10% tolerance, so treat any single number as a working figure that can sit up to 10% either side in practice. You will also notice the measured maximum for the 4″, 6″ and 8″ fans reads a little under the headline figure on the spec sheet. That gap falls inside the same 10% tolerance. The calculator above deliberately uses the lower measured figures, so its recommendation errs toward giving you more fan rather than less.

A worked example

A 2.4 × 1.2 × 2 m tent, with a carbon filter and about 3 m of ducting with one sharp bend.

  • Volume: 2.4 × 1.2 × 2 = 5.76 m³
  • One air change a minute: 5.76 × 60 = 346 m³/h
  • Carbon filter, ×1.25: 432 m³/h
  • 3 m of ducting, ×1.09: 471 m³/h
  • One 90° bend, ×1.20: 565 m³/h

565 m³/h. A 150 mm fan tops out around 683 m³/h measured, so it would cover it — but at about 83% of maximum, which is the loud end. A 200 mm fan does the same job at around 41%, near silent, with room for a hot February. That is the fan to buy.

These figures are a sizing guide, not a specification. Real airflow depends on how the ducting is actually run, how loaded the filter is, and the static pressure of the whole system. If you are close to a boundary, or the space is unusual, ask us — we would rather help you get it right than have you send a fan back.