# Calculate it properly: exhaust fan CFM, intake size and carbon filters

> Size an exhaust fan for the bigger of two numbers: one air change a minute (CFM = tent volume in ft³) or the heat it must carry out (CFM = BTU/h ÷ (1.08 × temperature rise in °F)). For a 4×4×6.5 ft tent that's 104 CFM for air changes but about 190 CFM to carry 600 W of heat out at a 10 °F rise. Then allow for the carbon filter, ducting and bends using the fan's curve. Use passive intakes about 2–3 times the exhaust duct area for slight negative pressure.

- Canonical: https://genetixbay.com/knowledge/equipment/calc-ventilation-and-carbon-filters.html
- Category: Equipment · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review
- Stage: Any stage · Systems: Any system
- Updated: 2026-11-01 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What size [exhaust fan](https://genetixbay.com/knowledge/glossary/exhaust-fan.md) do I need for my grow tent?** Take the larger of two numbers: tent volume in ft³ (one air change a minute) and the heat-removal airflow, [CFM](https://genetixbay.com/knowledge/glossary/cfm.md) = watts × 3.412 ÷ (1.08 × allowed temperature rise in °F). For a 4×4×6.5 ft tent with about 600 W of lights and [fans](https://genetixbay.com/knowledge/glossary/fan-leaf.md), that's 104 CFM versus about 190 CFM at a 10 °F rise. Then choose a fan that still delivers that with the [carbon filter](https://genetixbay.com/knowledge/glossary/carbon-filter.md) and ducting attached, using its fan curve.

**How big should a passive intake be?** Make passive intake openings about 2–3 times the cross-section of the exhaust duct, placed low on the opposite side. For a 6 in exhaust (28 in², 182 cm²), that's about 57–85 in² (365–545 cm²) of open intake, for example two or three open 6 in ports. The tent walls should pull in slightly when the fan runs; if they bow in hard, open more intake.

**Do I need an intake fan in my grow tent?** Most growers don't. GenetixBay recommends passive intakes for most tents: they keep the tent under slight [negative pressure](https://genetixbay.com/knowledge/glossary/negative-pressure.md) so smell goes through the carbon filter. Add an active intake fan for long or filtered intake ducts, intake from another room or outdoors, or large rooms, and keep it weaker than the exhaust so the space stays under negative pressure.

**How do I match a carbon filter to my fan?** Choose a carbon filter rated for at least the airflow your fan actually moves, and ideally a little more, with the same duct size. Pushing more air through a filter than its rating shortens the time air spends in the carbon, so smell gets through. A filter also adds resistance, so the fan moves less air than its free-air rating; check the fan curve at the filter's pressure drop.

**How much CFM do I need to cool a grow tent?** CFM to carry heat out = heat in BTU/h ÷ (1.08 × temperature rise in °F); in metric, litres per second = watts ÷ (1.21 × temperature rise in °C). For 600 W in a 4×4 tent, a 10 °F (5.6 °C) rise needs about 190 CFM (90 L/s) and a 5 °F (2.8 °C) rise needs about 380 CFM. If the incoming room air is already warm, exhaust alone can't cool the tent.

**Short answer, in plain words.** Your exhaust fan has two jobs: swap the tent's air about once a minute, and carry the lights' heat out. Work out both and use the bigger one. Then remember that a carbon filter, ducting and bends make the fan weaker than its box says, so pick one with headroom and a speed control. Let fresh air in through openings low on the other side, about **2–3 times the size of the exhaust duct**, so the tent walls pull in just slightly. For a **4 × 4 ft** tent with about 600 W, the heat job (about **190 CFM**) is bigger than the air-change job (104 CFM). *Supported (airflow physics; the derating and intake ratios are grower practice).*

Use the calculators: [fan CFM](https://genetixbay.com/knowledge/tools/#fan-cfm), [passive intake](https://genetixbay.com/knowledge/tools/#passive-intake), [carbon filter](https://genetixbay.com/knowledge/tools/#carbon-filter).

## What are the formulas?

**1. Air-change airflow.** CFM = tent volume (ft³) ÷ minutes per air change. For one change a minute, **CFM = volume in ft³**. Metric: L/s = volume (m³) × 1,000 ÷ 60. *Grower practice: one change every 1–3 minutes for a lit tent.*

**2. Heat-removal airflow** [1]:

- Imperial: **CFM = BTU/h ÷ (1.08 × ΔT)**, with ΔT the allowed rise in °F from intake air to exhaust air. BTU/h = watts × 3.412 [2].
- Metric: **L/s = W ÷ (1.21 × ΔT)**, with ΔT in °C (K).
- The 1.08 and 1.21 factors assume air near sea level and room temperature. At altitude, air is thinner: at about 5,000 ft (1,500 m) you need roughly 20% more CFM for the same heat [1].

**3. The fan you buy** must deliver the larger of 1 and 2 **at the system's static pressure**, with the filter, ducting and bends attached [1][3].

**Sizing an exhaust fan.** Tent volume (CFM for 1 change a minute) → Heat CFM (BTU/h ÷ (1.08 × ΔT)) → Take the larger → Allow for filter and duct: Check the fan curve → Passive intake 2–3× duct area. Source: [1][3]; intake is a GenetixBay house position [5].

## How much airflow for each size?

Lights at about 900 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) with a top-tier [LED](https://genetixbay.com/knowledge/glossary/led.md), plus fans (see [lighting sizing](https://genetixbay.com/knowledge/equipment/calc-lighting-sizing.md)). "Heat" assumes the exhaust carries all the heat out.

| Space | Volume | 1 change/min | Heat (lights + fans) | Heat CFM at 10 °F (5.6 °C) rise | Heat CFM at 5 °F (2.8 °C) rise |
|---|---|---|---|---|---|
| 2 × 4 × 6 ft | 48 ft³ (1.36 m³) | 48 CFM (23 L/s) | ~300 W (1,030 BTU/h) | ~95 CFM | ~190 CFM |
| 4 × 4 × 6.5 ft | 104 ft³ (2.94 m³) | 104 CFM (49 L/s) | ~605 W (2,060 BTU/h) | ~190 CFM | ~380 CFM |
| 5 × 5 × 6.5 ft | 163 ft³ (4.60 m³) | 163 CFM (77 L/s) | ~920 W (3,140 BTU/h) | ~290 CFM | ~580 CFM |
| 4 × 8 × 6.5 ft | 208 ft³ (5.89 m³) | 208 CFM (98 L/s) | ~1,170 W (3,990 BTU/h) | ~370 CFM | ~740 CFM |
| 10 × 10 × 8 ft room | 800 ft³ (22.65 m³) | 800 CFM (378 L/s) | ~3,580 W (12,210 BTU/h) | ~1,130 CFM | ~2,260 CFM |

**Chart: Exhaust airflow by space, one air change vs heat.** Heat usually sets the fan size: a 4 × 4 needs 104 CFM for one air change a minute but about 190 CFM to carry its heat out at a 10 °F rise; a 10 × 10 room needs 800 vs about 1,130.

| Space | 1 air change a minute (CFM) | Heat at a 10 °F rise (CFM) |
|---|---|---|
| 2 × 4 tent | 48 | 95 |
| 4 × 4 tent | 104 | 190 |
| 5 × 5 tent | 163 | 290 |
| 4 × 8 tent | 208 | 370 |
| 10 × 10 room | 800 | 1,130 |

Source: Table above; CFM = BTU/h ÷ (1.08 × ΔT) [1].

**4 × 4 tent, worked example.** 104 CFM: One air change a minute; ~191 CFM: Heat at a 10 °F rise; ~275 CFM: With filter and duct allowances; 57–85 in²: Passive intake for a 6 in duct. Source: Formulas above [1]; allowances and intake ratio are grower practice [5].

**The lesson:** with modern light levels, **heat usually sets the fan size, not the one-minute rule.** And exhaust can only cool to the temperature of the air coming in: if the room is 80 °F, no fan makes the tent 75 °F. That's when you need [AC](https://genetixbay.com/knowledge/equipment/calc-cooling-ac-sizing.md). Big rooms usually go sealed with AC rather than moving thousands of CFM.

Worked example, 4 × 4 tent: heat CFM at a 10 °F rise = 2,063 ÷ (1.08 × 10) ≈ **191 CFM**. Metric: 605 W ÷ (1.21 × 5.6) ≈ **90 L/s**.

## How much do filters, ducts and bends cost you?

A fan's headline CFM is measured with nothing attached (zero static pressure). Every filter, metre of duct, bend and silencer adds resistance, and the fan moves less air [1][3]. Two ways to handle it:

**The proper way: fan curve versus system curve.**

1. Get the fan's curve (CFM versus static pressure, in inches of water column or pascals) from the maker.
2. Add up your system's pressure drop at the airflow you need: the carbon filter's rated drop (from its datasheet), duct friction (from a duct friction chart; flexible duct is much worse than smooth rigid duct), and each fitting [1].
3. Where the two curves cross is what you'll actually get [3].

**The grower shortcut** (*Grower practice*, not a law): add about **25%** for a carbon filter, and about another **10–20%** for a run with a couple of bends or flexible duct. So for 191 CFM: 191 × 1.25 × 1.15 ≈ **275 CFM**. A 6 in fan rated around 400 CFM free-air, on a speed controller, covers it with room to spare.

**Keep ducting short, straight, smooth and pulled taut.** Crushed or saggy flexible duct can cut airflow a lot. Run the duct size the fan was made for. *Supported* [1].

## How big should the intake be?

**GenetixBay's house position: passive intakes are right for most growers** (founder's field experience) [5]. An exhaust fan pulls air out; open intake ports let fresh air in; the tent sits under **slight negative pressure**, so any smell leaks inward and leaves through the carbon filter (see [do I need an intake fan?](https://genetixbay.com/knowledge/answers/do-i-need-an-intake-fan.md)).

**Size: open intake area about 2–3 times the exhaust duct's cross-section.** *Grower practice.* A bigger opening lets the same air in at lower speed and lower pressure drop, so the fan isn't starved [1].

| Exhaust duct | Duct area | Passive intake, 2–3× |
|---|---|---|
| 4 in (100 mm) | 12.6 in² (81 cm²) | ~25–38 in² (160–245 cm²) |
| 6 in (150 mm) | 28.3 in² (182 cm²) | ~57–85 in² (365–545 cm²) |
| 8 in (200 mm) | 50.3 in² (324 cm²) | ~100–150 in² (650–970 cm²) |
| 10 in (250 mm) | 78.5 in² (507 cm²) | ~157–236 in² (1,010–1,520 cm²) |

Duct area = π × (diameter ÷ 2)². Bug screens and filters on intakes block part of the opening, so add more area when you screen them.

**The check:** with the exhaust running, the tent walls should **pull in slightly**. Flat walls: not enough exhaust or too much intake. Walls sucked in hard and the fan whistling: open more intake. *Grower practice.*

**When an active intake fan is actually needed:**

- Long intake ducts, or intake drawn from another room or outdoors through ducting.
- Filtered intakes (fine insect or HEPA filters) that add a lot of resistance.
- Large rooms, or rooms where doors and gaps can't supply enough make-up air.

Keep any intake fan **weaker than the exhaust** (often about 70–80% of its airflow) so the space stays under negative pressure. *Grower practice.*

## How do I match a carbon filter?

**Rule: the filter's rated airflow should be at least what your fan actually moves through it, ideally a bit more, with the same flange size.** *Supported (adsorption physics; filter ratings are the maker's).*

Why: activated carbon removes odour molecules by adsorption, and adsorption needs **contact time**, how long the air spends in the carbon bed [4]. Contact time ≈ carbon bed volume ÷ airflow. Push more air through than the filter is rated for and the time drops, so smell gets through. A deeper bed of good carbon gives more time at the same airflow.

**Placement:** most tents hang the filter inside at the top, pulling hot air through the filter, then the fan, then out. Keep [relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md) in check: very damp air shortens carbon life. Use a pre-filter sleeve. For life and replacement signs see [how long does a carbon filter last?](https://genetixbay.com/knowledge/answers/how-long-does-a-carbon-filter-last.md).

## Common mistakes

- **Sizing only by the one-minute rule** and fighting heat all summer.
- **Buying by free-air CFM** and losing a third or more to the filter and duct [3].
- **A filter rated below the fan's airflow,** so smell leaks out [4].
- **Starving the intake,** so the fan works hard and moves little air.
- **An intake fan stronger than the exhaust,** which pushes smell out of the tent's seams.
- **Crushed, long or kinked flexible duct.**
- **Running CO₂ with the exhaust on** (see [CO₂ calculations](https://genetixbay.com/knowledge/environment/calc-co2-enrichment.md)).

## Sources

1. ASHRAE. 2021. ASHRAE Handbook: Fundamentals, Chapter 1 Psychrometrics (air density, specific heat) and Chapter 21 Duct Design (friction, fittings, fan and system curves). https://www.ashrae.org/technical-resources/ashrae-handbook
2. Thompson A, Taylor BN. 2008. NIST Special Publication 811, Appendix B (unit conversions). https://www.nist.gov/pml/special-publication-811
3. AMCA International. AMCA Publication 201: Fans and Systems (fan curves, system curves and system effect). https://www.amca.org/
4. US EPA. 2018. Air Pollution Control Technology Fact Sheet: Carbon adsorber (adsorption depends on contact time and bed depth). https://www.epa.gov/air-emissions-monitoring-knowledge-base/monitor-control-and-air-pollution-control-technology-fact-sheets
5. GenetixBay founder. Field experience (first-hand account; GenetixBay house position on passive intakes).