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, passive intake, 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].
Tent volumeCFM for 1 change a minute- Heat CFMBTU/h ÷ (1.08 × ΔT)
- Take the larger
- Allow for filter and ductCheck the fan curve
- Passive intake 2–3× duct area
How much airflow for each size?
Lights at about 900 µmol/m²/s with a top-tier LED, plus fans (see lighting sizing). "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 |
- 104 CFMOne air change a minute
- ~191 CFMHeat at a 10 °F rise
- ~275 CFMWith filter and duct allowances
- 57–85 in²Passive 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. 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.
- Get the fan's curve (CFM versus static pressure, in inches of water column or pascals) from the maker.
- 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].
- 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?).
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 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?.
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).
