Free Grow Room Tool

Grow Tent Fan Calculator — Exhaust CFM, Carbon Filter & Duct Size

The rule is one full air exchange per minute — then reality adds a carbon filter, flexible duct, a hot LED or HPS light and a warm room. Enter the tent size and those factors; get the exhaust fan CFM you should buy, the fan size (4, 6 or 8 in), the intake opening and how many clip/oscillating fans to add for circulation.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-24

Grow Tent Exhaust Fan Sizing

Base CFM = tent volume (ft³) per minute × adjustment factors. Run an EC/PWM inline fan at 50–70 % most of the time.

Quick preset

Tent size

Length

Width

Height

Standard tents 5–7 ft (160–200 cm).

Grow light power (total)

Light type

Carbon filter Carbon filter on the exhaust (+25 %)

Exhaust duct length

+5 % per 5 ft of flex duct

Bends in the duct

+10 % each (90°)

Room where the tent stands

CO₂ enrichment Sealed room with CO₂ (fan only for heat purge / on timer)

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

How grow tent fans are sized

Base CFM = tent volume (ft³) × 1 air exchange per minute // = L × W × H in feet
Adjustments carbon filter +25 % · flex duct +5 % per 5 ft · each 90° bend +10 % · hot room +30 % · HPS light ×1.5 (CMH ×1.3)
Heat check CFM ≥ light heat (W) × 3.412 ÷ (1.08 × ΔT °F) // keep ΔT ≤ 9 °F (5 °C) over the room
Fan size ≤ 190 CFM → 4 in · ≤ 400 → 6 in · ≤ 750 → 8 in · ≤ 1,200 → 10 in
Intake passive opening ≥ 1 ft² per 400 CFM, or intake fan at ~80 % of exhaust (keeps negative pressure)

Tent	Volume (6.5 ft tall)	Base CFM	With filter + duct (typ.)	Fan
2 × 2	26 ft³	26	40–50	4 in
2 × 4	52 ft³	52	80–100	4 in
3 × 3	59 ft³	59	90–110	4 in
4 × 4	104 ft³	104	160–200	4–6 in
4 × 8	208 ft³	208	320–400	6 in
5 × 5	163 ft³	163	250–310	6 in
8 × 8	416 ft³	416	640–800	8 in
10 × 10	650 ft³	650	1,000–1,250	10 in

Why over-size and slow down: an inline fan at 60 % speed moves 60 % of the air for ~22 % of the power and about 11 dB less noise, and it still has full capacity for a July afternoon. A controller that tracks temperature and humidity (VPD) is the best money after the light.

Frequently Asked Questions

What size fan for a 4×4 grow tent?

A 4×4×6.5 ft tent is 104 ft³, so the base is ~105 CFM. With a carbon filter, 10 ft of duct and two bends you need about 190–200 CFM, which means a 6 in inline fan (or a strong 4 in EC fan) run on a controller. With a 480 W LED in a normal room that is also enough for heat.

How many CFM for a 2×2 grow tent?

About 26 ft³ → 40–50 CFM with a filter: a 4 in inline fan at low speed. Even the smallest 4 in fans (100–190 CFM) are oversized, which is fine — run them at 30–50 %.

Do I need an intake fan?

Usually not for tents up to 5×5: passive intake vents of at least 1 ft² per 400 CFM of exhaust work, and keep the tent under negative pressure so odour cannot leak. For 8×8 and larger, an intake fan at ~80 % of exhaust gives steadier airflow.

Should the carbon filter be inside or outside the tent?

Inside at the top (hottest, smelliest air), pulling through the filter into the fan, then out through the duct. Filter CFM rating must be equal to or above the fan's CFM. Replace the carbon every 12–18 months, sooner in humid conditions.

How much does a grow tent fan cost to run?

A 6 in EC inline fan draws 20–45 W at typical speeds: 18 h/day is 0.4–0.8 kWh, roughly $0.06–0.14 per day or $20–50 per year. See the fan electricity cost calculator.

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<a href="https://www.longwellfans.com/resources/grow-tent-fan-calculator/">Grow Tent Fan Size Calculator (CFM & Carbon Filter)</a> by LONGWELL Fans

APA: LONGWELL Fans. (2026). Grow Tent Fan Size Calculator (CFM & Carbon Filter). https://www.longwellfans.com/resources/grow-tent-fan-calculator/

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