Ventilation & Carbon Filter Sizing Calculator

Ventilation & Carbon Filter Sizing Calculator

Get the right inline fan and carbon filter CFM for your grow room or tent — in seconds. Enter your room size, set your air-exchange target, and the calculator accounts for every resistance loss in your duct run. Need help choosing? Talk to a Grow Expert.

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Ventilation & Carbon Filter Sizing Calculator

Enter your room dimensions and air-exchange target. Switch to Advanced to dial in duct losses, bends, and silencer resistance for a precise required CFM.

How to use this calculator
  1. Enter room dimensions — length, width, and height in feet.
  2. Set air exchange time — 2 minutes is the most common starting point. Lower for hot rooms, higher for sealed CO₂ setups.
  3. Advanced mode — add carbon filter loss, duct length and type, number of 90° bends, and silencer resistance for a precise result.
  4. Set headroom — 20% is standard. This accounts for filter aging and running the fan below maximum speed to reduce noise.
  5. Tap Calculate CFM — your required fan CFM and recommended duct size appear instantly.
  6. Match your fan to a unit rated at or above the Required CFM (with headroom). Size your carbon filter to match the fan's rated CFM.

Formula: Base CFM = Room Volume ÷ Exchange Time  ·  Required CFM = Base × Climate × (1 / (1 − total_loss_fraction)) × (1 + headroom)

Ventilation Quick Reference

Common grow tent sizes — recommended inline fan and carbon filter CFM
Tent Size Volume (ft³) Base CFM (2 min) Required CFM* Fan Size Filter Size
2×2×5 20 10 ~16 4-inch (190 CFM) 4-inch
3×3×6 54 27 ~41 4-inch (190 CFM) 4-inch
4×4×7 112 56 ~86 4-inch (190 CFM) 4-inch
5×5×8 200 100 ~153 6-inch (440 CFM) 6-inch
4×8×8 256 128 ~196 6-inch (440 CFM) 6-inch
10×10×8 800 400 ~613 8-inch (750 CFM) 8-inch
10×20×10 2000 1000 ~1533 Two 8-inch fans Dual 8-inch

*Required CFM includes 20% carbon filter loss, 10 ft flex duct, 1 bend, and 20% headroom.

📊 CFM Waterfall — See Every Loss Step Waterfall ▾
Calculate first to see your CFM breakdown chart.

Each bar shows how airflow is reduced from the Base CFM by each loss factor. The final green bar is your Required Fan CFM — the minimum rated CFM for your inline fan.

Frequently Asked Questions

How do I calculate CFM for a grow room?
Divide your grow room volume (length × width × height in feet) by the desired air exchange time in minutes. For a 10×10×8 room exchanging air every 2 minutes: 800 ft³ ÷ 2 = 400 CFM base. Then multiply by loss factors for your carbon filter, ducting, and bends to find the actual fan CFM you need.

What CFM fan do I need for a carbon filter?
Your fan must be rated at or above your required CFM — which already accounts for the filter's resistance. A carbon filter reduces effective airflow by 20–25%. If your base requirement is 200 CFM and you have a standard filter, your fan needs to be rated for at least 250 CFM. Always size your carbon filter to match the fan's rated CFM, not your base requirement.

How often should I exchange air in a grow room?
Most growers target a full air exchange every 1–3 minutes. Sealed rooms with CO₂ enrichment can extend to 3–5 minutes since you're not exhausting the enriched air constantly. Rooms with high-intensity lighting, hot climates, or dense canopies benefit from 1-minute exchanges to keep temperature and humidity under control.

What size carbon filter for a 4×4 grow tent?
A 4×4×7 tent has 112 ft³. At 2 minutes per exchange, base CFM is 56. After a 20% carbon filter loss, 10 ft of flex duct, and 1 bend, your required CFM is about 90. A 4-inch inline fan rated 190 CFM paired with a matching 4-inch carbon filter is the standard choice — it delivers comfortable headroom and lets you run the fan at 60–70% speed for quieter operation.

Should I run my carbon filter on the intake or exhaust?
Always run the carbon filter on the exhaust (negative pressure) side, pulling air through the filter before it exits the tent. This keeps odors from escaping into the room. Some growers run the filter inside the tent connected directly to the fan; others mount the fan outside with the filter inside. Either works — just make sure air flows through the carbon before being exhausted.

Ready to build the perfect ventilation setup?

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