Fan & CFM Calculator
Work out the exhaust airflow your grow space actually needs — from the heat going into it and the air changes you want — and what fan rating that means once a filter and ducting are in the way.
Your space
Every dimension must be greater than 0.
Watts drawn at the wall, not any “equivalent” figure on the box. Nearly all of it ends up as heat. “Other” covers pumps, dehumidifiers and anything else running inside the space.
How much warmer the room may sit than the air you draw in. Ventilation can only ever approach the intake temperature — it cannot go below it.
A rule of thumb rather than a measurement. It is the floor under your fan size; the heat figure usually sets the real one.
Leave at 0 near sea level. Higher up the air is thinner and carries less heat, so the same fan removes less.
Airflow needed
What this means
What would help
Shown based on what you entered, not a fixed list. As an Amazon Associate we earn from qualifying purchases.
Where these numbers come from
- Calculated
- Heat removal is the standard sensible-heat relation: CFM = BTU/hr ÷ (1.08 × ΔT°F). The 1.08 is 0.075 lb/ft³ air density × 0.24 BTU/lb/°F specific heat × 60 minutes. Watts become BTU/hr at 3.412142. Air changes are volume ÷ minutes. Elevation scales the 1.08 by the US Standard Atmosphere density ratio, because that constant only holds for sea-level air near room temperature.
- General guideline
- Exchanging the full volume every one to three minutes is a widely repeated rule of thumb, not a measured requirement. It is treated here as a floor under the fan size, never as the answer on its own.
- Manufacturer-specific
- The two published methods disagree badly, so we show both. One widely used set puts a carbon filter at 20–25%, flexible duct at 4.5% per 10 ft, rigid at 2.5%, each right-angle bend at about 5%, and a silencer at 10–15%. A major fan manufacturer’s own guidance instead multiplies by 1.6 for a filter, 1.2 for ducting and 1.2 for a silencer, and puts a single 90° bend at 60%. Those are not small differences and we will not pretend to resolve them. Your fan’s published airflow-versus-static-pressure curve and your filter’s rated airflow beat every figure here.
Bends are applied only in the lower method: the per-bend figure in the higher one has no published rule for combining several, and compounding it produces numbers no one should act on. This is a planning aid, not a substitute for the documentation that came with your equipment.
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Two numbers, and the bigger one wins
Most fan calculators do one thing: multiply your tent volume by a handful of vendor factors. That misses the point, because volume is rarely what decides the answer.
This one works out two independent figures. The first is air exchange — how much airflow it takes to replace the room’s volume every minute or three. The second is heat removal — how much airflow it takes to carry away the heat your lights and equipment are producing while holding the temperature rise you asked for. Whichever is larger is the one you have to satisfy.
Why heat almost always decides it
A 4 × 4 × 6.5 ft tent holds about 104 cubic feet, so a one-minute air change needs roughly 104 CFM. Put a 240 watt light in it and hold the rise to 5 °F, and the heat figure comes out near 152 CFM. Heat wins, and it is not close.
Turn the light up to 600 watts and the gap widens sharply. That is the useful thing this tells you: when heat is setting your fan size, a bigger fan is the expensive answer. Less wattage, or accepting a warmer room, moves the number far more.
One rule the arithmetic will not let you break: ventilation cannot cool a room below the air you are drawing into it. Ask for a zero temperature rise and the calculator refuses, because no fan on the market can do it. At that point you need cooler intake air or actual cooling, not more airflow.
What a filter and ducting really cost you
Here the honest answer is that nobody agrees. One widely used set of figures puts a carbon filter at a 20–25% loss, flexible duct at 4.5% per 10 feet and a right-angle bend at about 5%. A major fan manufacturer’s own guidance instead multiplies by 1.6 for the filter and puts a single 90° bend at 60%.
Those are not small differences, and picking one silently would be pretending to a precision that does not exist. So the tool runs both and gives you a range. Buy toward the upper end and get a fan with speed control — a fan running at half speed is quiet and long-lived, while one pinned at maximum is neither.
Three mistakes worth avoiding
- Using the wattage on the box. Marketing wattage and wall draw are different numbers. Use what the fixture actually pulls, which is usually on the label or your plug-in meter.
- Ignoring the filter’s own rating. If your filter is rated below your fan, the filter is your real ceiling no matter what the fan can move.
- Confusing exhaust with circulation. An inline fan exchanges the air in the room; a clip or circulator fan stops it sitting still around the leaves. They solve different problems and you generally want both.
Common questions
How many air changes per minute do I actually need?
Once a minute is the usual rule of thumb, and every two or three minutes is common in cooler rooms or where noise matters. Treat it as a floor under your fan size rather than the answer, because the heat figure normally lands higher anyway.
Does elevation change the answer?
Yes, and most calculators ignore it. Thinner air carries less heat per cubic foot, so the same fan removes less. Above roughly 2,000 feet the correction is worth applying, and the tool does it for you.
Why is the fan figure a range rather than one number?
Because the published guidance on what filters, duct and bends cost you differs by roughly tenfold. Any single number would be a guess dressed up as a calculation. The range shows you both published methods and lets you decide where to sit inside it.
Should I size the fan for lights-on or lights-off?
Lights-on. That is when the heat load is highest, and it is the condition the fan has to cope with. With speed control you can simply turn it down when the lights are off.
The other calculators
- VPD Calculator — once the airflow is right, this tells you whether the moisture in that air is where your plants want it.
- Grow Light / DLI Calculator — the wattage you entered above is also what sets your light levels. This works out whether your crop is getting enough.
- All grow tools