Free tool

Exhaust Fan Calculator

This exhaust fan calculator gives the CFM a grow tent, grow room or greenhouse needs, and the fan rating to buy once the carbon filter, bends and duct take their share.

Fan rating to buy

—

Airflow the room needs: —
Room volume: —
Air changes per hour: —
Left after filter and ducting: —
Fan size to look for: —

Next: Still too warm with the fan running? Size an air conditioner

How it works

The room needs its whole volume moved once per exchange. Each part of the system then takes a share of the fan’s rated airflow: the carbon filter, every bend and every length of duct. The rating to buy is the airflow you need divided by what is left.

Lights still making the room too warm? Use the air conditioner calculator.

Matching the filter, ducting and silencer

Use a carbon filter and duct of the same diameter as the fan, with a filter rated for at least the fan’s airflow. A smaller filter chokes the fan.
Run the fan on a speed controller. A size that does the job at 60–80% speed is quieter and has room for hot days.
A duct silencer adds a little loss, like another short length of duct.

Formula

Needed  = length × width × height ÷ minutes per exchange
Left    = (1 − filter) × (1 − bend)^bends × (1 − duct × length/10 ft)
Rating  = needed ÷ left

CFM chart for grow tents

For a full air change every 2 minutes, with a carbon filter, one 90° bend and 8 ft (2.4 m) of flexible duct. Faster exchange (every minute) doubles the airflow. Use the calculator above for your own ducting.

Exhaust fan airflow for common grow tent sizes
Tent	Volume	Airflow needed	Fan rating to buy	Every minute	Fan size
2×2 ft 60×60 cm, 4 ft tall	16 ft³	8 CFM	11 CFM	22 CFM	4″
2×4 ft 60×120 cm, 5 ft tall	40 ft³	20 CFM	27 CFM	55 CFM	4″
3×3 ft 90×90 cm, 6 ft tall	54 ft³	27 CFM	37 CFM	74 CFM	4″
4×4 ft 120×120 cm, 6.5 ft tall	104 ft³	52 CFM	71 CFM	142 CFM	4″
5×5 ft 150×150 cm, 6.5 ft tall	163 ft³	81 CFM	111 CFM	222 CFM	4″
4×8 ft 120×240 cm, 6.5 ft tall	208 ft³	104 CFM	142 CFM	284 CFM	4″
10×10 ft 300×300 cm, 6.5 ft tall	650 ft³	325 CFM	444 CFM	887 CFM	8″

Fan sizes use one maker’s ratings (4″ 226 CFM, 6″ 425 CFM, 8″ 857 CFM, 10″ 1,201 CFM, 12″ 1,604 CFM) and leave room to run the fan at 80% speed or less. Other makers differ a little.

Exhaust fan calculation formula, step by step

An exhaust fan does two jobs in a grow space: it takes out heat and moisture, and it brings in fresh air with carbon dioxide for the plants. How fast it must work depends on how often you want all the air replaced. Here is the formula worked through for a 4×4 tent, 6.5 ft tall:

Volume: 4 × 4 × 6.5 = 104 ft³
Airflow needed: 104 ÷ 2 minutes = 52 CFM (88 m³/h), or 30 air changes an hour
Share left: carbon filter 20%, one 90° bend 5%, 8 ft of flexible duct 3.6%, applied one after another, leaves 73% of the rating
Rating to buy: 52 ÷ 0.73 = 71 CFM

A 4-inch inline fan rated 226 CFM covers that at about 30% speed. Change the air every minute instead and you need 142 CFM rated, the same fan at about 65%.

How many CFM for a grow room

Rooms are bigger, so the fan size climbs quickly. With an 8 ft ceiling, a carbon filter, one bend and 8 ft of flexible duct, changing the air every 2 minutes:

10×10 room (800 ft³): 400 CFM needed, about 546 CFM rated; an 8-inch fan at about 65%
12×12 room (1,152 ft³): 576 CFM needed, about 786 CFM rated; a 10-inch fan at about 65%
200 sq ft room (1,600 ft³): 800 CFM needed, about 1,092 CFM rated; a 12-inch fan at about 70%

Changing the air every minute doubles each figure. A 12×12 room then needs 1,572 CFM rated, more than one 12-inch fan can give at 80% speed, so use two. These are grow-room figures, for heat from lights. A bathroom or kitchen fan follows different rules.

Greenhouse exhaust fan calculator

Choose Greenhouse and the calculator changes the air every minute. Alabama Extension gives 1 to 1.5 greenhouse volumes per minute. Use the average height, halfway between the eaves and the ridge, and set the carbon filter to No if there is none.

An 8×12 ft greenhouse with an average height of 7 ft holds 672 ft³. It needs 672 CFM at one volume a minute and 1,008 CFM at one and a half. With no filter or duct, nothing is lost on the way, so an 8-inch fan at about 80% covers the lower figure. A 12×24 ft greenhouse, 8 ft on average, needs 2,304 CFM. That is more than any single inline fan in the chart, so plan for two or more fans, or one larger commercial unit.

Common exhaust fan mistakes

Buying the airflow you need, not the rating. The filter and duct take their share first. In the 4×4 example, a fan rated exactly 52 CFM would deliver about 38.
Long, bendy duct runs. Three bends and 20 ft of flexible duct leave 62% of the rating, so the same tent needs 83 CFM. One short, straight run of 4 ft of rigid duct with no bends leaves 79%, and 66 CFM will do.
Fast exchange with added CO₂. Retailer guidance for a sealed room with CO₂ is an exchange every 3–5 minutes, not every 1–3. Fresh air dilutes the gas you paid for; the CO₂ calculator shows how much.
Expecting the fan to cool below the room. Fresh air can only bring the tent down to the temperature of the air coming in. If that is too warm, size cooling with the air conditioner calculator.
Drying out seedlings. Strong exchange pulls humidity down. The humidifier size calculator shows what it costs in water, and the VPD calculator whether it matters.

FAQ

Questions growers ask

How do I size an exhaust fan?

Divide the space's volume in cubic feet by the minutes per air change, then divide by the share of airflow left after the filter, bends and duct. A 4×4 tent, 6.5 ft tall, changed every 2 minutes needs 52 CFM. With a carbon filter, one bend and 8 ft of flexible duct, buy about 71 CFM rated.

How many CFM for a 4×4 tent?

About 52 CFM should reach a 4×4 tent, 6.5 ft tall, to change its air every 2 minutes. Buy a fan rated around 71 CFM to allow for a carbon filter, one bend and 8 ft of flexible duct. A 4-inch inline fan rated 226 CFM does that at about 30% speed. Every minute doubles both.

What size exhaust fan for a 12×12 room?

About 786 CFM rated for a 12×12 ft grow room with an 8 ft ceiling, changing its 1,152 cubic feet every 2 minutes through a carbon filter, one bend and 8 ft of duct. A 10-inch inline fan rated 1,201 CFM does it at about 65% speed. Once a minute needs 1,572 CFM rated, so two fans.

How many CFM for a 10×10 room?

400 CFM should reach a 10×10 ft room with an 8 ft ceiling to change its 800 cubic feet every 2 minutes. Allowing for a carbon filter, one bend and 8 ft of flexible duct, buy about 546 CFM rated: an 8-inch inline fan at about 65% speed. A 10×10 tent, 6.5 ft tall, needs about 444 CFM rated.

What size exhaust fan for 200 sq ft?

About 1,092 CFM rated for a 200 sq ft grow room with an 8 ft ceiling. Its 1,600 cubic feet changed every 2 minutes is 800 CFM, before a carbon filter, one bend and 8 ft of duct take their share. A 12-inch inline fan rated 1,604 CFM does it at about 70% speed. Bathroom fans follow other rules.

Does a grow tent need an intake fan?

Not always. Passive intake vents work when the exhaust fan pulls out more air than comes in, which keeps the tent under slight negative pressure: the walls draw in a little, and smells leave through the carbon filter rather than the seams. If you add an intake fan, set it to move less air than the exhaust.

How many CFM does a greenhouse need?

One to one and a half times its volume every minute, per Alabama Extension. An 8×12 ft greenhouse with an average height of 7 ft holds 672 cubic feet, so it needs 672 to 1,008 CFM. Choose the Greenhouse setting, which changes the air every minute, and set the carbon filter to No if you have none.

How often should the air change?

Growers usually aim for a full exchange every 1–3 minutes in tents and rooms, and 3–5 minutes in sealed rooms with CO₂. That is industry guidance, not research. For greenhouses, Alabama Extension gives 1–1.5 volumes per minute.

Sources

The formulas and target ranges on this page come from these references.

Hydrobuilder (retailer): Ventilation & carbon filter sizing (an exchange every 1–3 minutes, 3–5 in sealed CO₂ rooms; carbon filter −20–25%; ~5% per 90° bend; ~4.5% per 10 ft of flex duct, ~2.5% rigid). hydrobuilder.com/pages/ventilation-carbon-filter-sizing-calculator
Pickens & Wells (2022). Controlling the greenhouse environment for vegetable crops. Alabama Cooperative Extension ANR-2925 (exchange 1.0–1.5 greenhouse volumes per minute). www.aces.edu/blog/topics/crop-production/controlling-the-greenhouse-environment-for-vegetable-crops/
AC Infinity: CLOUDLINE PRO T4, T6, T8, T10 and T12 inline duct fans, product pages (total airflow 226, 425, 857, 1,201 and 1,604 CFM). acinfinity.com/hydroponics-growers/ventilation/inline-fan-systems/cloudline-pro-t6-quiet-inline-duct-fan-system-with-temperature-humidity-vpd-controller-6-inch/
NIST Special Publication 811, appendix B.9: conversion factors (footcandle to lux, 1.076 391 × 10¹). www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9

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