Grow Room Fan Size Calculator with Size-wise CFM

No matter it’s low ventilation or high, it is equal to dead(almost) grow plants. And the whole ventilation is mostly up to what size intake and exhaust fan you’re using for your grow system. 

So, how to calculate fan size for grow room? 

To determine the extractor fan size of your grow room or tent, the formula is to multiply grow room volume(cubic feet), carbon filter factor(+25%), insulation factor(±20%), ducting(+10% for 10 feet) and the light factor(+10% for 1000W). You’ll get the fan size in CFM

In case you want to work out what size intake fan for grow room to use, just deduce the extractor fan size by 15-20%. 

That was the quick grow room fan size calculator formula. Eventually, some of you might raise an eyebrow and think- What on earth do these ‘factors’ mean? And why do you have to take these many of them under your radar?

Hold your horses, dear grower. To answer these obvious questions of yours, we’ve crafted this entire 3000 words article. If you really want to learn how the math behind this formula works, provide yourself a few minutes till you’re through the entire post. 

Factors to Consider in Calculating Grow Room Fan Size

First things first, let’s clarify what these factors mean, how they affect the grow room fan size calculation, and what exact percentage they’ve to contribute in the calculation. 

Factor 1: The Volume of The Room/Tent

At first, work out how much space your fans have to deal with, a.k.a, the active grow space. In other words, that’s the space covered by your grow lights. And to keep the calculation simple, let’s assume that your fans will be able to replace the entire air within the space in 1 minute(explained in the next section).  

Rule of Thumb

Simply use your tent’s dimensions(length, width, height) and multiply them to get the total volume. The unit can be either Feet or Meter, based on the state norms. Without taking any other factors into our radar, this value is the fan CFM you need. 

Say, it’s a 10x10x7 feet tent that you’re dealing with. So, the fan size will be- 

Fan Size= (10 feet x 10 feet x 7 feet)/1 Minute= 700 CFM

Factor 2: Time of Complete Air Replacement

While calculating the fan size requirement for your grow room or tent, you’ve to find the amount of air you need to ventilate in each minute. Because you know, the size unit(CFM) sports the capacity of how much air the fan can ventilate in a minute. 

Rule of Thumb
Find out the volume of your grow space (length x width x height) and divide it by the minutes it takes to empty up entire air within it. As an example- if the room dimension is 10 feet(L) X 10 feet(W) X 7 feet(H), then the total volume comes 700 cubic feet. And if that takes ~2 minutes to take the entire air out, then the calculation would be- 

Fan Size= 700 cubic feet/ 2= 350 CFM

Factor 3: Presence of Carbon Filter

Having an inline carbon filter in the ventilation system drops the efficiency of the extractor fan. When air is passed through the layer of activated carbon, it’s speed deduces by a certain extent. Also, you’ve to keep the filter’s age, size, the thickness of the activated CO2 layer, etc as minor considerable. But for now, we’re keeping them aside. 

Rule of Thumb

Carbon filters deduce the extractor fan efficiency by 25%. So, if you use one in-line carbon filter in the system, and the generic CFM for the fan is 200 CFM, then the actual fan size requirement will be-

Fan Size= (200 CFM + (200×25%))= 250 CFM 

Factor 4: Insulation of the Room

If you grow in a good insulation environment that doesn’t have that much of heat exchange with the surroundings, you don’t need to consider this factor. 

But if it’s in a less-insulated/more sun-exposed areas like the attic, the basement, upstairs, south-facing bedroom, etc, then there are some important considerations.  In any of these cases, the temperature will be either more or less than what you are supposed to maintain in your grow room. 

Rule of Thumb

If the place is a warm attic(or any similarly warm place), add 20% extra on top of the required CFM. If it’s in a cold place like a basement, deduce 15% from the required CFM. 

If your generic CFM is 200, then- 

Fan Size for warm places=  (200 CFM + (200×20%))= 240 CFM

Fan Size for cold places=  (200 CFM – (200×15%))= 230 CFM

Factor 5: The Ductwork Curves and Length

It’s needless to say that, ducting bends and length will hinder the air extraction capacity of the fans. Based on your grow room size and plant numbers, there can be three ducting scenarios- 

  1. Minimal or no ducting with no bends. 
  2. Medium ducting runs(5-10 feet) with 2-3 bends. 
  3. Long ducting runs(10-20 feet) with 3-6 bends. 

Rule of thumb 

For every 5 feet of ducting, there will be an addition of 5% in the fan CFM. And for every ducting equivalent to 90 degrees of angle, there will be an addition of 20% to the CFM. 

If you’ve a 200CFM initial fan size, then- 

Fan Size considering ducting(15’)=  (200 CFM + (200×15%))= 230 CFM

Fan Size considering bends(3)=  (200 CFM + (200×60%))= 320 CFM

Factor 6: Lights and Other Heat Sources

As you know, lights and pumps are the two obvious heat sources in a lamp. If you use air-cooled lights in your grow room, that’s a different case. But if they’re not air-cooled, you’ve to take each of them in calculating your estimated fan size. And same goes to other heat generative devices like motors etc. 

Rule of thumb 

For each 1000W lamp power, add ~10% to the fan cfm requirement.

If you have 4 fans of 1000W each, and if the generic calculations say a 200CFM, then the above total will be-

Fan Size= (200 CFM + (200×40%))= 280 CFM

Grow Room Fan Size Calculator(Extractor & Intake Fan)

For this segment of grow room fan size guide, we’ll take a test case of grow space, imply some generic conditions, and work out the final size of both grow room ventilation fans of both extract and intake. 

Let’s say, we have an 8x8x7 grow tent at your basement, with an inline carbon filter set up with the 4″ duct system. As the room is large enough for ScrOG style training, there are 15 plants to grow(according to our size guide).  At a certain stage of growing, there are 4 600W HID lamps, which are air-cooled. And apart from the light, there are no other major heat sources. 

Extractor Fan

Let’s start with the grow room extractor and intake fan calculator formula-

Step 1: Calculate the Space Volume

The first step is rather simple. All you have to decide is the unit. There are two types of them- the European Standard and the North Americal Standard. In the first standard, the dimension is taken in Meter(m), and in the second kind, it’s in feet(f). For instance, let’s stick to the Americal Standard of measurement

So, the volume of the tent will be- 

Volume= 8 feet x 8 feet x 7 feet= 448 cubic feet 

Step 2: Calculate The Air Replacement Time

Let us assume that whatever fan we’ll end up with, will be able to replace the entire air of the tent/room within one minute. That reduces the complicacy of the calculation. 

Fan Size= Volume/Minutes to Empty Up Air

= 448 cubic feet/1 minute= 448 CFM(Cubic Feet Per Minute)

Step 3: Calculate Carbon Filter Allowance

As we’ve said before, there is a 4” carbon filter in our setup, and that’s in-lined with the ducting. And the exact rate at which it will reduce the fan efficiency is 25%

Therefore- 

Fan Size= 448 CFM + (448×25%)= 560 CFM

Step 4: Calculate the Insulation

The level of insulation has a lot to do with the environment of the grow room/tent. For instance, we’ve ended up with a grow setup in the basement, which is a rather cooler place then what it is in another room. So, we’ll consider a 15% fall in the required fan size for the room. 

Fan Size= 560 CFM – (560×15%)= 476 CFM

Step 5: Calculate the Ductwork

In our 8×8 grow tent setup with grow room ducting fan, we’re supposed to keep the ducting limited within 10 feet(including the bends). That’s not any rule or so, but let’s stick to that for the sake of calculation. And for several turnings and adjustments, there are about 2 bends in the ducting. 

Therefore- 

Fan Size= 560 CFM + (560×10%) + (560×40%)= 840 CFM

Step 6: Calculate the Lights

As heat sources, lights are supposed to raise the CFM requirement by 10% each. But as we have an air-cooled HID light set, these won’t heat the space up. And we don’t need to take it under calculation. 

Fan Size= 840 CFM + 0= 840 CFM

Finally, the grow room extractor/exhaust fan calculator suggests us to go with a fan of 840 CFM of size. 

Intake Fan

Done with the grow room exhaust fan size calculation? Let’s bring some fresh air in through the grow room air intake fan now.

Once you’ve figured out the size of exhaust fan, working out the grow room intake fan size is pretty simple. There should be a slightly negative pressure present in the grow room. That means you have to intake slightly less air in comparing to what goes out. Therefore, the CFM of the intake fan can not be as much as the CFM of the exhaust fan. 

The simply put rule is- The intake fan will be 15% less powerful than the exhaust fan size. So, from our previous data-

The Intake Fan Size= 840 CFM – (840×15%) = 714 CFM

So, the final is intake fan size is 714 CFM. Although it might deviate a lot based on fan placement in grow room.

Quick Recommendations(for Grow Tent)

There are a handful of grow tent sizes, and calculating the grow fan size can be a hectic process. Here in this segment of the grow room fan size calculator guide, we’ll provide you with a worked out fan size for typical grow tent sizes- 

What Size Fan for 2×2 Grow Tent?

In a 2x2x6 tent, there is usually 1 carbon filter, 4 lights of 400W(non-air cooled), no ducting inside and a passive air intake system. If you keep it insulated enough from the surrounding, then-

Fan size for 2x2x6 grow tent= (24 cubic feet/1 minute) x 1.25 x (1.2)= 36CFM

What Size Fan for 2×4 Grow Tent?

In a 2x4x6 tent, there are usually 1 carbon filter, 4 lights of 600W(non-air cooled), 3-6 feet of ducting inside and a passive air intake system. If you keep it insulated enough from the surrounding, then-

Fan size for 2x4x6 grow tent= (48 cubic feet/1 minute) x 1.25 x 1.05x 1.25= 78 CFM

What Size Fan for 3×3 Grow Tent?

In a 3x3x6 tent, there are usually 1 carbon filter, 4 lights of 600W(non-air cooled), 6-8 feet of ducting inside and a passive air intake system. If you keep it insulated enough from the surrounding, then-

Fan size for 3x3x6 grow tent= (54 cubic feet/1 minute) x 1.25 x 1.07x 1.25= 90 CFM

What Size Fan for 4×4 Grow Tent?

When it’s about a 4x4x6 tent, you get to have 1 carbon filter, 4 lights of 600W(non-air cooled), 8-10 feet of ducting inside and a passive air intake system. If you keep it insulated enough from the surrounding, then-

Fan size for 4x4x6 grow tent= (96 cubic feet/1 minute) x 1.25 x 1.08x 1.25= 130 CFM

What Size Fan for 4×8 Grow Tent?

When it’s about a 4x8x6 tent, you get to have 1 carbon filter, 4 lights of 600W(non-air cooled), 10-14 feet of ducting inside and an active air intake system. If you keep it insulated enough from the surrounding, then-

Exhaust Fan size for 4x8x6 grow tent= (192 cubic feet/1 minute) x 1.25 x 1.12x 1.25= 336 CFM

Intake Fan size for 4x8x6 grow tent= 285 CFM

What Size Fan for 5×10 Grow Tent?

When it’s about a 5x10x8 tent, you get to have 1 carbon filter, 4 lights of 800W(non-air cooled), 15-18 feet of ducting inside and an active air intake system. If you keep it insulated enough from the surrounding, then-

Extract Fan size for 5x10x8 grow tent= (400 cubic feet/1 minute) x 1.32 x 1.16x 1.25= 765 CFM

Intake Fan size for 5x10x8 grow tent= 650 CFM

What Size Fan for 8x8x8 Grow Tent?

When it’s about an 8x8x8 tent, you get to have 1 carbon filter, 4 lights of 1000W(non-air cooled), 18-20 feet of ducting inside and an active air intake system. If you keep it insulated enough from the surrounding, then-

Extract Fan size for 8x8x8 grow tent= (512 cubic feet/1 minute) x 1.4 x 1.19x 1.25= 1066 CFM

Intake Fan size for 8x8x8 grow tent= 906 CFM

What Size Fan for 10×10 Grow Tent?

When it’s about a 10x10x8 tent, you get to have 1 carbon filter, 6 lights of 800-1000W(non-air cooled), 20-22 feet of ducting inside and an active air intake system. If you keep it insulated enough from the surrounding, then-

Extract Fan size for 10x10x8 grow tent= (800 cubic feet/1 minute) x 1.54 x 1.22x 1.25= 1878 CFM

Intake Fan size for 10x10x8 grow tent= 1600 CFM

Boost Grow Room/Tent Fan Life By Doing These

You must be aware of how important is a fan in a grow room. Being a little careful about a few things might boost the lifespan of these devices. Let’s learn how-

Don’t Mismatch Your Ducting and Fan Diameters

Ducting is already a serious reason to bring the fan efficiency down. If your fan and the ducting have a diameter mismatch, that’s an even worse scenario. As an example, a combination of 6″ fan reducing to a 4″ ducting will choke off airflow. 

Summary line, make a perfect match in between your fan diameter and the ducting diameter. 

Induce Active Air Intake Instead of Passive

In case you are dealing with a medium-sized grow room or tent, a few air inlet holes won’t be able to drag enough of the fresh air. For grow spaces higher than 8×8 feet or so, use an active air intake fan for grow room to keep up with the wind pressure and air supply. Also, learn how to set up an exhaust fan in grow room for maximum efficiency.

Use A Centrifugal Fan

For small tents or rooms, squirrel type grow fans are sufficiently strong. But for larger spaces with more extraction power demand, centrifugal fans are better choices. In fact, all of these grow room fan calculator formulas of this post is crafted in this manner.

Centrifugal fans are those which are used in line with the air cooling panel, the ducting and the carbon filter. It will filter your air, cool the grow lights and ventilate the garden air. 

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Control Your Fan Speed

At different growth stage and different seasons, you’ve to induce different air ventilation rate for your plants. As an example, it should be higher in the vegetative period comparing to the seedling period.

And that can be done with a fan speed controller. With some of their advanced versions, this CFM variation can be pre-programmed based on timers. Also, ensure a proper on-off cycle for the fans

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Protect from Bugs

The inlet/intake fan is usually located near the ground area. Because the air near to the ground is cooler in temperature. But that also brings on the chance of the entry of bugs, dust and airborne germs. 

Keep your grow room fan setup proteced by using a bug mesh onto the inlet fans to get rid of this threat. It will also reduce the grow room exhaust fan noise by some extents. However, you’ve advised to stick to the quietest grow room exhaust fan. 

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When Exactly You Care About Fan Size?

We know that you, as a serious grower already have the answer in mind. But for the sake of the moment, let’s have a look at the importance of precise grow room fan cfm calculation– 

Growing Plants Demands More Airflow

When plants are at early stages of growth, the leaves are smaller in size. So, the CO2 that they extract from the surrounding air, is not that much in amount. A decent airflow around the leaves might feed them with the essential Co2 supply. 

But when they grow up, leaves become larger and the necessity of CO2 follows. Physiological processes like photosynthesis and transpiration take place at a higher extent. The required supply of CO2(for photosynthesis) and O2(for transpiration) raises higher. 

So, to keep plants alive, you’ve to vent more of the heavy CO2-rich air out and replace it with fresh air. That obviously, will ask for more powerful, and consistent extractor fans while keeping the passive/active air intake on. 

To Evacuate Excess Heat

Lamps, lights, and pumps– these are the common heat sources in any grow system. In a sealed grow room, they become the major source of heat and temperature. If you keep the space unventilated, they’ll quickly raise the temperature to 100+ degrees Fahrenheit.

With such a rise in temperature, plants start to grow slowly and can succumb to heat stress. In the worst case scenario, they might die as well. Because of the barring issues caused by the excess temperature, plants will use all the CO2 and O2 in the space. And that will also raise the humidity of the grow room.

The only way out is to keep the air circulating in the current manner, with the obvious help of extractor and grow room inline fans.

Final Words

Phew! It had been a long journey from the start to this point of the post. But by the end, we’re hopeful that you’ve learned what and how to work out fan size for grow room or any sealed grow space. Post your comments if you need any custom solution regarding this concern. We’ll be back with an appropriate answer. 

Happy growing!

smsaleh

I'm Saleh, a hobbyist DIYer, and a blogger. Whatforme.com is my little place on the web to express what I've learned first-hand, specially about home improvement.

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