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# How to Calculate CFM

## How to Calculate CFM

#1
02-29-08, 12:18 PM
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Join Date: Feb 2008
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How to Calculate CFM

Can anybody give me the correct formula for calculating the CFM.? I think it has something to do with coming up with the cubic square feet of the room??

I found a site that does it for you but it doesn't take into consideration that my bathroom is not a perfect square (or rectangle, or whatever). One part of the room has one set of measurements and another part of the room has another because of cabinets and a wall that comes in and makes it a much smaller area than the rest of the room.

Anyway, does anyone know the formula?

#2
02-29-08, 12:42 PM
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Dont know of any for in the home. But for an attic its sq ft X0.7==cfm.

#3
02-29-08, 01:39 PM
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What is it you are trying to determine? And what will you do with the information?

Exhaust fans are measured in CFM (Cubic Feet per Minute) if that's why your asking.

Determine the air volume of your room and decide how many "room exchanges" of air you desire in what period of time and you can match a fan to those requirements.

12" X 12" X 12" equals one cubic foot (1728 cubic inches.)

#4
02-29-08, 01:46 PM
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Originally Posted by Bud Cline
What is it you are trying to determine? And what will you do with the information?

Exhaust fans are measured in CFM (Cubic Feet per Minute) if that's why your asking.

Determine the air volume of your room and decide how many "room exchanges" of air you desire in what period of time and you can match a fan to those requirements.

12" X 12" X 12" equals one cubic foot (1728 cubic inches.)
I'm trying to determine what size ventilation fan I need to buy to install in my bathroom that currently has no fan.

#5
02-29-08, 02:14 PM
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And by the way, thank you for your response. You wouldn't believe how long I've been googling on the internet in search of this formula!

#6
02-29-08, 02:37 PM
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Calculate the air volume of your room, in cubic feet.
Then decide how many times you want that volume of air to exchange in one hour. Then you can match a fan to your needs.

For example:

A 5' X 7' bathroom with an 8' ceiling will contain 280 cubic feet of static air space.

5' X 7' = 35 sqft X 8'high = 280 cuft.

If you want that air to turn over three times in one hour you need to move a total of 840 cubic feet of air.

280 cuft X 3 exchanges = 840cuft.

840 cuft/60 min(one hour) = 14 CFM.

A small fan rated at 14 CFM would serve the needs of this illustration.

Nothin' to it, piece-o-cake.

#7
02-29-08, 02:52 PM
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How to Calculate CFM

Half bath, 3/4 bath, full bath or complete bath with a jacuzzi? - All have different effects on the air volume. Several factor enter into the practicality of ventilation of a bathroom -

1. Heavy, moist air is harder to move that dry air.

2. Odors can be noticed very easily.

Turming over three times in one hour does not mean everything is evacuated since there is always circulation problems and you are also exhausting fresh air.

The bottom line is to get a larger fan with two speeds that is quiet. You will only do it once (in this house) and the installation is probably not much different.

#8
02-29-08, 09:33 PM
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Join Date: Feb 2008
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Bud, Thanks again. That's exactly what I was looking for. I'd seen the formula before but couldn't remember it. Thank you so much. T

Originally Posted by Bud Cline
Calculate the air volume of your room, in cubic feet.
Then decide how many times you want that volume of air to exchange in one hour. Then you can match a fan to your needs.

For example:

A 5' X 7' bathroom with an 8' ceiling will contain 280 cubic feet of static air space.

5' X 7' = 35 sqft X 8'high = 280 cuft.

If you want that air to turn over three times in one hour you need to move a total of 840 cubic feet of air.

280 cuft X 3 exchanges = 840cuft.

840 cuft/60 min(one hour) = 14 CFM.

A small fan rated at 14 CFM would serve the needs of this illustration.

Nothin' to it, piece-o-cake.

#9
02-29-08, 09:34 PM
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Turning over three times in one hour does not mean everything is evacuated since there is always circulation problems and you are also exhausting fresh air.
This WAS NOT a recommendation, it was merely an example.

#10
03-27-14, 11:54 PM
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Join Date: Mar 2014
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CFM calc

i am an Electrical engineer , but somehow my boss wants me to design HVAC system also.

My question is ,

While calculating the CFM ,calculating Volume of room air is understood, but what is the factor which is referred as '' number of times air is exchanged'' i.e. 3 is assumed in this thread for toilet?

Can i get some description of this factor? what are the standard number of exchange rates of air in a place where i have to install an AC machine.

lets say , for a sample bedroom.

What will be CFM calculation for Cooling and heating.Assume we dont need ventilation having a window in the bedroom.a

Thanks!