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Sizing an Exhaust Fan

Sizing an Exhaust Fan TIPS

Size Is Everything!

Can you imagine trying to cool yourself on a hot summer day with just the air that's allowed to pass through a tiny cocktail straw? Just a tiny stream of air would pass through and it would be virtually worthless.

Move The Air

To cool something or provide enough air to keep things fresh, you have to move air. The room you're trying to ventilate dictates the amount of air that needs to be moved in a given amount of time.

CLICK HERE to get FREE & FAST BIDS from local contractors that install ventilation fans.

Air Movement Guidelines

For years the Home Ventilating Institute said that all the air in a kitchen should be changed out for new at least fifteen times an hour. They felt that bathrooms should have a complete air change eight times an hour.

Shhhhhhhhsss! Quiet Fans

Are you upset with how loud some bathroom or kitchen exhaust fans are?

Did you know you can get wonderful fans for bathroom exhaust where the fan is way up in the attic and there's no noise in the bathroom while the air is being sucked out of the room?

It's possible! Look at the following fan I installed in my own home:

bath exhaust fan

Here's a typical Fantech bath exhaust fan setup. The odd cone-shaped metal thing is the fan. The two boxes that have the round white covers are place up in the ceiling joists of the bathrooms. All you see are the sleek round white covers. They come with and without lights! I LOVED these in my last house. CLICK THE IMAGE TO BUY the SET NOW.

A Fast Example

Let's take a small kitchen for an example. Say the kitchen measures 10 feet by 12 feet with an 8-foot ceiling. Remember how to compute cubic feet in a room?

You multiply length times width times height:  10 feet X 12 feet X 8 feet

That computes to 960 cubic feet of air in the room.

The minimum requirements say that this air should be changed 15 times an hour.

You now need to multiply 960 X 15.

This means that a fan must have the ability to move 14,400 cubic feet of air in an hour.

Exhaust Fan Videos

Watch these helpful videos about exhaust fans and how to vent them out of roofs.

The following video shows you how to prevent roof leaks around bathroom exhaust vents.

Fans Are Rated Cubic Feet Per Minute Not Hour

Since there are 60 minutes in an hour, we must divide the 14,400 by 60 to get CFM (cubic feet per minute).

Divid 14,400 by 60 and you get 240 CFM. Nothing to it!

CLICK HERE to get FREE & FAST BIDS from local contractors that install ventilation fans.

Handy Table For Fan Values

The following table does lots of the math for you. All you need to do is calculate the square feet of a room and the table tells you the CFM for either a kitchen or bathroom.

Square feet is easy. Just multiply the length of the room by the width. The table assumes a common 8-foot ceiling height.

The room area is listed in square feet. All values below the listed room size represent the fan's minimum capacity shown in cubic feet per minute.

Room Area in Square Feet Fan Capacity in CFM for a KITCHEN Fan Capacity in CFM for a BATHROOM Fan Capacity in CFM for Other Rooms
35 N/A 40 N/A
45 N/A 50 N/A
55 N/A 60 45
65 N/A 70 55
75 150 80 60
85 170 90 70
95 190 100 80
100 200 110 80
125 250 140 100
150 300 160 120
175 350 N/A 140
200 400 N/A 160
225 450 N/A 180
250 500 N/A 200
310 620 N/A 250
375 750 N/A 300
435 870 N/A 350
500 1,000 N/A 400
560 1,120 N/A 450
625 1,250 N/A 500
685 1,370 N/A -

Table Values courtesy of Nutone, Inc.

CLICK HERE to get FREE & FAST BIDS from local contractors that install ventilation fans.

Column B98


11 Responses to Sizing an Exhaust Fan

  1. What size (CFM) remote kitchen exhaust fan should I use to properly vent a 4- burner gas cooktop? The distance from the exhaust above the stove to fan on the outside wall is 14 feet and there are 3 x 90 degree bends to get to the outer wall to vent.

    Does an induction cooktop require less CFM venting?


  2. kitchen is 8'x8' and 10 ft high.ie say 650 Ccu ft of air.air is to be changed 15 times an hour. therefore fan must move650x15 equuals to
    say10000times. on dividing by 60, we get160cfm
    pl advise will 150 cfm will be sufficent or shud igo higher cfm
    is dimension fan meaningfu like 6'/8'/10'/12' size or shape square / square/ is relevant
    Pl advise on urgent basi and oblige.
    Thank you

  3. Hi, I am working on a school project, how to design a heating and cooling for a commercial building of 60, 000 sq ft. Any commercial building will do, I am planning on design a system for a super market, I don't how to start this project, perhaps you can give me some ideas thanks.

  4. I want to design a machine , which needs exhaust fan . This machine should be placed in lawn or some open area to capture the air surrounding that machine. How will I know the capacity as amount of air is not known

  5. I have a 10 by 15 by 8 room that I would like to know how many CFM exhaust fan I will need. Will be used to clear cigar smoke


  6. Hi there I'm looking to put an exhaust fan in my garage above my poker table to clear cigarette smoke. My garage is 28'x28'x10' What size of fan would you recommend?

  7. I have cathedral ceilings in my home. The living room and dining room and kitchen are all cathedral. I would say about 900 sq ft. It gets really hot in the summer time and I'm thinking of putting an exhaust fan in one side of my peaks. I don't know right off hand what the cfm is but could you recommend something please? I'm at my wit's end at this point. I've burnt the blower motor in my ac unit and right now staying at a family members house until I can get it fixed. Thanks

  8. never understood this line of thinking. what would you rather hear? white noise from a fan or the sound of someone going number 2? i always choose the loudest fans out there

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