CFM Calculator

What CFM counts, and its metric equivalents

CFM stands for cubic feet per minute, a volume flow rate: how much air passes a point in one minute. On its own it says nothing about speed, pressure or temperature. A 200 CFM fan moves 200 cubic feet of air every minute whatever shape the duct has, though the speed inside the duct changes as the duct gets smaller.

The bridge to metric units comes from the National Institute of Standards and Technology. NIST Special Publication 811 (2008), Appendix B.8, lists one cubic foot per minute as 0.0004719474 cubic meters per second, printed there as 4.719474 x 10 to the minus 4, which is 0.471947 liters per second and 1.699 cubic meters per hour. The same table gives one cubic foot as 0.02831685 cubic meters, and dividing by 60 lands on the same factor.

Those two numbers cover most of the arithmetic in this page. 100 CFM is 169.9 cubic meters per hour and 47.2 liters per second. Going the other way, one cubic meter per hour is 0.5886 CFM.

CFM from room volume and air changes

A room does not care about CFM directly. It cares how often its air is replaced, and air changes per hour counts full volume swaps. The Department of Energy ventilation cooling guide writes the relation as ACH = 60 x CFM / volume, which rearranges to CFM = volume x ACH / 60.

Take a bedroom 12 feet by 14 feet with an 8 foot ceiling. The volume is 12 x 14 x 8 = 1,344 cubic feet. At 6 air changes per hour, 1,344 x 6 = 8,064 cubic feet per hour, and 8,064 / 60 = 134.4 CFM. In metric that is 134.4 x 1.699 = 228.3 cubic meters per hour, or 63.4 liters per second.

A second example comes from the same guide. A bathroom of 672 cubic feet at 8 air changes per hour needs 672 x 8 = 5,376 cubic feet per hour, and 5,376 / 60 = 89.6 CFM, so a 90 CFM fan is the smallest standard size that clears the bar.

Air change targets differ by room because the sources differ: moisture in a bathroom, grease in a kitchen, people and carbon dioxide in a bedroom.

Check the first row: 1,000 x 8 = 8,000, and 8,000 / 60 = 133.3 CFM. The whole-home row is small because 0.35 air changes spread over a house is a continuous trickle, not a fan you switch on.

CFM from duct size and velocity

Inside a duct the relation is short: CFM = velocity in feet per minute x cross-section area in square feet. The area of a round duct is pi times the radius squared, with the radius in feet, so an 8 inch duct has a radius of 4 inches, which is 0.3333 feet.

Work it through. The area is 3.1416 x 0.3333 x 0.3333 = 0.3491 square feet. At 700 feet per minute the flow is 0.3491 x 700 = 244.3 CFM. Run the same duct at 400 feet per minute and the flow drops to 139.6 CFM, because flow tracks velocity in a straight line.

Rectangular ducts use width times height. A 24 inch by 12 inch duct is 288 square inches, which is 2 square feet, so 500 feet per minute through it moves 1,000 CFM.

Velocity is what an anemometer reads at a grille and area is what a tape measure gives; the product is the airflow. A small duct at high velocity and a large duct at low velocity can carry the same CFM, but the small one will be louder and will lose more pressure along its length.

CFM from a sensible heat load

Heating and cooling both move heat with air. The standard airside relation is Q = 1.08 x CFM x delta T, where Q is sensible heat in BTU per hour and delta T is the temperature change across the coil in Fahrenheit degrees. The 1.08 is not a magic number: it is air density 0.075 pounds per cubic foot, times specific heat 0.24 BTU per pound per degree, times 60 minutes per hour, and 0.075 x 0.24 x 60 = 1.08.

Solve for airflow and you get CFM = Q / (1.08 x delta T). A system moving 24,000 BTU per hour with a 20 degree difference needs 24,000 / (1.08 x 20) = 24,000 / 21.6 = 1,111.1 CFM.

The same load with a 15 degree difference needs 24,000 / (1.08 x 15) = 24,000 / 16.2 = 1,481.5 CFM. A smaller temperature difference always costs more airflow for the same heat, which is why a system with a dirty coil and a low delta T feels weak at the registers.

At altitude the 1.08 falls, because air at 5,000 feet weighs less than air at sea level. The relation still holds, but the multiplier has to be rebuilt from the local density: density x 0.24 x 60. Using 1.08 at altitude undersizes the airflow for the load.

What the ventilation standards expect

The residential numbers come from one standard. ANSI/ASHRAE Standard 62.2-2019, Tables 5-1 and 5-2, sets local exhaust at 50 CFM for a bathroom on demand control, or 20 CFM when the fan runs continuously. A vented kitchen range hood needs 100 CFM on demand, and a kitchen exhaust that is not a vented hood needs 300 CFM or 5 air changes of the kitchen volume.

Whole-dwelling ventilation uses a different equation: Q = 0.03 x floor area + 7.5 x (bedrooms + 1), with floor area in square feet and Q in CFM. A 2,000 square foot house with three bedrooms needs 0.03 x 2,000 = 60, plus 7.5 x 4 = 30, for 90 CFM of continuous ventilation.

The California Energy Commission applies the same equation in its residential compliance manual. A 1,800 square foot townhouse with three bedrooms works out at 0.03 x 1,800 = 54, plus 7.5 x 4 = 30, for 84 CFM. The commission then subtracts an infiltration credit from a blower door test, which is how a real permit number ends up lower than the raw equation.

Edition matters. Older 62.2 editions used 0.01 per square foot, so a 1,500 square foot three bedroom house came out at 15 + 30 = 45 CFM under the 2010 rules and at 45 + 30 = 75 CFM under the 2013 rules. Match the edition your jurisdiction adopted.

Rated CFM is not delivered CFM

A fan's rating is measured at a stated static pressure, usually 0.1 inch of water column. Ducts, filters, louvers and terminations all push back harder than that, and a fan moves less air as the pressure rises. The Department of Energy's ventilation guide gives the size of the drop: a fan rated 50 CFM at 0.1 inch delivers about 23 to 31 CFM at 0.25 inch, and an 80 CFM fan delivers about 48 to 57 CFM at the same pressure.

That is a loss of 38 to 54 percent on the smaller fan, before a single elbow is counted. A long flex run, a roof cap and a backdraft damper can add as much pressure as the duct itself, and each one trims the number further.

The practical rule: compare the fan's rating at the pressure your install will see, not the big number on the box. Manufacturers publish a table or a curve for exactly this reason, and the 0.25 inch column is closer to a real bathroom install than the 0.1 inch headline.

Filter loading works the same way: a clogged filter raises pressure, flow falls, and the system runs longer to move the same heat.

Seven mistakes that change the answer

First, feeding feet per minute into a CFM formula. Velocity and flow are different quantities, and the conversion needs an area. Second, using floor area where volume belongs: air changes act on cubic feet, so a 10 foot ceiling needs 25 percent more airflow than an 8 foot ceiling over the same floor.

Third, applying one air change target to every room. A kitchen at 6 air changes will not clear grease, and a closet at 15 is wasted fan noise. Fourth, forgetting that the delivered flow is lower than the rating, which turns a correctly sized fan into an undersized one after installation.

Fifth, converting units in the middle of a calculation. Pick one system, convert the inputs first, and convert the answer at the end if you need it. Sixth, treating a design number as a measurement. A calculated 134.4 CFM is a target, and only a flow hood or a calibrated anemometer tells you what the grille delivers.

Seventh, ignoring make-up air. It has to come from somewhere: a tight house with a 200 CFM exhaust and no inlet path pulls against itself, and the flow at the grille drops even though the motor runs at full speed.

Sizing one bathroom fan, start to finish

Put the pieces together on a small bathroom, 10 feet by 10 feet with an 8 foot ceiling. The volume is 10 x 10 x 8 = 800 cubic feet. At 8 air changes per hour, 800 x 8 = 6,400 cubic feet per hour, and 6,400 / 60 = 106.7 CFM.

The ASHRAE 62.2 floor for a bathroom is 50 CFM on demand, so the air change target is the stricter of the two here. Now look at fan data sheets rather than the box. A 110 CFM fan rated at 0.1 inch may deliver only 70 CFM at 0.25 inch, which misses the 106.7 target. A 150 CFM fan rated at 0.25 inch, or a 110 CFM fan on a short straight duct, is the honest choice.

Then check the duct. A 4 inch duct at 106.7 CFM runs at 1,222 feet per minute, since 106.7 / 0.0873 = 1,222, and that is loud. A 6 inch duct brings the same flow down to 543 feet per minute, which is why the installation instructions ask for 6 inch duct on most bathroom fans even when the collar is 4 inch.

Finally, run the numbers in metric if the fan data sheet is metric. 106.7 CFM is 181.2 cubic meters per hour, and a fan listed at 180 m3/h is the same size.