CFM Calculator

CFM = length (ft) × width (ft) × ceiling height (ft) × air changes per hour ÷ 60 minutes per hour

L

Room length in feet — the page converts whatever unit you pick into feet first

W

Room width in feet

H

Floor to ceiling height in feet: the third side of the room, which is why this page asks for three lengths rather than a volume

ACH

Air changes per hour — how many times the whole room volume is replaced in an hour. The reference table gives the usual figure for each room type

÷ 60

Minutes in an hour, the step that turns an hourly volume into a per minute rate

1.699

Cubic metres per hour in one CFM, so the metric output is the same airflow written the other way

Use it when you are choosing a fan, a hood, a duct or a fresh air unit, and you need a number to match against a rating plate. The room types that get asked about most are kitchens and bathrooms, because those are where the fan is bought separately: a kitchen hood is rated in CFM, and a bathroom extract fan is too. It is also the page for checking whether an existing system is in the right range — measure the room, count the air changes it is supposed to achieve, and see whether the fan on the ceiling can deliver it. If the question is about the volume of the room rather than its airflow, the cubic feet calculator returns the same volume without the air changes, and the tank volume calculator does the same job for a cylinder.

1. A 12 × 15 ft bedroom with an 8 ft ceiling, six air changes an hour — the page's default

   1. Volume: 12 × 15 × 8 = 1,440 ft³
   2. Air moved per hour: 1,440 × 6 = 8,640 ft³ per hour
   3. Per minute: 8,640 ÷ 60 = 144 cfm
   4. In cubic metres: 1,440 × 0.0283168 = 40.78 m³
   5. Metric airflow: 40.78 × 6 = 245 m³ per hour

   144 cfm is the flow a supply register in this room should be able to deliver, and it is the sort of number that gets written on a duct schedule. The 245 cubic metres per hour beside it is the same airflow, not a second answer: one cfm is exactly 1.699 cubic metres per hour, so the two lines are one quantity in two units, and any difference between them is only the rounding to a whole number. If you are comparing a fan's rating plate against this, check which unit the plate uses before the numbers look like they disagree.

2. A 10 × 12 ft kitchen with an 8 ft ceiling, fifteen air changes

   1. Volume: 10 × 12 × 8 = 960 ft³
   2. Air moved per hour: 960 × 15 = 14,400 ft³ per hour
   3. Per minute: 14,400 ÷ 60 = 240 cfm
   4. In cubic metres: 960 × 0.0283168 = 27.18 m³
   5. Metric airflow: 27.18 × 15 = 408 m³ per hour

   240 cfm is a mid-range kitchen hood, which is the right order of magnitude — most domestic hoods sit between 150 and 600 cfm, and the fifteen air changes is how the sizing rule lands in that band. Note the direction: this is exhaust air leaving the room, not conditioned air arriving. Cooker hoods lose effectiveness fast when the duct is long, narrow or full of elbows, so a hood rated at 240 cfm on a short straight run does not deliver 240 cfm through four metres of flexible duct, and none of that is on this page.

3. A 4 × 4 ft basement plant room with a 6 ft ceiling, six air changes

   1. Volume: 4 × 4 × 6 = 96 ft³
   2. Air moved per hour: 96 × 6 = 576 ft³ per hour
   3. Per minute: 576 ÷ 60 = 10 cfm
   4. In cubic metres: 96 × 0.0283168 = 2.72 m³
   5. Metric airflow: 2.72 × 6 = 16 m³ per hour

   Ten cfm is tiny — less than a bathroom fan — and that is the honest answer for a space this small, which is worth seeing: air changes per hour is a rate, so a cupboard needs very little air and a hall needs a lot. The practical consequence is that the smallest fans on the market are already far bigger than this room requires, so the choice becomes about noise and about whether the space needs ventilation at all rather than about matching the number.

The air change figures are engineering rules of thumb rather than requirements from a code or a standard, and the number you should actually build to can differ a long way from them. Real ventilation design starts from the people in the room, the appliances in it and the contaminants being produced — a fresh air rate per person, a capture velocity at a hood, or a dilution rate for a particular gas — and those produce different numbers for the same room. The table also mixes two directions of airflow, supply for the habitable rooms and exhaust for the wet rooms, so a row is only a target for the kind of fan that room actually has. The page knows nothing about the system behind the fan: duct length, bends, filters and the pressure they add all reduce the air a given fan moves, and a fan rated at the figure returned here may deliver much less once it is installed. It also assumes the room is closed and that the air leaving has somewhere to come from — a fan that extracts air from a sealed room moves almost nothing. Finally, this is a flow rate, not a specification for a fan: ratings on a plate are usually given at a stated static pressure, and comparing them needs that figure as well.

How many CFM do I need for a bathroom fan?

The trade rule is about 1 cfm per square foot of floor area, with a floor of 50 cfm for the smallest rooms, so a 5 × 8 ft bathroom wants 40 by area and 50 in practice. The table above gets there from the other direction: ten air changes an hour in a room with an 8 ft ceiling works out at 1.33 cfm per square foot, which is 53 cfm for the same bathroom. The two rules agree closely enough that either is a fine starting point, and the 50 cfm minimum is what usually decides it. The important thing is that the fan is ducted to outside rather than into a loft.

How do I work out air changes per hour?

Divide the airflow by the room volume and multiply by 60: a 100 cfm fan in a 1,000 cubic foot room gives 100 ÷ 1,000 × 60 = 6 air changes an hour. This page does it the other way round, from air changes to airflow, which is the direction you need when you are choosing the fan rather than checking one. Both directions use only the volume, so getting the volume right — three measured lengths rather than an estimate from the floor plan — is the whole of the accuracy.

Do the CFM and the cubic metres per hour agree?

Yes, they are the same airflow written two ways: one cfm is 1.699 cubic metres per hour, exactly. 144 cfm and 245 cubic metres per hour in the first example differ by under a fifth of a percent, which is only the rounding to a whole number on both lines. This is unlike the paint calculator, where gallons and litres are rounded up to their own package sizes and so genuinely do not match — there is no pack size for air, so nothing is rounded here except the display.

Is air changes per hour the right way to size ventilation?

It is the quick way rather than the correct one. Building regulations and design guides mostly specify ventilation by activity instead — a fresh air rate per person, a rate per square metre of floor, or a capture velocity for a cooker hood — and those give different answers for the same room. Air changes per hour survives because it needs only a tape measure, which is why it is everywhere in practice and in the table above. Where the ventilation matters, work from the occupancy or the appliance and use this page as a sanity check rather than as the design.

Why does a kitchen need fifteen air changes and a bedroom only six?

Because the kitchen is producing something that has to be removed rather than because the room is bigger or the air is worse. Cooking puts moisture, grease and combustion products into the air, and a hood removes them at the source, so the target is set by what the hood has to capture. A bedroom's air changes are about delivering fresh air and removing carbon dioxide and moisture from people, which is a much smaller quantity. That is also why the two rows point in opposite directions: the bedroom figure is supply air and the kitchen figure is exhaust.

What about the airflow in cubic metres per hour?

Multiply cfm by 1.699, or work straight from the volume: 40.78 cubic metres multiplied by six air changes is 245 cubic metres per hour in the first example. Rating plates outside North America are usually in cubic metres per hour, and so are most fresh air units, which is why both lines are on the page rather than one. If a Chinese fresh air specification gives a figure per person instead — around 30 cubic metres an hour each — multiply that by the number of people and compare it against the airflow here; the two approaches are parallel ways of reaching a number, not the same calculation.

• Approximate Conversions from U.S. Customary Measures to Metric — the chart behind the cubic foot and cubic metre figures used here — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the metre and the cubic metre, the units the volume and the metric airflow are reported in — Bureau International des Poids et Mesures (BIPM)