CFM Calculator
Calculate required ventilation CFM from room dimensions, area, or volume and air changes per hour. Great for fans, ducts, and HVAC sizing.
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What the CFM calculator does
Sizing a fan or a duct comes down to one number: the cubic feet of air per minute, or CFM, a space needs to stay properly ventilated. This works it out. You give it the room size and how often the air should be replaced, and it returns the CFM the ventilation has to move.
It is the starting figure for choosing an exhaust fan, sizing ductwork, or planning HVAC. Below is how it works and the input that drives it.
How to use it
- Choose how to describe the room: by its dimensions, by its floor area and ceiling height, or by its volume directly.
- Enter those figures, each with its unit.
- Enter the air changes per hour for the room, then press Calculate, or Reset to clear it.
How the CFM is worked out
The calculator first finds the room's volume, then works out how much air per minute it takes to replace that volume as often as you asked:
CFM = (room volume in cubic feet × air changes per hour) ÷ 60
The air changes per hour say how many times the room's full volume of air is swapped out each hour, so multiplying gives the air per hour, and dividing by 60 turns that into air per minute. However you describe the room, it comes down to its volume and that rate.
What air changes per hour means
Air changes per hour, or ACH, is the heart of the calculation, so it is worth being clear on. It is the number of times in an hour that a volume of air equal to the whole room is brought in and the old air pushed out. Six air changes an hour means the room's worth of air is replaced six times, once every ten minutes.
The reason it drives everything is that a stuffy room and a fresh one differ by how fast their air turns over, not by their size alone. A bigger room at the same rate needs more airflow, and a busier or steamier room needs a higher rate, so the right CFM depends on both the volume and how hard the room is working.
Typical air change rates
The rate to enter depends on the room, and there are accepted ranges to work from. Ordinary living spaces, bedrooms, living rooms, and the like, generally want somewhere around five to seven changes an hour. Offices sit a little lower, around four to six, while busy or moisture-heavy rooms need more.
Kitchens and bathrooms are the demanding ones, since they make heat, steam, and smells that need clearing quickly, so they call for higher rates than a bedroom. The wetter, busier, or more polluted the air in a room, the more changes an hour it wants, so lean to the higher end for those and the lower end for quiet, dry spaces. Picking a rate that suits the room is what makes the CFM figure meaningful.
A worked example: a 15 by 12 room
Say the room is 15 feet by 12 feet with an 8 foot ceiling, and you want 6 air changes an hour.
The volume is 15 × 12 × 8 = 1,440 cubic feet. So the CFM is (1,440 × 6) ÷ 60 = 144 CFM.
That is the airflow to look for when choosing a fan for the room. Raise the rate to clear a steamier space faster and the CFM rises with it, in step.
Questions people ask
How do I calculate CFM for a room?
Multiply the room's volume in cubic feet by the air changes per hour, then divide by 60. A 1,440 cubic foot room at 6 changes an hour needs 144 CFM.
What is a good number of air changes per hour?
Around five to seven for ordinary living spaces, four to six for offices, and more for kitchens and bathrooms, which need their air cleared faster.
How does this help me size a fan?
The CFM is the airflow the fan needs to provide. Choose a fan rated at or above the figure for your room and rate.
Why divide by 60?
Because air changes are quoted per hour but fans are rated per minute, so dividing by the 60 minutes in an hour converts the rate.
References
A quick note on the numbers. The CFM formula, room volume times air changes per hour divided by 60, is the standard ventilation calculation, and the recommended air change rates for different room types follow the ventilation guidance of ASHRAE, the body that sets indoor air quality standards. The unit conversions follow the US National Institute of Standards and Technology guide.
- ASHRAE, Standards 62.1 and 62.2 (ventilation and acceptable indoor air quality). https://www.ashrae.org
- National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
Mahendra Thapaliya is a graduate student in Structural Engineering at the University of Bologna, with research interests in structural systems, FEM, earthquake engineering, and numerical modeling. At Eon Tools, he reviews construction tools.