HVAC CFM Calculator

Type in the room and the air changes per hour the design asks for, and you get the airflow that room needs in CFM (cubic feet of air per minute). It sizes air only. It is not a heat-loss calculation, it does not size equipment, and it is not a code check.

Measurement unit

The unit you measured the room in. All three dimensions below use it.

Room length

Inside wall to inside wall, the long way.

Room width

Inside wall to inside wall, the short way.

Ceiling height

Floor to ceiling. On a sloped or vaulted ceiling use the average height, not the low side.

Air changes per hour this room needs

One air change means all the air in the room has been replaced once, so this is how many times an hour that has to happen. Take the figure from the mechanical drawings, the design spec or your local code. Nothing here fills it in. A bathroom, a bedroom and a spray booth are three different numbers, and they change from place to place.

Result

Fill in a few more boxes and the answer will show here.

Opens your print dialog. Choose Save as PDF. Your figures stay on this device.

Loading the calculator.

This is a general measurement calculator. Confirm dimensions, product coverage and job requirements before ordering or installing materials.

How the airflow a room needs is worked out

Airflow starts as a volume, not an area. A room needs a certain amount of air moved through it every hour, and that amount depends on how much air the room holds: length by width by ceiling height. Multiply that volume by the air changes per hour the design asks for, divide by sixty, and you have the cubic feet per minute the room needs.

The air-changes figure is the one number this tool will not supply. A bedroom, a bathroom, a welding shop and a spray booth are four different targets, and they move with the jurisdiction. Take yours from the mechanical drawings, the design spec or your local code and type it in. Everything else here is geometry you can do with a tape.

Read the CFM as a requirement, not a verdict. If you also enter what the equipment is rated for, the tool shows the air changes that rating actually delivers, so you can see which way the gap runs and by how much before anything is ordered.

The common mistake is sizing from floor area and forgetting the ceiling. Twelve by ten under an eight-foot ceiling holds 960 cubic feet; the same floor under a fourteen-foot vault holds 1,680, and the airflow it needs rises with it. Measure the height every time, and use the average on a sloped ceiling rather than the low side. Take the measurement once and keep it somewhere it will still be there in two years. Zeus holds job records per address, so the room you measured today is the room you read back on the next call.

Built for HVAC work: see the HVAC job app.

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