Air Changes per Hour Calculator

The system is in and running. This works out how often the air in the room is actually replaced. Enter the room and the airflow reaching it. It reads a measurement; it is not a ventilation design 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.

Airflow the system delivers

What actually reaches this room. A reading taken at the grille beats a nameplate figure every time. The plate is what the fan moves on a bench, not what survives a long flex run and a loaded filter.

Airflow is in

Read it off the instrument or the nameplate. The two differ by about a factor of two, so check before you type.

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.

Reading an air-change rate off a room that is already running

This is the check at the other end of the job. The equipment is in, the fan is running, and the question is whether the air in the room is genuinely being replaced often enough. Take the airflow the room is getting, multiply it out to an hour, and divide by the volume of the room. That is air changes per hour.

Two inputs decide the answer and one of them is usually guessed. The room volume is a tape measure and a minute of your time. The airflow is not. The figure on the nameplate is what the fan moves on a test bench, not what survives thirty feet of kinked flex, a dirty filter and a damper somebody closed two winters ago. Measure at the grille where you can, and note which figure you used.

Minutes per air change is the same answer said in a way a client understands. Six minutes means the air in that room is replaced ten times an hour. It is also the number that makes a shortfall obvious without any arithmetic.

Enter the target you were given and the tool shows the airflow that target needs and how far off you are. Leave it blank and it stays quiet, because there is no universally correct rate and inventing one would be worse than saying nothing. Watch the units: CFM and liters per second turn up on the same job and differ by roughly a factor of two. Zeus keeps photos and notes on the job record for that address, so a reading taken today is still findable when someone asks about it next season.

Built for HVAC work: see the HVAC job app.

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