# How do I calculate the CFM of air flowing through a tube?

How do I calculate the CFM of air flowing through a tube?

Many things contribute to the cfm flowing through a tube. The main numbers that contribute are the size of the tube and the pressure of the air.

For instance, a 1/4″ hole at 20 psi will flow 31.4 cfm of air, while at 100psi it will flow 104 cfm.

You should be able to find charts on the internet to help you with this.

This depends on whether you are “designing” a system from scratch, or whether you are trying to measure a flowrate in an existing system.
If you are just trying to find the flowrate of an existing system, you will need a manometer and a pitot tube. The pitot tube goes into the duct through a hole in the duct and connect to the manometer. There will be a chart that will correlate the “inches of water” pressure reading on the manometer to a velocity (feet per second) inside the duct. Take this reading and multiply it by the cross sectional area of the duct, and you get cubc feet per second. Multiply by 60 seconds to get CFM.
If you are designing a system, it depends on what variables that you have to work within. Generally, there is a desired velocity of the air to keep particulates suspended or to avoid “whistling”, or to supply some process requirement or some other requirement. Frictional losses are calculated from the diameter and length of the duct as well as the pressure of the