HOW COMPUTER CASE FANS ARE MEASURED
Standard PC case fans are square in shape.
They are measured from edge-to-edge. I.E. an 80mm fan will measure exactly 80mm from end to end.
2) FAN SPEED
Fan speed is measured in rounds per minute (RPM).
How fast the blades spin per minute. Not really a useful spec.
3) AIR FLOW
Measured in cubic feet per minute (CFM).
A higher number means more air getting moved.
PC fans generally move about 20 to 70 CFM.
Generally measured in decibels relative to air (dBA).
25dBA is a quiet library, and PC fans are generally around 20 to 30 dBA.
Standard PC case fans run on 12V.
.But there are also USB 5~12V fans.