how to measure CFM in digital animometer
Depending on the tool you're using, it may or may not directly display the value you're after (cubic feet per minute). You'll have to check the documentation for the tool you're using, to determine if, and how to output the proper value.

If the anemometer only displays the air speed (fpm [feet per minute]), you can use that value, along with the area of the register, to calculate the volume of air.  The formula is as follows.

Q = A*V

Where:

Q = Air flow (cubic feet per minute [CFM])
A = Area of register (square feet [sq. ft.])
V = Air velocity  (feet per minute [fpm])

EXAMPLE:

With a 10" x 6" register, and an average air speed of 275 fpm.

Q = ((10" * 6") / 144) * 275 fpm
Q = (60 sq. in./ 144) * 275 fpm
Q = 0.4166666667 sq. ft. * 275 fpm
Q = 114.5833333333 CFM

NOTE:


When measuring the speed of the air coming from the register, take a few measurements from different positions in front of the register. Take the average of all the collected values, and use that in the formula.