I have been told 75% of the grille size is the free area, is that what you mean by Ak?

 

No Ak is the effective area and is used to calculate cfm from a velocity reading. Free area is not the same as effective area since the air undergoes compression and deflection as it passes through the grills so multiplying the velocity by the free area won't give you an accurate result.

My understanding is that Ak is measured though lab testing. Since we don't always have this info I wondered how people measure airflow through a grill if they don't have the specs or access to a balometer.

 

I am pretty sure the 75% is the (rule of thumb) that corrects for things like blade widths and angle, dirt on the vanes, etc. - the free area would be higher like around 90%, your right you don't want to use the higher number in when calculating cfm.

 

Okay maybe I misunderstood your first post since you mentioned free area - but I think your suggestion is good - 75% sounds like a good rule of thumb for effective area and 90% for free area.