Measuring the CFM of a fan

Maybe OT, but not as much as many.
I have a fairly large centrifugal blower (squirrel cage) & I'd like to know its CFM. It has a well defined opening, so knowing the velocity of
the air is what I need.
I can think of a couple of approaches. The first that comes to mind is placing a surface in the air stream & measuring the force exerted on it. I know that there is a relationship between the air velocity, the surface area and the force (drag), and I could find a formula on the web, but I worry that a little bit of knowledge could be a dangerous thing. I.e., I suspect that this is a complicated subject and an understanding should accompany formula use. But if someone here has the understanding & could give me the right formula, that would be great <G>.
Another approach is to measure the pressure in the stream & derive velocity from that. I think that this would be subject to even more complications than using force, but maybe not. Anyone know? If it's a reasonable approach, what about a formula?
Or, I could get/make a _really big_ poly bag & time how long it takes to inflate. Much too crude for my taste.
Advice anyone?
Thanks, Bob
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