how to measure fan cfm the cheap diy way?

I have a lot of old 80 mm fans lying around. I am using a micro grow box of 2' x 1' x 2.5' (h) .. therefore 5 cu ft.

I am planning to use 3 or 4 cfls of 23 watts..

I need to find out which fans have the higher cfm.

Also.suggestions needed for fan placement using carbon filters.. I dont have space for a cylindrical filter so will be using flat ones like in ACs.

The 80mm fans are all different and you need considerable information and calculation to determine CFM. Average about 30 cfm for an 80mm fan could be 80cfm for high quality high speed??? If you have them try 2 for exhaust at top with 2 equal openings at bottom for intake and air flow through box.

I have a few 100mm inline fans for heat transfer/bathroom extraction and they do 93cfm according to the specs so 30-80 sounds about right. mine isnt a great quality one either

Just look around it and see a model/number/smthn like that and search on the internet for specs.

PC fans are shit for anything other than cooling a PC. Get a cheap inline DUCT fan from home depot.