Uhm, at least 40? Well, here are the apparent sizes. With this you can say a range, but not really an average without the most common one... Let's just make a formula to chuck it into... let x = horizontal length in mm of filter let y = vertical length in mm of filter 12mm=1.076*10-5 foot (xy4.51012)/1.07610-5 The largest at apparently 20 * 8 mm (2084.51012)/1.07610-5 and (1454.51012)/1.07610-5 So about 6.69 billion square feet to 2.9billion. This can vary in square feet though... since it can be squished down to different sizes and so on... but this is if you lay them side by side without changing them A volume one shouldn't be that much harder to do though (Pir2x4.51012)/3.531*10-8 Substitute in the values and it'll be between 12.8-3.5mill cubic feet I imagine it'd fall far more towards the bigger side of these ranges because as far as I know the larger ones are the most common TL;DR 3.5mill-12.8mill cubic feet or 2.9bill-6.7bill square feet.
