The easiest thing to do would be to visually compare the 8.5kg bag of vermiculite to something of a similar size with cubic feet on it. This is what the book recommends - I did it with compost and it worked out great.
~plantgirl of Plantgirl's Square Foot Garden Blog

I went the wrong way with the math. Sorry.......It's a little more than 4 cu. ft.

I shorted myself with the vermiculite. I bought 6 big bags for four 4' by 8' beds. Used a 18 gallon bucket to measure everything. In the end, my bed were low.

Since the vermiculite was an hour away, I decided to just add peat and compost to bring the soil level to the top of my beds. Everything is thriving.

Don't stress too much about the mix. If you can't find vermiculite, use perlite.

So if I reading this right, 25 pounds are MORE than 4 cube feet of vermiculite.

example:https://www.tractorsupply.com/tsc/product/sweet-pdz-stall-refresher-25-lb-5065990?cm_mmc=feed--GoogleShopping--Product-_-5065990&gclid=Cj0KCQjwgtWDBhDZARIsADEKwgPZ-1a1V7OmY-D5XrU2Bx8ahlNnjnvFxhex Stall refresh

I measured 4 cups vermiculite which weighed 162 grams, or 162 grams/quart or .36 pounds or 5.76 oz/quart