Amount of Perlite for 36" Floor

[david s]

4 x 4 x 0.33= 5.28 cu ft
But as the stuff reduces in volume on mixing you need to add 20% so you’ll need 6.32 so better get at least 6.5.
5 parts Verm or perlite, 1part cement mix dry then add 3 parts water. Mix in a barrow not a mixer.

[Penngrove]

I'm working on the 36" forno bravo brick plan, and as I figure I need a perlite/cement layer on the base that is about 4'X4' and 4" thick, with a 5:1 portland cement mix. How many cubic feet of perlite/vermiculite do I need to order?