I like jardin's suggestion of approximating by halves and quarters, so follow him before taking off on my wild goose chase; I used to teach math....
and, yet again, I will inveigh against the "customary system" of measurement and say that this entire situation would be so trivial an 8-year-old could figure it out in the metric system.
OK. Breathing normally again.
1 cubic foot = 7.48 gallons (but we'll call it 7.5 gallons). Therefore,
4 cubic feet = 30 gallons, or six 5-gallon buckets full of vermiculite.
Since you only want half of what's available, you only need three 5-gallon buckets full of vermiculite (pretty close to jardin's recommendation and your own arithmetic!
Be sure to work with the vermiculite in a wind-free area
; the stuff is very light and fly-away. If you can't find a wind-free area, or if the wind comes up while you're working, tie a handkerchief or scarf around your nose and mouth or wear a surgical mask (a la serious allergies) while working with it. It can make a lot of dust.
What we did was to mix the compost, potting soil, and vermiculite together on a tarp. Rolling the stuff by using the edges of the tarp was quite easy, and everything got thoroughly mixed together. Then we used--yes--our
5-gallon buckets to transfer the mix into Bed #1--the cement block bed. By the end of the process, we were using trowels to fill the buckets, but the whole thing was fairly quick once we got the "tarp-mixing" moving along.
BTW, back-calculating from your total of 6 yd3
of materials (vermiculite + peat + potting soil), it looks as if you plan to make a 4'x4'x6-inch
container; correct? My experience in Spring and Summer 2008 with the 6-inch depth was pitiful. My bok choy, chard (!), peas, zukes, tomatoes, leeks, carrots (oh, my, the poor carrots), beets, and parsnips were uniformly un-impressed with 6 inches of "improved" planting medium. Only the lettuce managed to grow to a normal size. Since the newspaper to block off weed development was new, the veggie roots had trouble penetrating it that year at the tender length of 6 inches.
As we "harvested" the very small collection of plants, it became clear that a greater depth was needed for basic root development. I measured the cement blocks again, and confirmed that they could hold 10 inches' worth of planting medium. I added a boatload of compost--couldn't afford vermiculite that fall, and haven't purchased peat again--and mixed it in as plants came out, building up the depth over a few weeks.
The fall/winter plants were much happier: more chard, kale, fava beans, cabbage, broccoli romanesco, and similar veggies. Thus my recommendation for 10 inches minimum.
Even the carrots were happier with 10 inches than 6, but now I plant them in the 12-inch box we have. I have a grand total of 92 square feet in veggies, all of it in raised boxes. There are a few containers (fewer after the house-painting of the last while) with herbs and other veggies; those containers need revamping for the Spring....