Bricks, Masonry, Asphalt and Concrete - Sand/gravel volume for patio?

I'm totally lost here, please help.

I am planning on laying bricks (8x16x2) for a patio in two areas. Have the first one, 5.8x5.25 dug out to 8 inches.

I poured four .5 cubic feet bags of pea gravel in, and it seems to take up 2 inches. So that should mean another 4 bags of pea gravel and then four bags of sand/paver base, right?

Yet the calculations I've tried say I will need a lot more.

The calculations I've seen are in cubic yards but they sell the bags in cubic feet, so that was my first problem :-(

There are 27 cubic feet in 1 cubic yard. Assuming you meant 5.8 feet x 5.25 feet:

Four 0.5cf bags of pea gravel should yield:
(4 x 0.5cf)/(5.8ft x 5.25ft) x 12in/ft = 0.788" or a little over 3/4"

For 4" of pea gravel you need:
(4"/12 in/ft) x 5.8ft x 5.25 ft = 10.15 cubic feet
10.15cf/0.5cf/bag = 20.3 or 21 bags

For 2" of sand:
[(2"/12 in /ft) x 5.8ft x 5.25 ft]/0.5 cf/bag = 10.15 or 11 bags

Does this agree with your numbers?

Yes that sounds about right. The project has now been scaled back ;-)

I backfilled the existing hole to @4 inches. Will go with existing rock and sand, maybe adding more sand (paver base).

Then for the 26 sq. foot area we will buy 4 more bags of stone and 5 of sand to have some left over.

This will be a light-use patio. The first area adjuts a porch, not the foundation itself.

I am in Chicago, so there's a freeze/thaw cycle. I'm using a plastic edger on the bricks. One area will probably have substantial root encroachment from a lilac bush and an alleged dwarf apple (way overgrown).

Anything I need to watch out for with not having 4 inches of drainage? Our soil is hard-packed black dirt.