how do I deduct for bracing and divders

Like the topic say, how do I do this is there a formula?

Thanks in advance.

L * W * H = cu.in.

Here is a demo, you have a 3/4" divider in your box that is 12" high and 16" wide (internal dimensions)

.75 * 12 * 16 = 144 cubic inces or .083 cubic feet

ok one more question please

how do I turn 144 cubic inces to .083 cubic feet

L * W * H = cubic inches

12 * 12 * 12 = 1728 cubic inches = 1 cubic foot

.75 * 12 * 16 = 144 cubic inches = .083 cubic feet

144 / 1728 = .083 cubic feet