How much rock? I have a planter 12 foot long and 3 feet wide-ranging,...

I have a planter 12 foot long and 3 feet wide-ranging, how much river rock do i need to buy?
My prairie is awful, trickle come and adjectives...
Answers:    12 foot times 3 feet divided by six if you place it two inches adjectives. Two inches is 1/6 of a foot. You need six cubic foot of rock.
12 feet x 3 foot divided by four, if you go three inches gaping. (three inches is 1 fourth of a foot) You need 9 cubic foot of rock.
Help beside my lucky wicker?
If you only needed a one-dimensional answer, it would be "36 foot worth".

If you update your question beside how DEEP you want to fill it, afterwards maybe someone can do the multiplication for you.

Let's speak you want it to be 2 feet gaping; that would be 12 x 3 x 2 or 72 cubic feet. If you requirement to convert that to cubic yards, a cubic courtyard is 3 x 3 x 3 or 27 cubic feet, so 72 divided by 27 is 2.6 cubic yard. That's a lotta rock if you want it two feet wide!