I'm trying to convert 500 sq ft x 2" deep into square inches.

You are mixing 2 dimensions with 3.
500 square feet is 144 x 500 = 720,000 square inches.
2" deep would then turn it into a problem of cubic inches or feet....
I'm wondering if you have 2" tiles (since you asked in Home and Garden)? And you have 500 linear feet of them, or 500 square feet as you said?
Answer below what you are trying to do and we can help further.

I mean 72,000

1 square feet = 144 square inch

500 square feet = 72 000 square inch

https://www.google.com/search?client=ubuntu&channel=fs&q=square+feet+to+s qure+inch&ie=utf-8&oe=utf-8#hl=en&client=ubuntu&hs=jsP&channel=fs&sa=X&ei=Nr jTTvziPIfb0QHK-bQW&ved=0CBcQBSgA&q=500+square+feet+to+square+inch &spell=1&bav=on.2,or.r_gc.r_pw.r_cp. cf.osb&fp=69fb868bb4597fab&biw=1280&bih=762

I am filling a play area with rubber mulch. So we want to fill it 2 inches deep in a 500 sq ft area.

So that's cubic inches, not square.
144,000 cubic inches (2 x 72,000 at one inch deep)

So that's 83 cubic ft. now find how many cubic ft of mulch are in a bag or load and divide that into the 83. A 3 cubic ft. bag would be 28 bags.