mook Johnson wrote:
I'm going to be pouring a 3 x 5 slab 8" thick. 2" below ground and 6" above.
It will need to support about 500lbs.

I'll be using quikrete for this. Do you think I'll need rebar for this
small slab?

I'm w/ Dan--why in the world use an 8" pour for such a small load?
What is the footprint of the object(s) to be mounted on the slab? If
the purpose of the thickness is to get the height above surrounding
grade, I'd recommend strongly considering accomplishing that by grade
adjustment and a resulting thinner slab. A standard nominal 4" slab
would easily support the load unless it is a very small footprint (and
in that case it would make a lot more sense to make local mounting
piers thicker rather than the whole slab).

As for the bar, I wouldn't worry about bar, but I'd probably use 6"
square mesh just for doing it (and because I've got a roll remnant
around ).

HTH, and better or more specific answers would undoubtedly be
forthcoming w/ some more info on what the objective actually is...

BTW, iirc (and I think I do) the 80-lb Quikrete sack is 2/3 cu-ft so
your proposal would be a roughly 3x5x(2/3) cu-ft * (3/2) bags/cu-ft --
15 bags. That's a lot of hand mixing considering the time element
since you presumably want a continuous pour.