Depends on your soil, and also if you are planning to put a cmall chain wall in it or not. Where i live i poured a 6 inch slab 14 x 24 3 years ago. No change wall, all 1/2 and 3/8 rebar on 12 inch centers, suppose to be 4000lbs mix with activator in it. I have sand ground but this area had been packed hard from being a previous dirt parking area. I have had mine and many full sized pickups with loads over it, bad drought last year and lots of rain now and the year before. One small crack on the off lip as it tapers down to 3 1/2 inches there and only aboput 9 inches long is the crack. This has No chain wall on it, wish i had put one at least where i drive on and off of this slab. If you are ever going to put any posts or walls on it put a chain wall!! At least a post base where the posts will be.