The factors that affect concrete slab thickness are climate, soil conditions and intended use. The slab thickness numbers in this article are guidelines from the IRC and Local cities, consult with your local building department and/or a structural engineer for your local recommended concrete slab thickness.
Garage slabs: 3 1/2 inches thick
Home Interior Slabs: 3 1/2 inches thick, (The 2009 International Residential Code 506.1, says 3 1/2" thick with #6 wire mesh in the top 1/3 of the concrete slab, 506.2.4 and a 6 mil plastic barrier under the slab, 506.2.3)
Driveways: 4 inches, some areas require 5 inches.
Porch Caps: Spanning over a cold storage area, 6-8 inches thick with #4 rebar at 12" on center each way. This should be verified with a structural engineer because every porch span is different.
Porch Caps: On compacted soil, 4-5 inches
Sidewalks: 5 inches; this is required by many cities. Check with your local building department because even though you are paying for the concrete the city usually owns it.
Driveway Approaches: 6-8 inches, Check with your local building department.
Concrete poured on the exterior: driveways, foundations, sidewalks etc., is usually 6 bag mix . = 6000 psi.
Concrete poured on the interior: garages, houses, basement floors etc., is usually 5 bag mix.= 5000 psi.