Four inches carries light storage and vehicles. Five inches is the general-purpose shop standard. Six inches, or thickened sections, go where lifts, heavy equipment, or loaded trailers live, with edges and column footings sized to the building's drawings. Concrete at 3,500 psi or better is the common spec.
More detail
The right shop slab answers two questions: what will the floor carry, and what will the building bolted to it demand. For the floor, 4 inches handles mowers, four-wheelers, and light storage; 5 inches is the workhorse spec for a general shop with pickups and a welder; 6 inches (or 5 with thickened sections) belongs under two-post lifts, tractors and implements, gooseneck trailers, and any plan that includes a future lathe or mill. Concrete at 3,500 to 4,000 psi with #3 or #4 bar on proper chairs is the standard companion spec, and thickness only works over a compacted, moisture-conditioned pad, which is the line item that decides everything on clay. For the building, a pre-engineered metal building publishes column reactions and anchor-bolt layouts, and the slab thickens into footings under those columns per the drawings; matching that template within fractions of an inch at pour time is what makes erection day boring, which is the goal. Two additions worth deciding before the pour rather than after: conduit stubs for a future mini-split, compressor, or 240-volt welder circuit, and a vapor barrier if any part of the shop might someday be conditioned space. The written scope states thickness by area, the reinforcement schedule, the anchor plan, and the pad spec underneath it all.