Thickness is the simplest structural decision on a slab, and the rule is short: 4 inches carries people, 6 inches carries vehicles. Pour a patio at 6 inches and you buried money in the ground. Pour a driveway at 4 and you gambled the whole job on nobody ever backing a loaded truck onto it.
The geometry, before any prices
We do not carry separate per-thickness rates in our examples, so here is the part that is pure arithmetic: a 6 inch slab holds 50 percent more concrete than a 4 inch slab of the same footprint. On a 20 by 20 pad, 4 inches works out to about 5 cubic yards of concrete; 6 inches is about 7.4. More yards means more material and a bigger truck order, though the rest of the job (forming, grading, finishing) costs roughly the same either way. That is why going thicker never raises the total price by the full 50 percent. If you are checking your own pour, how to figure concrete yardage walks the math.
What actually needs 6 inches
Wheels and point loads. Driveways, garage floors, RV pads, and anything a tractor, trailer, or work truck will sit on. A parked vehicle concentrates weight in four small patches, and an RV or loaded trailer tongue is worse. That is a different problem than a hundred people standing on a patio.
One thing thickness does not fix: soft ground. A thick slab over a bad base still settles and cracks. If the subgrade is the worry, the money belongs in excavation and compacted base first, thickness second.
Reinforcement rides along with the choice
The steel decision usually tracks the thickness decision. Foot-traffic slabs at 4 inches typically get wire mesh, which does not stop cracks but holds them tight so they stay hairlines. Vehicle slabs at 6 inches get rebar, which adds real structural capacity. In both cases the steel has to be held up mid-slab on chairs; steel lying on the dirt is decoration.
Footpath or driveway: make the call once
Patios, sidewalks, and shed bases: 4 inches. Driveways, garages, RV pads: 5 to 6. The honest tiebreaker is the future, because you cannot thicken a slab later. If a vehicle will ever park on it, even once a year, build for the vehicle now. Run your dimensions through the concrete slab cost calculator to see how the square footage prices out at example rates.
If you pour slabs for a living, PriceDesk runs that calculator with your own rates and add-ons, in the truck. Free for 14 days, no credit card.