How thick should a concrete patio slab be?

What does the four-inch figure assume?

It assumes the preparation. A four-inch slab on a compacted, well-drained sub-base outperforms a six-inch slab poured onto soft fill, because a slab doesn't float. It's supported by what's under it, and cracking is usually a sub-base failure that the concrete gets blamed for.

So the questions that actually change the answer are what's going on it, what's under it, and what the ground does when it freezes.

What pushes it thicker?

• Vehicle loads. A patio a car or trailer will ever cross isn't a patio for this purpose. Driveway slabs are routinely specified thicker and often reinforced.
• A hot tub, an outdoor kitchen, anything with a concentrated point load. Manufacturers publish a required slab specification for spa installations, and that specification is the one to build to.
• Poor or expansive soils, and deep frost. Ground that moves seasonally moves the slab with it. In cold regions the frost line and the drainage of the sub-base matter more than an inch of concrete.
• Anything a future structure will sit on. A slab that will one day carry a roof, a wall or a screened enclosure is a foundation, and foundations are engineered.

What actually stops it cracking?

Concrete shrinks as it cures, and shrinkage cracks are normal. The craft is deciding where they go.

Control joints are deliberate weakened lines that give the crack somewhere to happen. The convention reported by the US Bureau of Reclamation from the American Concrete Institute's guidance is a groove about a quarter of the slab depth, spaced at twenty-four to thirty-six times the slab thickness. That is the same thing as two to three times the thickness in inches read off as feet, so a four-inch slab wants joints somewhere in the eight-to-twelve foot range. Aim for square panels. Rectangles up to about one-and-a-half to one are still within the recommendation; longer and thinner than that is outside it.

Isolation joints separate the slab from anything rigid it touches, so the two can move independently. Curing matters more than most people expect: concrete gains strength by staying damp, not by drying out, and a slab kept wet or covered for the first several days is meaningfully stronger than the same mix left in the sun. Reinforcement controls the width of a crack after it forms. It doesn't prevent cracking, and mesh left lying on the sub-base does little or nothing, because it has to sit within the slab to work.

Common questions

Do I need a permit?

Ask your building department before you order concrete. In the United States, residential slab requirements come from the jurisdiction's adopted building code, typically a version of the International Residential Code, sometimes alongside the American Concrete Institute's residential code, ACI 332, and jurisdictions amend them. An unattached patio slab is permit-exempt in many places, in which case there's no required number at all and the decision is yours. Outside the United States the governing document will be a different one entirely.

Is this engineering advice?

No, and it can't be. This page explains what the standard answer assumes so you know when it stops applying. Anything carrying a structure needs an engineer.

How much concrete will I need?

Once you've settled the thickness, work out volume, and remember your excavation is never as flat as your drawing. The sub-base high spots are where the slab ends up thin. The concrete calculator turns that thickness into cubic yards and bag counts for a rectangular slab, with the leftover of the last bag shown because counts round up.

Related

The concrete calculator gives volume and bag counts, how much waste factor to add covers why ordering to the exact number is the wrong target, and the home project tools hub has the rest of the material calculators.