How Thick Should Your Concrete Slab Be
Thickness is the cheapest strength you can buy. Here is how much each use needs.
Use 4 inches minimum for patios, walkways, and shed floors; 5 to 6 inches for driveways and anything carrying vehicles. Thickness must be uniform: dips in the subgrade silently add concrete and cost. Pair with compacted gravel base and control joints.
Thickness by use
Patios, walkways, and shed floors: 4 inches minimum. Driveways and RV pads: 5 to 6 inches. Garage floors: 4 inches is common, 6 if you park heavy trucks. Hot tub pads: 4 inches of well-reinforced concrete on compacted base, or 6 inches for large spas. Anything structural or supporting a building: follow the engineer's spec, not this page.
Uniform thickness matters more than the number
A slab specced at 4 inches that dips to 6 inches in low spots uses 50 percent more concrete there and still has a 4-inch weak point everywhere else. Screed to a uniform depth, and measure the subgrade in several places before you order. This is the number one reason DIY pours run short.
Base and reinforcement
Concrete needs 4 to 6 inches of compacted gravel underneath to drain and to stop the slab from rocking on soft spots. Reinforcement (wire mesh or rebar) does not add much strength, but it holds cracks tight so they stay hairlines instead of separating. Control joints every 8 to 12 feet give shrinkage somewhere planned to go.
How thickness changes your order
Going from 4 to 5 inches adds 25 percent more concrete; 4 to 6 inches adds 50 percent. On a 400 square foot driveway that is the difference between 5 and 7.5 yards. Thickness is the fastest way to blow a concrete budget, so decide it before you calculate, not after.
Skip the arithmetic
Enter your slab dimensions and thickness to get yards and bags.
Slab thickness questions
How thick should a concrete patio be?
A residential patio should be at least 4 inches thick, poured on 4 inches of compacted gravel, with control joints to manage cracking.
How thick for a concrete driveway?
Driveways need 5 to 6 inches of concrete with wire mesh or rebar, on a compacted base, to handle vehicle weight without cracking.
Can I pour concrete 2 inches thick?
No, not as a freestanding slab. Two inches works only as a bonded overlay on existing sound concrete. Standalone slabs need 4 inches minimum.
Does thicker concrete crack less?
Somewhat, but cracking is controlled more by base preparation, control joints, and curing than by thickness alone.