How Thick Should a Concrete Slab Be?

Published guidance puts sidewalks at 4 inches or more and driveways at 4 inches for passenger cars, or 5 or 6 inches where trucks and other heavy vehicles will use them. The driveway approach next to the street can be a separate case: of the two city handouts quoted below, one requires a minimum of 6 inches there and the other requires 6 inches.

The sections below take each kind of slab in turn and say who published each number, so you can open the source and read it yourself.

What the building code sets (IRC R506)

Section R506 of the residential code is titled “Concrete Floors (On Ground).” In the pages of the 2020 Indiana Residential Code posted by the building commission for Evansville and Vanderburgh County, R506.1 says concrete slab-on-ground floors must be at least 3½ inches (89 mm) thick.

That sentence is about floors. R506.1 does not give a thickness for a patio, a walk, or a driveway. For those, the numbers in the rest of this guide come from a manufacturer, a concrete industry association, and two city handouts, not from R506.

The text quoted here is the one Indiana adopted. Your state or city may have adopted a different edition or changed it, so for any slab that needs a permit, ask your local building department which rule applies before you set the forms.

Patios and sidewalks: at least 4 inches for sidewalks

QUIKRETE’s project guide for sidewalks and small slabs says sidewalks should be at least 4 inches (100 mm) thick.

None of the sources used for this guide gives a thickness for a patio by name. The figures QUIKRETE’s guide does give are that 4-inch minimum for sidewalks and “normally” 4 to 6 inches for simple slabs for small foundations and bases, covered under shed bases below. If your patio needs a permit, or sits where your city has its own rules, the building department’s number is the one to use.

Driveways: 4 inches for cars, 5 or 6 for heavier vehicles, and the city’s number at the street

QUIKRETE’s driveway guide says driveway thickness depends solely on the weight of the vehicles that will use it. For a driveway that serves only passenger cars, it says 4 inches is sufficient. Where trucks and other heavy vehicles will use the driveway, it recommends 5 or 6 inches.

The driveway guidelines posted on the Illinois Ready Mixed Concrete Association’s website start from the same number: set the forms to give a minimum slab depth of 4 inches.

Where a driveway crosses the public right-of-way to reach the street, the city may set the thickness. Two examples:

• Ankeny, Iowa: the city’s guidelines for single-family sidewalks and driveways in the public right-of-way say driveway approaches must be a minimum of 6 inches thick.
• Bloomington, Minnesota: the city’s handout on residential driveway approaches says concrete depth must be 6 inches for the approach, the apron, and the sidewalk within the driveway or driveway approach.

These are two cities, not a national rule. Your city may publish a different number, or none. Before you pour the part of a driveway nearest the street, ask your city’s engineering or public works department what it requires there.

Shed bases: normally 4 to 6 inches for small bases, depending on the load

QUIKRETE’s guide does not mention sheds by name. What it says is that simple slabs for small foundations and bases are normally 4 to 6 inches (100 to 150 mm) thick, depending on the load they must bear.

The guide does not say which loads call for which end of that range, so this page does not pick one for you. If your shed came as a kit, check the foundation instructions from its maker. If the shed needs a permit, the building department can tell you what it expects under it.

The published figures side by side:

Slab	Thickness	Published by
Slab-on-ground floor	At least 3½ in	2020 Indiana Residential Code, R506.1
Sidewalk	At least 4 in	QUIKRETE
Simple slab for a small foundation or base	Normally 4 to 6 in, depending on load	QUIKRETE
Driveway, passenger cars only	4 in	QUIKRETE
Driveway, trucks and other heavy vehicles	5 or 6 in	QUIKRETE
Driveway	At least 4 in	Guidelines posted by the Illinois Ready Mixed Concrete Association
Driveway approach in the public right-of-way	At least 6 in	City of Ankeny, Iowa
Driveway approach and apron	6 in	City of Bloomington, Minnesota

What else matters besides thickness: joints and strength

Thickness is one of several things the same sources specify. Two others are where the control joints go and how strong the concrete is.

Control joints are there so the slab cracks at the base of the joint rather than across its face. QUIKRETE says they should penetrate roughly ¼ of the slab thickness and be spaced every 6 to 8 feet in 4-inch-thick slab work. On a 4-inch slab, a quarter of the thickness is 1 inch.

The driveway guidelines on the Illinois Ready Mixed Concrete Association’s website give the same fraction, a cut of at least ¼ of the slab thickness, and a wider spacing: joints at intervals not exceeding 10 feet. The two sources name the same fraction for depth, one as “roughly” and one as a minimum, and differ on how far apart. The 6-to-8-foot figure is from a guide to sidewalks and small slabs, and the 10-foot limit is from guidelines for driveways.

For strength, the same guidelines call for a driveway mix with a compressive strength of at least 4,000 psi at 28 days, and tell the reader to refer to local code requirements. If you order ready-mix, that is a number to give the supplier.

Thickness also sets how much concrete you buy, because volume rises in step with it: a 6-inch slab takes one and a half times the concrete of a 4-inch slab of the same length and width. With this site’s defaults of 10% extra and 0.60 cu ft per 80 lb bag, a slab 10 ft 6 in by 12 ft takes 77 bags at 4 inches thick and 116 bags at 6 inches.