The size of rebar you need for a concrete slab depends on several factors, including the load the slab will bear, thickness of slab, the spacing of the rebar, load-bearing requirements, the type of structure, and local building codes. Here are some general guidelines: rebar size #3 is used for a 4-inch slab, #4 rebar used for a 5-inch slab and #5 rebar used for a 6-inch concrete slab.

Concrete is mixture of cement, sand, gravel with some recommended additives. When we study about properties of concrete, it is very strong under compressive and shear stress, however it perform very poorly under tensile strength. In fact the tensile strength of concrete is only about 10 to 15% of its compressive strength.

Reinforcing bars, also called rebar or steel rods, are used as composite material with concrete to increase the tensile strength of concrete. Rebar comes in a variety of lengths and thicknesses and typically has ridges or projections so they adhere well to concrete components.

The thickness of a concrete slab for residential applications like patios and driveways is about a 4-inch thick slab is common. For heavier loads or commercial applications, the thickness may be 5 inches or 6 inches to over a foot.

**What size rebar do I need for a concrete slab**

In general, for residential application, the minimum diameter of rebar used for concrete slabs is typically #3 rebar with a diameter of 3/8 inch, while the maximum diameter of rebar used for concrete slabs is typically #5 rebar with a diameter of 5/8 inch. However, in some cases, smaller diameter bars such as 8mm (1/4 inch) or 6mm (1/8 inch) may be used in specific applications or for light loads.

For concrete slabs, the contractors often use the 1/8 rule for determining the size of rebar for a concrete slab. For example, for a 3-inch slab, a #3 rebar will be used, while for a 4-inch slab, a #4 rebar will be used. Likewise, for a 5-inch slab, a #5 rebar will be used, while for a 6-inch depth of concrete slab, a #6 rebar will be used.

**However, as a general guideline for what size rebar do I need for a 4-inch, 5-inch and 6-inch concrete slab:**

- For a 4-inch concrete slab, you might need #3 rebar, which typically has a diameter of 3/8 inch.
- For a 5-inch concrete slab, you might use #4 rebar, which typically has a diameter of 1/2 inch.
- For a 6-inch concrete slab, you may need #5 rebar, which is typically 5/8-inch in diameter.

**Conclusions**:

Typically you will need #3 rebar with a diameter of 3/8 inch for a 4-inch concrete slab, #4 rebar with a diameter of 1/2 inch for a 5-inch concrete slab, and #5 rebar with a diameter of 5/8 inch for a 6-inch concrete slab.