How to Determine Rebar Size and Spacing in a Concrete Pad
When a vehicle drives over a concrete slab, the concrete holds up very well against the weight of the vehicle compressing it. However, depending on how the middle of the slab is supported, it may also flex in the middle due to tensile forces that pull apart, rather than compress.
Concrete is unable to withstand such forces. Steel, on the other hand, resists tensile forces well, so concrete workers reinforce concrete with steel rebar.

Decide on the percentage of steel to concrete. Structural concrete needs to be professionally engineered, so these calculations are just a general guideline. Typical values for continuously reinforced concrete pavement, for instance, are between 0.6 and 0.7 percent.

Find the rebartospacing multiplier with the formula M = 0.9_sqrt(Pt), with "P" representing the percentage of steel required and "t" representing the thickness of the slab. For example, if the percentage is 0.65 and the thickness of the slab is 6 inches, M = 0.9_sqrt(0.65*6) = 1.78.

Insert common spacing distances into the formula n = M_sqrt(s), with "s" being the spacing in inches. Round up the value of "n" to the next whole number to find the nominal rebar size required for that spacing. You can use any value of "s" you want, but for easier installation, "s" usually is a value that is a factor of 12, such as 3, 4, 6 or 12. For example, with spacing at four inches on center, n = 1.78_sqrt(4) = 3.56, so No. 4 rebar would be required. A layout with six inches on center calls for No. 5, and 12 inches on center requires No. 7.
Tip
You can derive these equations from the formula P = 100_As/Ac. "As" is the crosssectional area of a stick of rebar, and "Ac" is the crosssectional area of the concrete for each stick of rebar. The radius of a stick of rebar is equal to n/16, so As = pi_(n/16)². The crosssectional area of the concrete is the slab thickness multiplied by the spacing: Ac = ts. Therefore, P = 100_pi_n²/(16²_ts). Solve for "n" to get n = 0.9_sqrt(Pt)*sqrt(s).
Warning
Have structural concrete properly engineered and comply with all building codes applicable to your area.