Rebar, short for reinforcing bar, is a long steel rod mainly used to reinforce concrete. Rebar provides tensile strength for the concrete and is an ideal material because steel and concrete contract and expand at the same rate. The rods typically have raised surfaces or "deformities" that provide extra surface area for bonding with the concrete. Suppliers typically sell rebar in 20 and 60-foot lengths. There are 11 standard thicknesses of rebar in the United States.
Use a ruler to measure the diameter of the rebar. Do not measure the raised portion of any ridges or deformities.
Round the measurement to the nearest 1/8 inch.
Divide the diameter by 1/8 inch to determine the bar size. A piece of rebar with a 3/8-inch diameter is size No. 3. A piece of rebar with a 1-inch diameter is a size No. 8. In the United States, rebar comes in 11 standard sizes of Nos. 3-11, No. 14 and No. 18.
Many United States construction projects require metric specifications for materials. To accommodate this, a "soft" metric system is available for sizing rebar products, which do not conform to metric standards used in Canada and Europe. The "soft" metric system labels rebar according to the approximate number of millimetres in the diameter of the rebar. For example, rebar size No. 4 has a diameter of 1/2 inch. There are 12.7 millimetres in 1/2 inch, so the "soft" metric designation is No. 13.