Imperial bar sizes
Before metrication, the generally available size range for both round and square or chamfered square bars was ¼ inch to 1½ inches in the sizes shown below:
¼, 5/16, 3/8, 7/16, ½, 5/8, ¾, 7/8, 1 1/8, 1¼, 1 3/8, 1½
A 3/16 square area bar was also available for a time as were round area bars in sizes up to 2¼ inches.
Prior to 1964 most square and chamfered square bars were of square area so that nominal 1 inch bars had an area of 1 sq. inch. During the late 1960s a transition to round areas occurred with nominal 1 inch bars having an area of 0.785 sq. inches.
The nominal size range for twin twisted bars was from ¼ inch to 1¼ inches in the increments shown above. In addition smaller twin twisted bars were produced from 12, 10, 8, 6 and 5 SWG (Standard Wire Gauge) wires. The nominal size of a twin twisted bar referred to the size of one bar in the pair and a nominal 1 inch bar provided a steel area of 1.571 sq. in.
Further information on how reinforcement has developed over the years is given in Concrete Society Technical Report 70, Historical approaches to the design of concrete buildings and structures.