What do the numbers for reinforcing bars signify, how can I figure the area from this number, and what is the weight per foot?

The numbers are the designations that are always used in detailing and they represent the diameter, approximately, in eighths of an inch. A No. 5 bar is approximately 5/8 inch in diameter and a No. 9 bar approximately 1 1/8 inch. The ASTM sizes and weights are given in the table. Nominal dimensions of a deformed bar, without including deformations, are equivalent to the dimensions that a plain round bar would have if its weight per foot were the same as the deformed bar.

Nominal dimensions-round section

Bar size number

Weight, pounds per foot

Diameter, inches

Cross-sectional area, square inches

Perimeter, inches

3

0.376

0.375

0.11

1.178

4

0.668

0.500

0.20

1.571

5

1.043

0.625

0.31

1.963

6

1.502

0.750

0.44

2.356

7

2.044

0.875

0.60

2.749

2.670

1.000

0.79

3.142

9

3.400

1.128

1.00

3.544

10

4.303

1.270

1.27

3.990

11

5.313

1.410

1.56

4.430

14

7.65

1.693

2.25

5.32

18

13.60

2.257

4.00

7.09