The specified yield stresses specified in the Standards for reinforcement have varied over the years. Some values are given below.

Plain round mild steel bars
BS 785: 1938 & BS 785: Part 1, 1967
36,000 psi for bars up to 1 ½ inches

BS 4449: 1969, 1978 & 1988
250 N/mm²

Cold worked deformed bars
BS 1144: 1943
70,000 psi for bars less than 3/8 inch
60,000 psi for bars over 3/8 inch
50,000 psi for twin twisted bars

BS 1144: 1967
66,000 psi for bars up to 5/8 inch
60,000 psi for bars over 5/8 inch
72,000 psi for 72 Grade ribbed bars with rolling mark

BS 4461: 1969
460 N/mm² for bars up to 16mm
425 N/mm² for bars over 16mm

Hot rolled deformed bars
BS 785: Part 1, 1967
60,000 psi for medium tensile bars
70,000 psi for high tensile bars

BS 4449: 1969
410 N/mm² for all sizes

BS 4449: 1978
460 N/mm² for bars up to 16mm
425 N/mm² for bars over 16mm