The basic principle for the control of cracking is to provide a minimum amount of reinforcement such that its tensile capacity exceeds that of the concrete when it cracks. Thus:

p_crit = f_ct/f_y

where:

p_crit = critical (minimum) ratio of area of steel reinforcement to area of concrete
f_ct = tensile strength of the immature concrete
f_y = characteristic strength of the reinforcement.

To control maximum crack widths, the steel ratio required is:

p = (kRT₁aD)/(3w_max)

where:

T₁ = drop between peak temperature after casting and ambient temperature, °C
a = coefficient of thermal expansion, per °C
k = modification factor
R = restraint factor
D = bar diameter
w_max = maximum crack width.

Further guidance is given in BS 8007.