A simple model is suggested which enables the calculation of the relationship between the domain length and the domain wall spacing in constructional steel. The magnetic interaction between domain in neighboring grains is measured from the observed domain patterns, and a coefficient is proposed which makes possible such a calculation for antiparallel domains. The calculated results compare favorably with experiments. However, the observed scatter of experimental results is due to the complex magnetic interaction of ferrite grains with pearlite, cementite and nonmetallic inclusions.