Arch length (AL), intercanine width (ICW), and intermolar width (IMW) are essential for diagnosis and treatment planning and are closely related factors in orthodontics. The aim of the present study was to determine correlations between these measurements and to predict some of these measurements based on others. The dental casts of 197 Spanish patients (119 females and 78 males) with a mean age of 18 years (11-26 years) in the permanent dentition attending the Orthodontic Department of the University of Valencia, Spain, were selected. ICW, IMW, and AL on each dental cast were measured using a previously tested digital method. Correlation between variables was determined using Pearson's correlation coefficient. Linear regression analysis was applied and the 95 per cent confidence intervals (CIs) for slope and intercept were determined. The data showed very high correlations between ICW and AL, both for the upper and lower arches and for males and females, r = 0.925. This coefficient was very close to 1, indicating a linear relationship. The regression equation for AL and ICW was AL = 1.36 ICW + 29.39 for both arches. There was very high correlations between AL and ICW for the upper and lower arches and a regression equation between both magnitudes was established indicating that the size of one factor can be predicted by knowing the other. For an increase of 1 mm in ICW, the AL increases approximately 1.36 mm with a 95 per cent CI (1.30-1.42).