The conventional units for the solubility parameter are (calories per cm), or cal cm. The SI units are J m, equivalent to the pascal. 1 calorie is equal to 4.184 J.
1 cal cm = (523/125 J) (10 m) = (4.184 J) (0.01 m) = 2.045483 10 J m = 2.045483 (10 J/m)= 2.045483 MPa.
Given the non-exact nature of the use of δ, it is often sufficient to say that the number in MPa is about twice the number in cal cm.
Where the units are not given, for example, in older books, it is usually safe to assume the non-SI unit.