To the estimation of the wetting behaviour of solids, the contact angle on the boundary solid/fluid plays an important role. The paper proposes a new approach to estimate the contact angle size without experimental expenditure. Exclusively we use the available expert knowledge about the relationships between the polar portion of solids and fluids surface tension to the contact angle expressed in unprecise, linguistic terms like ＇＇The more polar is the solid the smaller is the contact angle to water＇＇. The fuzzy logic based on the theory of fuzzy sets is the well-known mathematical fool to handle linguistic terms and sentences. Using the methods of fuzzy theory and logic to a mathematical modelling and evaluation of expert knowledge, we finally get a fuzzy controller to the estimation of the contact angle size. A great advantage of the fuzzy method is that without consideration of experimental methods and additional assumptions expressed in mathematical equalities, an approximate estimation of the contact angle on the boundary solid/fluid is possible.