In this paper, the conformal mapping technique is applied to obtained closed-form expressions for calculating the characteristic impedance and the effective dielectric constant of the slotline. The presented expressions take into account the finite width of the strips of the slotline, the case often omitted in other works. The validity of the presented expressions is tested in a number of examples. The obtained results are compared with those obtained from the full-wave electromagnetic analyses. Good agreement is noted in all of the considered cases.