Vertical and adiabatic ionization potentials of small tin clusters having upto 20 atoms are numerically estimated using ab initio quantum mechanical methods. The ground state geometries and energetics of neutral and singly positively charged clusters are obtained using ultrasoft pseudopotential plane wave method with generalized gradient approximations. The calculated ionization potentials (IPs) show good agreement with the experimental results. The IPs of Sn7 and Sn10 show local maxima that corroborate the magic behavior of these clusters as found in the mass abundance spectra and photoionization experiments. A comparative study is also made to understand the effect of different exchange correlation functionals on the IPs using Gaussian molecular orbital methods.