We investigate condition numbers of matrices that appear during solving systems of linear equations. We consider iterative methods to solve the equations, namely Jacobi and Gauss-Seidel methods. We examine the influence of the condition number on convergence of these iterative methods. We study numerical aspects of relations between the condition number and the size of the matrix and the number of iterations experimentally. We analyze random matrices, the Hilbert matrix and a strictly diagonally dominant matrix.