  We show that numerical quasi-one-dimensional renormalization group allows
accurate study of weakly coupled chains with modest computational effort. We
perform a systematic comparison with exact diagonalization results in two and
three-leg spin ladders with a transverse Hamiltonian that can involve
frustration. Due to the variational nature of the algorithm, the accuracy can
be arbitrarily improved enlarging the basis of eigenstates of the density
matrix defined in the transverse direction. We observe that the precision of
the algorithm is directly correlated to the binding of the chains. We also show
that the method performs especially well in frustrated systems.
