  We calculate formation probabilities of the ground state of the finite size
quantum critical chains using conformal field theory (CFT) techniques. In
particular, we calculate the formation probability of one interval in the
finite open chain and also formation probability of two disjoint intervals in a
finite periodic system. The presented formulas can be also interpreted as the
Casimir energy of needles in particular geometries. We numerically check the
validity of the exact CFT results in the case of the transverse field Ising
chain.
