  In finite volume the partition function of QCD with a given $\theta$ is a sum
of different topological sectors with a weight primarily determined by the
topological susceptibility. If a physical observable is evaluated only in a
fixed topological sector, the result deviates from the true expectation value
by an amount proportional to the inverse space-time volume 1/V. Using the
saddle point expansion, we derive formulas to express the correction due to the
fixed topological charge in terms of a 1/V expansion. Applying this formula, we
propose a class of methods to determine the topological susceptibility in QCD
from various correlation functions calculated in a fixed topological sector.
