  Particle number conservation in fermionic systems restricts the allowed local
operations on bi-partite systems. We show how this restriction is related to
measurement entropy of particle fluctuations and compute it for several regimes
of practical relevance. The accessible entanglement entropy restricted by
particle number conservation is equal, to leading order, to the full
entanglement entropy. The correction is bounded by the log of the variance of
particle number fluctuations. The results are applied to generic Fermi systems
in dimension d>1 as well as to several critical systems in d=1.
